Frequent eruptions of Mount Rainier over the last 2,600 years · magmatic eruption of Mount...
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RESEARCH ARTICLE Frequent eruptions of Mount Rainier over the last ∼2,600 years T. W. Sisson & J. W. Vallance Received: 2 October 2007 / Accepted: 15 August 2008 # Springer-Verlag 2008 Abstract Field, geochronologic, and geochemical evidence from proximal fine-grained tephras, and from limited exposures of Holocene lava flows and a small pyroclastic flow document ten–12 eruptions of Mount Rainier over the last 2,600 years, contrasting with previously published evidence for only 11–12 eruptions of the volcano for all of the Holocene. Except for the pumiceous subplinian C event of 2,200 cal year BP, the late-Holocene eruptions were weakly explosive, involving lava effusions and at least two block-and-ash pyroclastic flows. Eruptions were clustered from ∼2,600 to ∼2,200 cal year BP, an interval referred to as the Summerland eruptive period that includes the youngest lava effusion from the volcano. Thin, fine-grained tephras are the only known primary volcanic products from eruptions near 1,500 and 1,000 cal year BP, but these and earlier eruptions were penecontemporaneous with far- traveled lahars, probably created from newly erupted materials melting snow and glacial ice. The most recent magmatic eruption of Mount Rainier, documented geo- chemically, was the 1,000 cal year BP event. Products from a proposed eruption of Mount Rainier between AD 1820 and 1854 (X tephra of Mullineaux (US Geol Surv Bull 1326:1–83, 1974)) are redeposited C tephra, probably transported onto young moraines by snow avalanches, and do not record a nineteenth century eruption. We found no conclusive evidence for an eruption associated with the clay-rich Electron Mudflow of ∼500 cal year BP, and though rare, non-eruptive collapse of unstable edifice flanks remains as a potential hazard from Mount Rainier. Keywords Mount Rainier . Eruptions . Holocene . Tephra . Glass . Lahar . Hazards Introduction Mount Rainier is an active volcano of the Cascade Range in Washington State, 50–70 km southeast of the major metropolitan areas of Seattle and Tacoma. With one exception, rivers heading on Mount Rainier flow to the west and northwest into the southern Puget Sound lowland where more than 150,000 people live on deposits from lahars and related floods released from Mount Rainier over the last 5,600 years (Crandell and Waldron 1956; Scott et al. 1995; Vallance and Scott 1997; Sisson et al. 2001). Mount Rainier produced few voluminous tephra-fall depos- its in the Holocene, and glaciers cover much of the upper mountain, hindering detailed estimates of its recent eruptive history. This history is important because it has been unclear if major lahars formed mainly during times of eruptive activity (due to dislodgement of edifice flanks and by newly erupted hot materials transiting and melting glaciers), or if large lahars took place during non-eruptive times (by spontaneous gravitational collapse of unstable Bull Volcanol DOI 10.1007/s00445-008-0245-7 T. W. Sisson and J. W. Vallance contributed equally to this study. Electronic supplementary material The online version of this article (doi:10.1007/s00445-008-0245-7) contains supplementary material, which is available to authorized users. T. W. Sisson (*) Volcano Hazards Team, USGS, 345 Middlefield Road, Menlo Park, CA 94025, USA e-mail: [email protected] J. W. Vallance Cascades Volcano Observatory, USGS, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683-9589, USA e-mail: [email protected]
Transcript of Frequent eruptions of Mount Rainier over the last 2,600 years · magmatic eruption of Mount...
RESEARCH ARTICLE
Frequent eruptions of Mount Rainierover the last ∼2,600 years
T. W. Sisson & J. W. Vallance
Received: 2 October 2007 /Accepted: 15 August 2008# Springer-Verlag 2008
Abstract Field, geochronologic, and geochemical evidencefrom proximal fine-grained tephras, and from limitedexposures of Holocene lava flows and a small pyroclasticflow document ten–12 eruptions of Mount Rainier over thelast 2,600 years, contrasting with previously publishedevidence for only 11–12 eruptions of the volcano for all ofthe Holocene. Except for the pumiceous subplinian C eventof 2,200 cal year BP, the late-Holocene eruptions wereweakly explosive, involving lava effusions and at least twoblock-and-ash pyroclastic flows. Eruptions were clusteredfrom ∼2,600 to ∼2,200 cal year BP, an interval referred toas the Summerland eruptive period that includes theyoungest lava effusion from the volcano. Thin, fine-grainedtephras are the only known primary volcanic products fromeruptions near 1,500 and 1,000 cal year BP, but these andearlier eruptions were penecontemporaneous with far-traveled lahars, probably created from newly eruptedmaterials melting snow and glacial ice. The most recentmagmatic eruption of Mount Rainier, documented geo-
chemically, was the 1,000 cal year BP event. Products froma proposed eruption of Mount Rainier between AD 1820and 1854 (X tephra of Mullineaux (US Geol Surv Bull1326:1–83, 1974)) are redeposited C tephra, probablytransported onto young moraines by snow avalanches, anddo not record a nineteenth century eruption. We found noconclusive evidence for an eruption associated with theclay-rich Electron Mudflow of ∼500 cal year BP, andthough rare, non-eruptive collapse of unstable edifice flanksremains as a potential hazard from Mount Rainier.
Keywords Mount Rainier . Eruptions . Holocene . Tephra .
Glass . Lahar . Hazards
Introduction
Mount Rainier is an active volcano of the Cascade Range inWashington State, 50–70 km southeast of the majormetropolitan areas of Seattle and Tacoma. With oneexception, rivers heading on Mount Rainier flow to thewest and northwest into the southern Puget Sound lowlandwhere more than 150,000 people live on deposits fromlahars and related floods released from Mount Rainier overthe last 5,600 years (Crandell and Waldron 1956; Scott etal. 1995; Vallance and Scott 1997; Sisson et al. 2001).Mount Rainier produced few voluminous tephra-fall depos-its in the Holocene, and glaciers cover much of the uppermountain, hindering detailed estimates of its recent eruptivehistory. This history is important because it has beenunclear if major lahars formed mainly during times oferuptive activity (due to dislodgement of edifice flanks andby newly erupted hot materials transiting and meltingglaciers), or if large lahars took place during non-eruptivetimes (by spontaneous gravitational collapse of unstable
Bull VolcanolDOI 10.1007/s00445-008-0245-7
T. W. Sisson and J. W. Vallance contributed equally to this study.
Electronic supplementary material The online version of this article(doi:10.1007/s00445-008-0245-7) contains supplementary material,which is available to authorized users.
T. W. Sisson (*)Volcano Hazards Team, USGS,345 Middlefield Road,Menlo Park, CA 94025, USAe-mail: [email protected]
J. W. VallanceCascades Volcano Observatory, USGS,1300 SE Cardinal Court, Building 10, Suite 100,Vancouver, WA 98683-9589, USAe-mail: [email protected]
edifice flanks, or by triggering from tectonic or severerainfall events (Scott 2004)). Accurate estimates of thenumber, timing, and style of Holocene eruptions can help toevaluate links between eruptions and lahars, and therebyguide monitoring and event-response planning.
To better determine Mount Rainier’s late-Holoceneeruptive history, we undertook a detailed stratigraphic andgeochemical study of thin apparent tephra-fall deposits onthe volcano’s proximal edifice flanks, as well as of limitedexposures of young lava and pyroclastic-flow deposits.Here we present evidence for ten–12 temporally andcompositionally distinguishable eruptions over the last2,600 years. Most eruptions probably involved multipleexplosive ejections of ash, and some were accompanied bypyroclastic or (and) lava flows. This estimate supercedesprevious published evidence for only three or four eruptionsover that time span, and for only 11 or 12 eruptions for allof the Holocene (Mullineaux 1974; Hoblitt et al. 1998). Thelast 2,600 years were studied in detail because theyencompass an interval of frequent eruptions and laharsfrom ∼2,600 to ∼2,200 cal year BP, referred to here as theSummerland eruptive period, that followed a period ofapparent dormancy from ∼4,400 to ∼2,600 cal year BP, andbecause major lahars are also known at ∼1,500, ∼1,000,and ∼500 cal year BP that might have been caused by
eruptions (Crandell 1971; Scott et al. 1995; Zehfuss et al.2003) (cal year BP refers to radiocarbon ages calibrated tocalendar years, 14C year BP refers to radiocarbon ages asmeasured, year BP refers to interpolated ages, all relativeto AD 1950). In the following we first describe limitedexposures of late-Holocene lava and pyroclastic flow deposits,beginning with the volcano’s summit, and then describetephra-fall deposits chiefly from a particularly completeexposure near Summerland on the volcano’s east flank(Fig. 1). Tephra and flowage deposits are then correlatedusing glass and mineral compositions to arrive at the likelynumber and style of eruptions over the last 2,600 years.
Late-Holocene lava and pyroclastic flow deposits
East summit crater lava flows
The present summit cone of Mount Rainier grew subse-quent to the major Osceola collapse of 5,600 cal year BP(Crandell and Waldron 1956; Vallance and Scott 1997) thatremoved the volcano’s summit and east–northeast flankduring a period of magmatic eruptions. The Osceola collapseexcavated a horseshoe-shaped crater 1.5 km across, open tothe east–northeast. The collapse amphitheater is now almost
47o 0
0' N
121o 44' W121o 59' W
46o 4
5' N
706
410
123
Carbon River
Whi
teR
iver
Nisqually River
Puyallup River
Wes
t For
kW
hit e
Riv
er
Summerlandtephra section
FanLakeParadise
Glacier Basin
South Puyallupblock-and-ash flow
deposit
Kau
tzCre
ek
White Riverash exposure
Figure 2
MowichLake
Puy. Clvr.
TG
10 km
Fig. 1 Map of Mount Rainiervolcano and vicinity showingdiscussed localities, extent ofQuaternary volcanic rocks(green), flank vents (yellowstars), glaciers (white), and Ter-tiary basement (gray). Rectangleshows area of Fig. 2. Solid blacklines show roads, some withWashington State Route numb-ers. TG is Tahoma Glacier, Puy.Clvr. is Puyallup Cleaver. Mar-ginal ticks and internal crossesmark latitude and longitude
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entirely filled by younger lava flows that constructed a newsummit cone, but the amphitheater’s upper margin can berecognized as an arc of small peaks flanking the true summiton the north, northwest, and southwest (Figs. 1 and 2). Theformer outlet of the collapse amphitheater is now replacedby a smooth dip-slope of ice-buried young lava flows thatspans more than 1,500 m (4,800 ft) elevation from thesummit to Camp Schurman and Steamboat Prow on thevolcano’s east–northeast flank. The smooth constructionalslope of the young lava cone contrasts with the steep,irregular incised headwalls elsewhere on the upper volcano.The young cone forms the volcano’s summit and is cappedby two overlapping craters, each 0.4 km across, with theeastern crater younger and truncating the western crater.The east summit crater consists of one or two conformable,radially outward-dipping lava flows of compositionallyuniform porphyritic andesite (plagioclase–hypersthene–au-gite–Fe-Ti oxides) (Table 1), overlain at the true summit byabout 5 m of poorly bedded unconsolidated (explosion)breccia. The east summit crater rim exposes the youngestlavas erupted from the volcano, and paleomagnetic meas-urements of these are consistent with an eruption age of2,000–2,200 year BP (Vallance et al., in prep). The flowsare beheaded and exposed in cross section along the innercrater wall, indicating that explosions enlarged the cratersubsequent to the youngest lava effusion. Rocks formingthe western half of the east summit crater, as well as those
of the earlier west summit crater, are pervasively replacedby clays and other secondary minerals due to acid-steamalteration (Frank 1995; Zimbelman 1996; Crowley andZimbelman 1997; Finn et al. 2001; John et al. 2008), thoughblocks of intensely altered lava can still be distinguished.
Emmons–Winthrop lava flows
Isolated windows through the large Emmons and WinthropGlaciers expose lava flows that form the surface of theyoung summit cone in the former breach of the Osceolacollapse amphitheater (Figs. 1 and 2). These rock exposuresare directly down-slope of the east summit crater, consistentwith a late-cone-building age for the exposed lava flows.Outcrops at 2,900–3,000 m (9,500–9,800 ft) elevationconsist of minimally incised, variably glassy, polygonal-jointed ice-chilled andesite, and a steeply flow-banded lavalevee of the same material. Lava samples are composition-ally uniform porphyritic andesite (plagioclase–hyper-sthene–augite–Fe-Ti oxides, one rounded amphibole foundin thin sections from five localities), similar in majorelement concentrations (Table 1) to andesite samples fromthe east summit crater. They are distinguished from thesummit lavas, however, by their much higher Sr concen-trations (∼800 vs. 550 ppm), and to a lesser degree by theirhigher concentrations of Th (6.8 vs. 5.3 ppm), U (2.1 vs.1.6 ppm), and light rare earth elements (57 vs. 45 ppm Ce),
10000
750012500
Emmons Glacier
CarbonGlacier
WinthropGlacier
pm
pm
pm
pm
Summerlandtephra section
121ο 45' W 121ο 40' W
46ο
52' 3
0" N
46ο
50' N
5 km
SteamboatProw
CampSchurman
TG
Fig. 2 Detailed map of Mount Rainier’s summit and northeast slopeshowing upper perimeter of Osceola collapse amphitheater (hachuredline), approximate area of young summit cone (dashed line andshaded), Emmons–Winthrop high-Sr lava flows (orange), east summitcrater lava flows (red and dotted line where concealed), west summitcrater (mauve), area of summit hydrothermal alteration (cross pattern),
Pleistocene lava flows (green), Tertiary basement (gray), and glacialdeposits (yellow). Paleomagnetic measurement sites (pm) fromVallance et al., in prep. TG is Tahoma Glacier. Contour interval500 ft (152 m), index contours every 2,500 ft (762 m). Marginal ticksand internal crosses mark latitude and longitude
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Tab
le1
Representativewho
le-rockchem
ical
compo
sitio
nsof
late-H
oloceneMou
ntRainier
lava
flow
sandblock-and-ashbo
mbs
eastsummitcrater
lava
flow
Emmons–WinthropGlacier
lava
flow
sSouth
Puyallupblock-and-ashflow
breadcrustbo
mbs
Sam
ple
93RW81
93RW17
493
RW177
03RW90
595
RE50
595
RE50
695
SR50
795
RE50
893
MW68
93MW71
93RW169
00SMN806
Latitu
dea
46.851
646
.850
346
.852
246
.851
846
.864
046
.859
646
.876
646
.870
746
.808
246
.808
046
.843
947
.140
1Lon
gitude
a−1
21.754
0−1
21.756
5−1
21.753
6−1
21.754
7−1
21.728
0−1
21.725
2−1
21.741
0−1
21.731
5−1
21.893
6−1
21.894
6−1
21.816
8−1
22.232
9SiO
2(w
t.%)
61.72
62.15
61.68
61.78
60.65
61.15
60.66
60.42
60.76
60.81
60.77
60.66
TiO
20.89
0.87
0.88
0.89
0.91
0.86
0.88
0.91
0.94
0.92
0.94
0.95
Al 2O3
17.18
17.24
17.19
17.33
17.52
17.64
17.62
17.56
17.41
17.57
17.45
17.46
FeO
b5.32
5.29
5.34
5.26
5.57
5.40
5.48
5.56
5.61
5.48
5.59
5.61
MnO
0.10
0.10
0.10
0.09
0.10
0.10
0.10
0.10
0.10
0.10
0.10
0.10
MgO
2.89
2.88
2.90
2.96
3.02
2.71
2.94
3.12
3.09
2.98
3.06
3.15
CaO
5.82
5.61
5.86
5.84
6.06
5.94
6.11
6.16
6.09
6.11
6.08
6.12
Na 2O
4.14
3.93
4.09
3.96
4.17
4.18
4.17
4.16
4.14
4.14
4.12
4.10
K2O
1.68
1.70
1.69
1.65
1.70
1.72
1.72
1.70
1.59
1.60
1.61
1.56
P2O5
0.27
0.23
0.27
0.25
0.31
0.31
0.32
0.32
0.29
0.28
0.28
0.29
Totalb
99.01
98.32
98.97
97.53
99.87
99.66
99.71
99.75
99.44
99.12
99.40
99.70
Rb(X
RF,
ppm)
4444
4439
3634
3636
4443
4239
Sr
550
540
560
550
744
815
806
761
540
540
540
563
Y20
2020
1614
1312
1320
2322
17Zr
188
190
190
181
182
189
187
183
178
176
182
174
Nb
1010
1211
87
88
<10
1210
12Ba
455
445
460
438
503
478
492
501
460
445
460
445
Ni
1013
1218
33
35
<10
<10
<10
12Cu
25<10
2030
1014
2019
2018
1621
Zn
6868
6774
6265
6366
7068
6774
V91
101
9693
109
116
Rb(INAA,pp
m)
4442
4242
3936
3838
4244
40Cs
1.8
1.9
1.8
1.9
0.8
1.4
1.5
1.2
1.8
1.7
1.7
Th
5.24
5.42
5.37
5.40
6.80
6.71
7.01
6.61
5.11
5.00
5.05
U1.6
1.6
1.6
2.0
2.2
1.9
2.1
2.1
1.5
1.6
1.5
La
22.6
21.8
22.3
23.1
27.3
27.1
27.8
27.8
22.2
21.7
22.2
Ce
45.2
43.5
45.3
50.8
56.8
57.1
58.1
56.4
43.3
42.9
44.3
Nd
2321
2323
.928
.629
.030
.227
.821
2022
Sm
4.54
4.41
4.63
5.00
5.65
5.58
5.78
5.61
4.52
4.45
4.56
Eu
1.2
1.1
1.25
1.27
1.42
1.42
1.46
1.45
1.21
1.19
1.24
Tb
0.54
0.52
0.57
0.60
0.58
0.55
0.57
0.58
0.55
0.53
0.54
Yb
1.7
1.6
1.6
1.73
1.66
1.65
1.63
1.66
1.4
1.4
1.5
Lu
0.23
0.22
0.23
0.24
0.24
0.25
0.24
0.23
0.21
0.22
0.23
Hf
4.35
4.37
4.42
4.52
4.56
4.75
4.66
4.51
4.05
4.01
4.06
Ta0.77
0.79
0.77
0.81
0.71
0.64
0.67
0.69
0.76
0.77
0.78
Sc
12.2
12.1
12.4
12.8
12.6
10.5
12.0
12.6
13.7
13.4
13.7
Cr
28.8
29.6
28.8
29.8
18.9
11.4
15.7
18.6
27.5
27.4
26.9
Co
15.2
14.9
15.5
15.7
16.3
14.6
15.9
16.4
16.1
15.7
16.2
aNAD27
CONUSgeog
raph
icdatum
bAnalysesno
rmalized
to10
0wt.%
with
allFeas
FeO
,totalgivesoriginal
total
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and lower Cr concentrations (18 vs. 30 ppm). Such high Srconcentrations are uncommon for Mount Rainier eruptiveproducts (McKenna 1994; Stockstill et al. 2002; Sisson,unpublished), facilitating correlations with Holocenetephras.
The broad geographic extent of the high-Sr andesitesbeneath the Emmons and Winthrop Glaciers is evidencethat the lavas are multiple flows or flow lobes from a single,widespread effusive episode that predated eruption of theeast summit crater andesite lavas. Paleomagnetic orienta-tions from four Emmons–Winthrop high-Sr lava flowlocalities are indistinguishable, differ from those from thesummit lava flows, and match that of a late-Holocenepyroclastic flow deposit in the valley of the South PuyallupRiver on the volcano’s lower southwest flank (Vallanceet al., in prep; Hagstrum and Champion 2002). Thesegeologic, compositional, and paleomagnetic results areconsistent with the Emmons–Winthrop high-Sr andesiteshaving erupted late in the growth of the post-Osceolasummit cone, but before the eruption of the normal-Srandesites of the east summit crater. Contacts between thehigh-Sr and the younger summit crater lava flows areconcealed by thick glacial ice.
South Puyallup block-and-ash flow
A non-welded, sandy block-and-ash flow deposit contain-ing prominent dark brown-to-black breadcrust bombs, 0.2–1.25 m diameter, is exposed where the West Side Road ofMount Rainier National Park crosses the valley of theSouth Puyallup River (Crandell 1971) (Fig. 1). Uniformpaleomagnetic orientations of undisturbed breadcrustbombs indicate that the flow was emplaced hot and cooledin the same magnetic field direction as the high-Sr lavaflows exposed through the Emmons and Winthrop Glaciers(Crandell 1971; Hagstrum and Champion 2002; Vallance etal., in prep.). Radiocarbon dating of a carbonized log(Crandell 1971; Table 2) indicates eruption of the pyroclas-tic flow at 2,580±150 cal year BP (calendar ages withuncertainties report the midpoint and range of the mostprobable calibration-age window from OxCal 4.0 (BronkRamsey 1995, 2001); thus, 2,580±150 represents a cali-bration window of 2,730–2,430 cal year BP). Roadcutincisions show that the pyroclastic deposit is no more than15 m thick and forms a small (∼0.3 km2) valley-flooringterrace near 1,130 m (3,700 ft) elevation. Erosion hasremoved the deposit down-valley, but glassy breadcrustbombs are scattered discontinuously up the valley of theSouth Puyallup River to at least has high as 1,300 m(4,300 ft) elevation, and isolated breadcrust bombs areperched atop narrow Puyallup Cleaver as high as 2,800 m(9,200 ft) elevation; rare breadcrust bombs are also in theupper valley of Tahoma Creek to the south of the South
Puyallup locality. This distribution indicates that thepyroclastic flow descended southwest from the summitcone through a low notch in the rear wall of the Osceolacollapse amphitheater at the head of the Tahoma Glacier(Figs. 1 and 2), and then traveled into the valley of theSouth Puyallup River system, with a small amount alsospilling south into Tahoma Creek.
The South Puyallup breadcrust bombs are composition-ally uniform (Table 1) porphryitic andesites (plagioclase–hypersthene–augite–Fe-Ti oxides, one resorbed amphibolefound in thin sections of 11 bombs) that are slightly lessevolved in major element abundances (lower SiO2, higherFeO*, MgO, CaO concentrations) than andesites from theeast summit crater or the Emmons–Winthrop Glaciers. Thebombs’ trace element abundances are unremarkable forMount Rainier andesites and are similar to those of samplesof the east summit crater (Table 1). The bombs are thusdistinct from the high-Sr lava flows exposed through theEmmons and Winthrop Glaciers, despite sharing a commonpaleomagnetic orientation. This paleomagnetic direction isatypical for the late Holocene (Hagstrum and Champion2002), which is evidence that the block-and-ash flow andEmmons–Winthrop lava flows resulted from separateeruptions of compositionally distinct magmas over a brieftime period (<100 years, D. Champion 2005, personalcommunication). Relative ages of the Emmons–Winthropand South Puyallup eruptives are interpreted in the section:Correlation of flow and fall deposits based on glass andmineral compositions.
Table 2 Calibrated 14C ages establishing timing of Mount Rainier’sSummerland period and younger eruptions
Stratigraphic unit Number Age (cal year BP) ±
White River tephra 1 1,040 410Above TC2 1 1,420 100Below TC1 1 1,610 90Above C tephra 2 2,190 130Below C tephra (SL7) 2 2,270 220SL6 1 2,420 80SL5 1 2,240 250Top of SL4 1 2,160 150SL4 1 2,540 180S. Puyallup block & ash flow 1 2,580 150SL2 3 2,610 90SL1 1 2,450 100Round pass mudflow 5 2,590 40Between MSH Pu, Ps 1 2,870 90Base of MSH Pm 2 3,030 70
Reported ages are weighted means of n samples, using calibrated agesdetermined as midpoint of highest probability results from OxCal 4.0(Bronk Ramsey 1995, 2001); ± represents acceptable age window.Raw age measurements mainly from Vallance et al., in prep., alsoCrandell (1971), Mullineaux (1974), Scott et al. (1995), Hoblittwritten comm. (2008)
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Crandell (1971) notes an abundance of black andesitebombs in the ∼500 cal year BP Electron Mudflow in thevicinity of the towns of Orting and McMillan, 40–50 kmNW of Mount Rainier’s summit. Bombs collected from thatarea (SMN sample in Table 1) are identical in chemicalcomposition and appearance to breadcrust bombs collecteddirectly from the South Puyallup pyroclastic flow deposit,indicating that those bombs in the Electron Mudflow wereentrained from South Puyallup-related deposits and are nota juvenile magmatic component of the Electron Mudflowevent.
Late-Holocene Mount Rainier tephra deposits
Mullineaux (1974) identified, dated, and named 11 tephra-fall deposits interpreted as products of explosive Holoceneeruptions of Mount Rainier (Fig. 3). These Mount Rainiertephras consist chiefly of juvenile pumiceous-to-scoria-ceous lapilli, ash, and bombs, generally deposited as lobesextending east from the volcano, with the exception of twounits that are a local directed blast deposit of dense lithicblocks (layer S) and a mixed fall deposit of clay and pumice(layer F), both associated with the Osceola collapse eventof 5,600 cal year BP (Vallance and Scott 1997). To these11, Hoblitt et al. (1998) added a 1,080±250 14C year BP(1,040±410 cal year BP) fine-grained tephra that isrestricted to the upper valley of the White River (Fig. 1).Five of the named tephras, in order layers H (∼4,700 yearBP), B (∼4,500 year BP), C (∼2,200 cal year BP), WhiteRiver (∼1,000 cal year BP) and X (AD 1820–1854), weredeposited subsequent to the Osceola collapse event andrecord some of the eruptions that rebuilt Mount Rainier’ssummit cone (excepting layer X, for which we presentevidence following that this is not a true eruption deposit).
Mullineaux (1974) noted that additional eruptions mightbe recorded by numerous thin (mm–cm), dark strataconsisting of sparsely vesicular ash grains and crystalfragments that are interlayered with the prominent namedtephras, but that the thin dark ashes could not, at that time,be distinguished from local accumulations of ash redepos-ited by wind and water. Much effort in the present study hasbeen directed at developing a stratigraphic sequence forthese dark tephra layers, investigating if the layers areproducts of eruptions, and correlating some with thepreviously described young lava and pyroclastic flowdeposits. A consistent sequence of dark, fine-grained tephralayers or groups of layers is preserved in sub-alpinemeadows, generally 6–12 km from the volcano’s summit,as would result if most of those ash deposits were productsof eruptions.
To further investigate if the dark, fine-grained tephrasresult from eruptions, multiple glassy ash grains were hand
picked from samples of the various ash deposits, and theirglass compositions were measured by electron-microprobe(see Electronic Supplementary Materials for analyticalmethods). Most glass-rich grains are blocky and sparselyvesicular, consistent with quenching and explosive frag-mentation of largely degassed magma traversing the edificehydrothermal system, but grains range with increasingvesicularity to scoriaceous with fluidal shapes, or lesscommonly, pumiceous (Fig. 4), indicative of some syn-eruptive vesiculation. Visibly glassy grains constitute aminority of ash (<30%) in each of the fine-grained darktephra deposits, with the majority of grains being well-crystallized angular volcanic rock fragments and brokenphenocrysts. Phenocrysts and microphenocrysts of pla-
X - probably non-eruptive
MSH W - 471 ybp
C - 2200 ybp
MSH P - 2600-3000 ybp
MSH Y - 3600-4400 ybp
B
H
F - 5600 ybp Osceola collapse
N
D
L - ~7300 ybp
A - ~7500 ybpMazama O - 7700 ybp
R - ~10000 ybp
Summerland eruptive period(see Figs 5, 6, 7)
0 0
30 12C
EN
TIM
ET
ER
S
INC
HE
S
Fig. 3 Summary stratigraphic section of thick, prominent Holocenetephra-fall deposits in subalpine meadows near Mount Rainier,modified from Mullineaux (1974). Yellow tinted intervals are depositsof ash-sized tephra erupted from Mount St. Helens (MSH). Orangetinted interval is ash-sized tephra from the paroxysmal eruption ofMount Mazama, Oregon, that created Crater Lake. Pink tintedintervals are deposits of pumiceous-to-scoriaceous ash, lapilli, andbombs from Mount Rainier eruptions. Red tinted interval is pumice-bearing clay-rich ashfall deposit from the Osceola collapse event; notshown from that event is the layer S directed blast block layer. Grayintervals contain both thin, dark, poorly vesicular tephras from MountRainier (not distinguished at this scale), and non-eruptive accumu-lations of ash-sized sediments reworked from earlier tephra deposits orcarried by water and wind from nearby till
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gioclase, pyroxenes, and Fe-Ti oxides are ubiquitous inthe glassy grains. Microlites of these mineral phases arealso present in varying amounts in the blocky-to-scoriaceous grains, probably due to syn-ascent crystal-lization driven by degassing, but are less abundant inpumiceous grains. Variable growth of microlites con-tributed to differences in glass composition between grainsin single deposits. Nevertheless, we show that in most cases,field-based stratigraphic boundaries between fine-grainedtephra units correspond with shifts in the most commonglass composition, in the range of compositions, or in other
aspects of composition or mineralogy, as would result if mostof the glassy grains were juvenile magmatic ejecta, and thedifferent dark, fine-grained tephra units resulted from distincteruptions of magma, rather than by reworking of earliersediments.
The Summerland eruptive period
Tephra-fall products from the Summerland eruptive periodare defined as a succession of eight stratified deposits.These consist of six thin, fine-grained ash units (SL1–SL6)
200 µm 200 µm 200 µm
pxn
ox
pl
gl
pl
plpxn
ox
gl
pl
ox
pxn
ox
pl
gl
Fig. 4 Backscattered electron micrographs of representative Summer-land eruptive period glassy ash grains. Brightness corresponds toaverage atomic number, decreasing in the order Fe-Ti oxides (ox),
pyroxenes (pxn), plagioclase (pl), glass (gl), and voids. Left grain—blocky, middle grain—scoriaceous-fluidal, right grain—pumiceous
pumiceous C - tephra ~2200 yr BP = SL7
SL6SL5
SL4
SL2
SL1
SL3
MSH-P tephras ~2900 - 2600 yr BP
MSH-Yn ~3700 yr BP
reworked pumice lenses
mixed MSH-P & SL1
organic-richsilty sediment
25
cm
(covered)
Fig. 5 Annotated photograph ofSummerland eruptive periodtephra section exposed nearSummerland campsite, east–northeast flank of Mount Raini-er. Figure 7 shows upper portionof exposure including SL8deposit
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that commence between the upper two of four cream-colored Mount St. Helens P tephras that are widespread atMount Rainier. Summerland products also include thepumiceous-to-scoriaceous C tephra (= SL7) that immedi-ately overlies the SL6 deposit, and may include aninconspicuous fine-grained ash (SL8) that overlies the Ctephra. The SL1 to SL6, C, and several subsequent tephrasare well developed in the upper half of a ∼2.5 m thickstream bank exposure near the Summerland campsite onMount Rainier’s east flank (NAD27 latitude 46.8646°longitude −121.6611°), at a distance of 7.5 km from thevent. This exposure (Figs. 5, 6 and 7) was sampled bothwith a spatula at spaced intervals and as a continuous seriesof box cores that were impregnated with epoxy andpolished to reveal fine stratigraphic details. In addition,isolated pumice lapilli were discovered by excavating largeamounts of the SL1 deposit at Summerland, from the SL2deposit near Paradise on the volcano’s south flank, andfrom the SL4 deposit in an exposure near Fan Lake on thevolcano’s southeast flank (Fig. 1). The following descrip-tions mainly from the Summerland locality characterizetephra products of the Summerland eruptive period andsequentially younger tephras, and with glass compositionsand radiometric ages, are the primary evidence for thenumber ofMount Rainier eruptions over the last ∼2,600 years.Representative tephra glass compositions are presented inTable 3 (see Electron Supplementary Materials for allanalyses). Lahars spread from Mount Rainier during theSummerland eruptions, including the collapse-generatedRound Pass Mudflow, numerous unnamed non-cohesivelahars of the Summerland lahar assemblage, and probablythe pumice-rich National Lahar that may have been triggeredby the C-tephra eruption (Crandell 1971; Scott et al. 1995;Zehfuss et al. 2003). Key tephra deposit features, correlativeeruptive products, and associated lahars are summarized inTable 4.
Pre-summerland deposits The Summerland eruptive periodwas preceded by an interval of apparent dormancy atMount Rainier (Fig. 3) starting at about the onset of theMount St. Helens Y tephra eruptions (≤4,400 cal year BP)and ending late in the time of eruption of the Mount St.Helens P tephras (∼2,700 cal year BP) (Mullineaux 1974,1996; Crandell et al. 1981; M. Clynne 2007, unpublishedages). This dormant interval was preceded at Mount Rainierby the Osceola collapse and an early phase of summitregrowth, recorded at the Summerland locality by thecollapse-associated clay-rich F tephra, the shortly followingscoriaceous-to-pumiceous H and B tephras, and numerousunnamed thin, dark, fine-grained tephras (combined thick-ness of 30–35 cm). These juvenile Mount Rainier tephrasare capped by strata of yellowish pumiceous Mount St.Helens Y ash (MSH-Y, 4–6 cm), mainly of the voluminous
Yn event of ∼3,700 cal year BP (Crandell et al. 1981;Mullineaux 1996). Sediment from the dormant intervalbegins above the MSH-Y deposits as 9–10 cm of brownishsilty ash with common horizontally oriented plant needles,probably from heather, and scattered grains of reworked Yash. The lower pair (3–4 cm) of four cream-coloredMount St. Helens P pumiceous ashes interrupts thebrown, non-eruptive sediments. These lower two light-colored ash deposits are probably units MSH-Pm andMSH-Ps (Mullineaux 1996). This lower P doublet isoverlain by an additional 14–16 cm of brownish, plantneedle-rich, silty sediment that includes at least two lensesof reworked Mount Rainier D pumice lapilli in its upperhalf at the Summerland locality. A radiocarbon samplefrom beneath these pumice lenses gives an age of 2,870±
25 C
ENTI
MET
ERS
MSH - Y
MSH - Pm?
MSH - Ps?
MSH - Pu?
organic-rich silty sedimentwith lenses of reworkedtephra lapilli
organic-rich silty sediment
organic-rich silty sediment
organic-rich silty sedimentreworked MSH-P & SL1
MSH - Py?
SL1
SL2
SL3
SL4
SL5
SL6
C tephra (SL7) to 35 cm thickness
Fig. 6 Line drawing from photograph of tephras and non-eruptivesediments exposed near Summerland campsite, spanning fromdeposition of Mount St. Helens (MSH) Y to Mount Rainier C.Subdivisions of MSH-P tephras (MSH-Pu, etc.) are inferred fromthickness and stratigraphic position (Mullineaux 1996) but have notbeen verified geochemically. Illustrated upper portion of MSH-Y isreworked
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90 cal year BP. Such pumice lenses have not been found atthis exposure level elsewhere around Mount Rainier,indicating they are a local depositional feature. Theorganic-rich, non-eruptive sediment and reworked pumicelenses are capped by the upper doublet of Mount St.Helens P ash deposits, probably units MSH-Pu and MSH-Py (Mullineaux 1996), which are separated at theSummerland locality by unit SL1, the first Summerlanderuptive period tephra (combined MSH-Pu, SL1, andMSH-Py thickness of 5–7 cm).
SL1 The SL1 tephras are the earliest recognized fall depositof the Summerland eruptive period and lie between theupper two of four widespread Mount St. Helens P ashdeposits (Figs. 5 and 6). At the Summerland locality unitSL1 consists of 1–4 cm of fine-to-medium-grained, poorlysorted, brownish-gray ash bounded above and below by thewhite-to-cream colored, amphibole-bearing, pumiceousMSH-P ashes. Contacts with the MSH-P tephras aregenerally sharp, but with minor interfingering, and up to1 cm of SL1-like ash locally overlies the upper P deposit,
possibly due to minor post-depositional reworking. TheSL1 deposit is considerably thicker (8–9 cm) near GlacierBasin, on the volcano’s northeast flank (Fig. 1), where itcan be divided into three subunits that probably recordmultiple explosive events, though these have not beenstudied in detail.
The SL1 tephra contains common dark, microlite-bearing glassy grains, 0.2–1.5 mm, ranging from poorlyvesicular and blocky to scoriaceous. Angular gray lithics to1.5 mm predominate. Reworked white pumiceous MSH-Pash grains, some with fresh brown amphibole phenocrysts,are scattered in the SL1 deposit, indicating minor post-depositional reworking. Isolated lapilli of brownish, micro-lite-poor pumice (plagioclase–hypersthene–augite–Fe-Tioxides) and similar brownish pumiceous ash grains arealso present in the SL1 deposit but are uncommon (tephra-fall deposits originating from Mount St. Helens consistexclusively of ash-sized grains at the distance of MountRainier (Mullineaux 1974), so these and other pumiceousand scoriaceous lapilli are Mount Rainier eruptive prod-ucts). Glasses in dense-to-scoriaceous SL1 grains (Table 3)are mainly rhyolitic and differ in composition betweengrains (70–78 wt.% SiO2), whereas the pumice-lapilliglasses are dacitic (66.8 wt.% SiO2) and uniform fromgrain to grain (Table 3, Fig. 8). A radiocarbon sample(twig) collected from the SL1 unit yields 2,450±100 calyear BP (Table 2).
SL2 The SL2 tephras form a relatively thick (6–8 cm at theSummerland exposure) fine-to-medium-grained ash depositoverlying the uppermost Mount St. Helens P ash. The SL2deposit is distinctive in that it darkens upward graduallyfrom a brownish base to uniform, dark brownish-gray ashin its upper half (Figs. 5 and 6). The lowest part of the SL2unit (to 1 cm at Summerland) is brown, fine-grained siltyash with abundant horizontally oriented plant needles,scattered angular lithics (to 2.5 mm), and few dark glassygrains. This brownish interval probably records a hiatusbetween the SL1 and SL2 eruptions, with the brownish siltyash being windblown sediment trapped by plant communi-ties that dropped the abundant needles. Above this basalzone the SL2 tephras are increasingly dominated by blocky-to-scoriaceous glassy grains (commonly 0.1–1 mm, rarelyto 3.5 mm), accompanied by more abundant angular graylithics (0.1–1.5 mm) and phenocryst fragments. The SL2tephras are also faintly internally stratified on a scale of0.25–1.5 cm. Stratification in the unit’s lower half is mainlydefined by alternations between apparently non-eruptivesediments rich in brown, silty ash with abundant plantneedles, similar to the basal layer, and coarser zones rich inlithics, crystal fragments, and glassy grains. Zones ofbrownish silty ash are absent in the upper half of the SL2deposit where stratification is defined by slight variations in
MSH-XMSH-XMSH-WMSH-W
CIgW3CIgW3
CIgW2CIgW2CIgW1CIgW1
TC2C2
TC1C1
SL8SL8fine ash on C fine ash on C tephephra
pumipumiceous C eous C tephephra = SL7a = SL7
Fig. 7 Annotated photograph of post-C tephra ash deposits exposednear Summerland campsite, east–northeast flank of Mount Rainier.MSH-W and MSH-X are thin but widespread Mount St. Helenstephras. CIgW1, 2, 3, and fine ash between the Mount Rainier C andSL8 tephras are probably non-eruptive deposits of fine-grainedreworked ash
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Table 3 Representative electron-microprobe major-oxide analyses of glasses from late-Holocene Mount Rainier tephras
Sample Number SiO2 TiO2 Al2O3 FeOa MnO MgO CaO Na2O K2O P2O5 Cl SO3 Orig. total
White River: ∼1,000 cal year BP ashSR713–13 15 72.0 0.97 12.7 3.86 0.06 0.53 1.90 4.10 3.54 0.20 0.05 0.01 99.0SR713–14 10 72.9 0.98 12.4 3.75 0.07 0.42 1.59 4.27 3.39 0.18 0.09 0.02 99.4SR713–15 10 73.3 1.02 12.0 3.55 0.05 0.34 1.48 3.35 4.59 0.25 0.08 0.02 97.4SR713–20 10 72.4 0.92 12.5 3.74 0.05 0.56 1.87 4.27 3.35 0.20 0.10 0.01 99.0SR713–29 15 72.8 1.01 12.4 3.49 0.06 0.43 1.75 4.16 3.55 0.21 0.07 0.02 98.5
TC2: ∼1,500 cal year BP ashL12–1 15 73.0 0.94 12.5 3.65 0.04 0.45 1.79 4.11 3.24 0.18 0.06 0.03 99.3L12–3 10 73.4 1.01 12.1 3.74 0.02 0.34 1.52 4.07 3.46 0.23 0.06 0.02 99.9L12–4 10 73.9 0.86 12.0 3.21 0.03 0.35 1.50 3.75 4.18 0.11 0.06 0.02 99.5L12–10 10 73.5 0.94 12.1 3.37 0.06 0.41 1.62 4.15 3.54 0.24 0.06 0.01 100.0L12–12 15 74.0 0.95 12.0 3.33 0.05 0.26 1.35 4.15 3.58 0.19 0.09 0.01 99.7TC1: ∼1,500 cal year BP ashL11–2 10 75.5 0.72 12.0 2.43 0.03 0.26 1.04 3.98 3.83 0.09 0.09 0.01 99.4L11–3 10 76.2 0.64 11.8 2.25 0.04 0.22 0.89 3.47 4.45 0.11 0.01 0.01 99.2L11–4 10 75.1 0.83 12.1 2.67 0.03 0.30 1.02 3.70 4.07 0.11 0.04 0.00 99.1L11–5 9 74.5 0.87 12.2 2.92 0.05 0.32 1.24 3.72 3.91 0.14 0.06 0.01 99.3L11–11 10 75.2 0.72 11.9 2.46 0.04 0.26 1.00 3.00 5.22 0.11 0.07 0.00 99.0
SL8: fine ash above C tephraCIgC2–1 5 72.7 0.60 13.5 2.97 0.04 0.59 2.03 4.00 3.31 0.13 0.09 0.01 99.5CIgC2–2 8 72.9 0.57 13.4 3.14 0.06 0.51 1.93 3.93 3.29 0.13 0.11 0.00 99.0CIgC2–3 10 73.2 0.64 13.3 2.95 0.02 0.50 1.87 3.84 3.40 0.10 0.11 0.05 98.8CIgC2–11 10 73.3 0.65 13.3 2.98 0.07 0.48 1.77 3.74 3.50 0.14 0.09 0.01 99.2CIgC2-fine 10 73.4 0.59 13.5 2.47 0.08 0.33 1.67 4.07 3.70 0.09 0.13 0.04 97.2
C-tephra (SL7), brown pumice lapilli98RE692-P1 17 64.3 1.02 16.5 4.87 0.07 1.74 4.28 4.54 2.29 0.27 0.08 0.03 100.098RE692-P2 20 64.3 0.93 16.7 4.77 0.09 1.75 4.27 4.57 2.23 0.27 0.08 0.03 99.898RE692-P3 24 64.7 1.12 16.2 5.09 0.11 1.63 4.11 4.38 2.29 0.30 0.07 0.01 98.998RE692-P3-hbl 6 64.4 1.02 16.2 5.21 0.11 1.67 4.32 4.49 2.17 0.31 0.07 0.01 99.2C-tephra (SL7), dense gray pumice lapilliJV506C-G1 15 74.6 0.46 12.6 2.31 0.09 0.33 1.26 4.20 3.96 0.04 0.21 0.01 99.7JV506C-G2 20 74.5 0.54 12.6 2.32 0.05 0.34 1.33 4.17 3.86 0.06 0.25 0.01 99.9C-tephra (SL7), white pumiceous streak in brown pumice lapilli98RE692-P3 W 12 76.3 0.45 12.0 1.82 0.03 0.24 0.98 3.92 4.04 0.05 0.21 0.01 97.8
Upper SL6SL6A-II-1 5 77.6 0.60 11.4 1.50 0.00 0.07 0.33 2.87 5.53 0.03 0.06 0.00 99.7SL6A-II-3 pum 6 69.5 1.27 13.3 4.64 0.10 0.73 2.58 4.11 3.33 0.32 0.09 0.02 99.6SL6A-II-7 10 70.2 0.88 13.9 3.99 0.07 0.87 2.73 4.05 3.00 0.19 0.12 0.02 99.5SL6A-II-13a pum 5 69.1 1.24 13.6 4.82 0.09 0.88 2.80 4.03 3.09 0.28 0.11 0.01 99.3SL6A-3 10 73.8 0.56 13.2 2.58 0.05 0.44 1.29 4.05 3.97 0.04 0.08 0.02 99.4Middle SL6SL6B-11 9 73.0 0.97 12.9 3.18 0.05 0.36 1.47 3.94 3.85 0.17 0.13 0.02 99.5SL6B-14 10 72.5 0.95 13.1 3.20 0.07 0.42 1.67 4.09 3.66 0.20 0.13 0.01 99.7SL6B-15 14 74.6 0.75 12.4 2.61 0.04 0.31 1.07 4.02 4.10 0.08 0.11 0.02 99.9SL6B-19 9 73.7 0.88 12.8 2.87 0.07 0.33 1.29 3.96 3.90 0.14 0.08 0.02 99.2SL6B-22 10 74.0 0.91 12.6 2.52 0.05 0.29 1.20 4.01 4.28 0.09 0.14 0.01 99.8Lower SL6SL6C-2 7 73.1 0.96 12.8 2.92 0.05 0.37 1.38 4.11 3.99 0.15 0.11 0.03 99.5SL6C-3 4 72.0 1.03 13.1 3.36 0.10 0.48 1.74 4.16 3.74 0.18 0.19 0.03 99.5SL6C-5 15 73.7 0.83 12.4 2.92 0.06 0.35 1.17 4.08 4.24 0.13 0.20 0.01 99.7SL6C-9 4 72.6 1.03 12.6 3.24 0.07 0.42 1.64 3.93 4.15 0.24 0.14 0.01 99.3SL6C-17 7 74.8 0.77 12.2 2.46 0.05 0.31 0.85 3.93 4.42 0.09 0.15 0.01 99.7
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Table 3 (continued)
Sample Number SiO2 TiO2 Al2O3 FeOa MnO MgO CaO Na2O K2O P2O5 Cl SO3 Orig. total
Upper SL5SL5A1–5 5 67.4 1.07 14.7 4.60 0.07 1.35 3.38 4.29 2.86 0.27 0.10 0.02 98.3SL5A1–6 5 67.0 1.06 14.9 4.68 0.09 1.24 3.61 4.32 2.73 0.34 0.12 0.00 99.7SL5A1-e 5 67.8 1.03 14.6 4.11 0.10 1.31 3.35 4.52 2.89 0.24 0.10 0.01 98.1SL5A1–4 8 67.8 1.11 14.6 4.23 0.15 1.07 3.00 4.04 3.56 0.27 0.20 0.04 98.5SL5A1–13 5 69.9 1.13 13.7 4.16 0.07 0.76 2.31 4.11 3.45 0.29 0.18 0.01 99.0Upper-middle SL5SL5B1–3 pum 5 68.3 0.98 14.4 4.16 0.10 1.21 2.84 4.44 3.12 0.27 0.19 0.00 99.1SL5B1–4 pum 5 68.1 1.05 14.5 4.21 0.12 1.12 2.99 4.39 2.98 0.28 0.24 0.05 99.4SL5B-6 3 67.4 1.14 14.7 4.36 0.25 1.09 2.84 4.46 3.20 0.28 0.23 0.07 99.3SL5B-7 2 73.1 0.84 12.7 2.99 0.10 0.47 1.38 3.89 4.32 0.10 0.13 0.04 99.2SL5B-9 3 69.4 1.16 14.0 4.01 0.06 0.79 2.49 4.10 3.54 0.30 0.23 0.02 98.6Lower-middle SL5SL5C-1 3 73.6 0.89 12.8 2.66 0.06 0.27 1.43 3.96 4.05 0.16 0.14 0.00 99.3SL5C-2 9 67.7 1.07 14.7 4.35 0.09 1.18 3.03 4.25 3.22 0.26 0.17 0.01 98.2SL5C-3 6 72.9 1.06 12.7 3.10 0.05 0.44 1.39 3.99 4.15 0.15 0.12 0.03 96.9SL5C-7 2 69.2 1.13 14.1 4.18 0.04 0.78 2.43 4.52 3.11 0.31 0.14 0.03 98.1SL5C-16 4 73.2 0.96 12.7 2.85 0.04 0.41 1.42 3.95 4.31 0.10 0.11 0.01 98.2Lower SL5SL5D-5 7 75.7 0.77 12.3 2.13 0.01 0.18 0.78 3.58 4.44 0.08 0.12 0.03 99.2SL5D-8 10 73.1 0.91 12.7 2.96 0.02 0.40 1.32 4.25 4.08 0.14 0.09 0.00 98.8SL5D-13 9 73.6 0.91 12.5 2.79 0.04 0.37 1.14 4.11 4.32 0.10 0.13 0.01 99.2SL5D-14 9 74.3 0.89 12.7 2.48 0.04 0.18 0.94 3.85 4.35 0.14 0.16 0.04 99.5SL5D-17 10 72.9 0.94 12.9 3.00 0.06 0.43 1.52 4.10 3.93 0.12 0.13 0.01 99.6
Upper SL4SL4A-1 pum 5 68.5 1.10 14.0 4.44 0.10 1.01 2.97 4.47 2.91 0.27 0.16 0.05 99.8SL4A-2 pum 5 68.2 1.13 14.2 4.58 0.10 1.24 3.16 4.19 2.90 0.24 0.14 0.02 99.6SL4A-3 pum 10 68.7 1.20 14.3 4.35 0.06 1.15 3.06 3.96 2.85 0.29 0.13 0.01 99.3SL4A-4 pum 5 67.7 1.23 14.3 4.80 0.07 1.19 3.14 4.19 2.88 0.34 0.17 0.07 100.1SL4A-8–4 4 70.9 1.11 13.4 3.85 0.02 0.56 1.93 4.16 3.61 0.32 0.13 0.02 98.9Upper-middle SL4 pumice lapilliSL4-upr pum1 30 68.2 1.02 14.2 4.46 0.07 1.18 3.19 4.42 2.82 0.23 0.13 0.02 98.6SL4-upr pum2 28 68.1 1.07 14.2 4.39 0.06 1.16 3.25 4.49 2.80 0.22 0.14 0.06 98.5Upper-middle SL4SL4B-8–10 pum 10 67.5 1.05 14.3 4.59 0.09 1.35 3.64 4.32 2.79 0.25 0.14 0.02 100.0SL4B-9–5 5 70.6 1.15 13.1 4.22 0.07 0.71 2.19 4.12 3.39 0.30 0.07 0.01 99.6SL4B-11 9 67.9 1.19 14.3 5.15 0.08 1.26 3.43 3.44 2.84 0.30 0.12 0.01 98.9SL4B-13 pum 10 69.8 0.88 14.2 4.24 0.06 1.04 3.07 3.55 2.88 0.26 0.12 0.01 99.2SL4B-14 pum 9 69.1 1.05 14.4 4.41 0.07 1.09 3.14 3.53 2.85 0.26 0.12 0.02 98.4Lower-middle SL4SL4C-12 10 69.3 1.15 14.1 4.48 0.07 1.00 3.06 3.60 2.87 0.26 0.10 0.01 98.9SL4C-14 5 69.7 1.00 14.3 4.31 0.07 0.94 3.02 3.39 2.91 0.29 0.09 0.01 99.0SL4C-15 9 69.2 1.19 14.3 4.41 0.08 0.99 3.06 3.57 2.89 0.22 0.10 0.01 98.6SL4C-17 10 68.7 1.11 14.4 4.75 0.04 1.12 3.28 3.53 2.77 0.27 0.10 0.01 99.3SL4C-18 10 69.4 0.93 14.2 4.46 0.06 1.03 3.00 3.68 2.88 0.25 0.09 0.02 98.8Lower-middle SL4 pumice lapilliSL4-lwr pum2 27 68.1 1.01 14.2 4.34 0.10 1.17 3.29 4.56 2.82 0.26 0.15 0.04 97.8SL4-lwr pum3 29 68.3 1.04 14.1 4.42 0.07 1.17 3.22 4.43 2.85 0.28 0.13 0.02 98.2Lower SL4SL4D-4 10 67.6 1.19 14.5 5.11 0.07 1.36 3.65 3.63 2.56 0.31 0.09 0.02 99.2SL4D-5 5 67.4 1.15 14.4 5.37 0.08 1.35 3.62 3.66 2.52 0.38 0.09 0.00 99.6SL4D-11 5 68.9 1.21 13.8 4.80 0.08 0.95 2.93 3.70 3.11 0.35 0.11 0.01 99.6SL4D-12 5 67.9 1.28 13.7 5.44 0.05 1.08 3.21 4.09 2.76 0.35 0.09 0.02 99.8SL4D-17 5 69.4 1.10 13.2 4.75 0.11 1.04 2.75 3.94 3.23 0.29 0.11 0.05 99.3
Bull Volcanol
Table 3 (continued)
Sample Number SiO2 TiO2 Al2O3 FeOa MnO MgO CaO Na2O K2O P2O5 Cl SO3 Orig. total
SL3SL3–1 10 66.6 1.16 14.4 5.37 0.07 1.48 3.85 4.24 2.46 0.33 0.10 0.01 99.6SL3–3 10 66.7 1.34 14.4 5.18 0.10 1.33 3.59 4.36 2.52 0.36 0.08 0.04 100.5SL3–4 10 66.8 1.18 14.2 5.40 0.10 1.36 3.64 4.30 2.49 0.38 0.09 0.00 100.1SL3–7 10 66.4 1.32 14.2 5.47 0.10 1.46 3.76 4.36 2.46 0.37 0.11 0.02 99.5SL3–13 5 68.5 1.51 13.0 5.49 0.11 1.03 3.03 3.99 2.91 0.31 0.09 0.03 100.4
Upper SL2SL2A-1 5 75.5 0.69 11.8 2.63 0.10 0.23 1.09 3.96 3.86 0.11 0.12 0.01 99.9SL2A-2 5 72.4 0.88 12.3 4.00 0.02 0.53 1.86 3.96 3.83 0.14 0.05 0.00 99.8SL2A-5 5 74.0 0.80 12.2 3.19 0.06 0.41 1.31 3.77 4.06 0.12 0.07 0.00 99.2SL2A-6 5 73.4 0.73 12.3 3.59 0.08 0.46 1.63 3.87 3.79 0.17 0.09 0.00 99.3SL2A-7 10 59.5 1.65 15.2 8.43 0.09 2.88 5.82 4.61 1.47 0.32 0.06 0.03 99.0Upper-middle SL2 hornblende pumice lapilliSL2-A/B pum1 20 64.2 0.96 16.5 4.95 0.10 1.78 4.30 4.66 2.22 0.27 0.07 0.03 99.2SL2-A/B pum2 20 64.2 0.96 16.4 5.02 0.11 1.77 4.32 4.64 2.19 0.30 0.08 0.04 98.8SL2-A/B pum3 20 64.0 0.99 16.4 5.05 0.08 1.80 4.41 4.69 2.21 0.27 0.08 0.02 98.9Upper-middle SL2SL2B-1 5 72.3 1.13 12.2 4.59 0.00 0.39 1.86 3.47 3.76 0.26 0.06 0.00 99.8SL2B-2 5 73.4 0.75 12.3 3.51 0.07 0.45 1.77 3.24 4.34 0.16 0.06 0.00 99.0SL2B-3 5 74.1 0.76 12.1 3.36 0.01 0.41 1.42 3.97 3.69 0.10 0.05 0.02 99.5SL2B-5 5 74.5 1.02 11.7 3.31 0.07 0.30 1.21 3.89 3.79 0.17 0.04 0.00 99.5SL2B-6 5 72.9 1.07 12.3 3.53 0.04 0.50 1.84 3.81 3.80 0.19 0.03 0.04 99.4Middle SL2SL2C-1 5 74.0 0.98 12.1 3.07 0.03 0.29 1.48 3.30 4.53 0.20 0.08 0.00 98.9SL2C-3 10 72.3 0.87 12.4 4.15 0.08 0.50 1.38 3.71 4.32 0.21 0.09 0.02 99.7SL2C-7 5 72.6 0.75 12.5 3.92 0.04 0.59 2.07 3.95 3.38 0.11 0.08 0.00 99.0SL2C-10 5 71.1 0.87 12.8 4.43 0.09 0.73 2.26 4.13 3.26 0.24 0.07 0.01 99.4SL2C-13 5 73.4 0.85 12.2 3.50 0.01 0.48 1.66 4.12 3.51 0.20 0.08 0.00 99.1Lower-middle SL2SL2D-4 9 75.9 0.70 11.7 2.38 0.02 0.25 0.97 4.10 3.85 0.12 0.02 0.01 98.9SL2D-5 5 73.0 0.88 12.8 3.16 0.07 0.36 1.82 4.29 3.42 0.11 0.06 0.01 99.5SL2D-6 8 76.7 0.70 11.7 1.94 0.02 0.10 0.60 3.96 4.16 0.10 0.03 0.02 99.0SL2D-8 10 75.7 0.66 11.6 2.40 0.06 0.27 0.92 3.57 4.72 0.08 0.05 0.00 99.1SL2D-10 5 76.2 0.74 11.8 2.27 0.00 0.12 0.62 4.10 4.08 0.13 0.03 0.01 98.6Lower SL2SL2E-2 5 70.7 0.90 13.5 3.71 0.05 0.63 2.28 4.52 3.33 0.23 0.08 0.02 98.9SL2E-5 5 71.0 1.18 12.7 3.96 0.09 0.79 1.87 4.17 3.87 0.31 0.07 0.00 98.7SL2E-6 3 74.2 0.74 12.4 3.03 0.03 0.48 1.40 3.95 3.60 0.10 0.08 0.00 97.0SL2E-17 4 71.5 0.92 13.4 3.70 0.10 0.63 2.18 3.92 3.31 0.26 0.08 0.00 98.4
SL1SL1–3 10 75.0 0.49 12.5 2.18 0.06 0.27 1.23 3.36 4.86 0.04 0.07 0.00 100.2SL1–5 10 74.4 0.61 12.7 2.39 0.08 0.34 1.33 4.12 3.81 0.06 0.09 0.02 100.4SL1–6 10 75.7 0.42 12.2 2.00 0.05 0.28 1.11 4.06 4.01 0.06 0.05 0.02 99.9SL1–8 10 74.7 0.62 12.8 2.12 0.03 0.17 0.95 4.29 4.12 0.05 0.10 0.01 99.7SL1–12 10 70.3 1.14 13.1 4.03 0.13 0.62 2.12 4.70 3.41 0.28 0.09 0.04 100.3SL1 pumice lapilliSL1 pum-lg 12 66.8 1.06 15.0 4.61 0.06 1.34 3.57 4.23 2.88 0.30 0.10 0.02 99.1SL1 pum-sm 8 66.8 1.07 14.9 4.68 0.07 1.33 3.61 4.32 2.88 0.28 0.10 0.01 99.5
a Analyses are averaged glass compositions for individual tephra grains normalized to 100 wt.% with all Fe as FeO, total gives original total,number gives number of point analyses averaged. Units are arranged in approximate stratigraphic order (younger upward), open rows marksignificant time breaks. See Electronic Supplementary Materials for additional analyses
Bull Volcanol
Tab
le4
Sum
merland
erup
tiveperiod
andsubsequent
teph
radepo
sitsat
Mou
ntRainier
Eruptive
period
Tephra
unit
Color
Grain
size
Dom
inant
juvenile
type
Other
juvenile
type
Thickness
b
(cm)
VEIc
Com
ment
Age
eCorrelativ
eeruptiv
eunits
Correlativ
elahars
Set
Layer
MR-X
aBrown
Lapilli
None
None
discontin
uous
RedepositedC
tephra
Post-
neoglacial
None
None
MSH-W
nWhite
Ash
Shards
Pum
iceous
2MSHderuptio
n471
Unnam
edWhite
River
ash
Ltbrow
nAsh
Dense,blocky
Scoriaceous
1–2
2ContainsredepositedC
tephra
1,000
Nonepreserved
Fryingpan
Creek
Twin
Creeks
TC
TC2
Dkbrow
nish
gray
Ash
Dense,blocky
Scoriaceous
12
1,500–
1,600
Nonepreserved
Twin
Creeks
TC1
Ltbrow
nish
gray
Ash
Dense,blocky
Scoriaceous
2–3
2
Sum
merland
SL
SL8
Ltbrow
nish
gray
Ash
Glass-coated
phenos
Dense,
blocky
1–2
22,100(?)
Sum
mitlava
(?)
Ctephra
(SL7)
Brown
Lapilli
Pum
ice
Scoria
bombs
354
2,200
National(?)
SL6
Dk-brow
nAsh
Dense,blocky
Scoriaceous
5–9
2Organic-rich
2,300–
2,400
Multip
lein
White,Puyallup,
&Nisqually
drainages
SL5
Med-brown
Ash
Scoriaceous
Dense,
blocky
5–9
22,400–
2,500
Emmons–Winthrophigh-
Srlavas
SL4
Med-brown
Ash
Dense,blocky
Pum
ice
lapilli
4–6
2–3
2,500–
2,600
South
Puyallupblock-and-
ashflow
SL3
Med-brown
Ash
Dense,blocky
Scoriaceous
12
2,500–
2,600
Noneexposed
SL2
Brown-dk
gray
Ash
Dense,blocky
Pum
ice
lapilli
6–8
2–3
2,500–
2,600
Noneexposed
MSH-Py
White
Ash
Shards
Pum
iceous
1–3
MSH
eruptio
nSL1
Med-brown
Ash
Dense,blocky
Pum
ice
lapilli
1–4
22,500–
2,600
Noneexposed
Round
Pass(?)
MSH-Pu
White
Ash
Shards
Pum
iceous
1–3
MSH
eruptio
n
aRedepositedC-tephra,
notprod
uced
byan
erup
tion
bThickness
atSum
merland
expo
sure
ornear
upperm
ostroad
bridge
across
White
River
(White
River
ash)
cVolcanicExp
losivity
Indexestim
ated
from
depo
sitthickn
essandabun
danceof
juvenile
material
dMou
ntStHelenserup
tiondepo
sit
ePermissibleage,
orage-windo
w,in
calend
aryearsbefore
1950
Bull Volcanol
overall grain size. This upward increase in juvenile eruptivecomponents typifies the SL2 unit and probably results fromshortened repose periods, and perhaps eruption of greateramounts of magma, for successive SL2 explosive events.Isolated brown pumice lapilli (to 1 cm) are present but rarein the upper half of the SL2 tephra deposit in exposuresnear Paradise on the south flank of the volcano (Fig. 1), but
have not been found at the Summerland locality. Thesepumice lapilli differ from those in the underlying SL1 unit inthat they contain phenocrysts of fresh, non-resorbed brownamphibole in addition to plagioclase, hypersthene, augite, andFe-Ti oxides. The weighted mean age of three radiocarbonsamples (twigs) from the SL2 deposit is 2,610±90 cal year BP(Table 2; for averaging, calibrated ages are weighted by(±value)−2 and reported weighted uncertainties are the largerof the weighted sample variance or (Σ weights)−0.5). Thedark SL2 tephra set is overlain abruptly by another intervalof non-eruptive silty brown sediment containing scattered,horizontally oriented twigs and plant needles that marks thebase of the SL3 deposit.
Because of its thickness and vertical zonation, the SL2tephra was sampled at five evenly spaced levels (Table 3).Glasses in the common, blocky-to-scoriaceous microlite-bearing ash grains have variable but mainly rhyoliticcompositions that do not change systematically withheight in the deposit (Fig. 8). Their compositions aresimilar to those of glasses in blocky-to-scoriaceous grainsfrom the underlying SL1 deposit, but glasses in the SL2amphibole-bearing pumice lapilli are dacitic and aredistinctly less evolved (64.2 wt.% SiO2) than glasses inthe amphibole-free pumice lapilli of the underlying SL1tephra (66.8 wt.% SiO2). Glass compositions in the SL2hornblende pumice lapilli are also uniform from grain tograin.
SL3–SL4 The SL3 and SL4 tephras are sequential depositsthat can be distinguished by superposition but that probablyrepresent the initial and main stages of a single eruptiveepisode. A non-eruptive zone of silty fine-grained brownash with abundant horizontally oriented plant needles andtwigs (1–1.5 cm thick at the Summerland locality) caps theunderlying SL2 tephra and marks the base of the combinedSL3–SL4 tephra set. The SL3 unit is the lowermost andrelatively continuous stratum (1–1.5 cm thick) of fine-to-medium-grained brownish-gray ash immediately above thetwig-bearing zone, whereas the SL4 tephras consist of athicker overlying interval (4–6 cm) of similar but discon-
58 60 62 64 66 68 70 72 74 76 78 80
SiO2 (wt.%)
rela
tive
str
atig
rap
hic
po
siti
on
White River
TC2TC1SL8C (SL7)
SL6
SL5
SL3
SL4, pumice,
SL2 & pumice
SL1 & pumice
S. Puyallup bombs&
Fig. 8 Stratigraphic succession of glass SiO2 concentrations for late-Holocene Mount Rainier deposits, subdivided as ash grains (smallblue diamonds), pumice lapilli (large circles), and rinds on SouthPuyallup breadcrust bombs (yellow squares). Glasses from the sub-plinian Mount Rainier C tephra are subdivided into matrix glass fromdominant brown pumice (tan), matrix glass in poorly inflated dacitelapilli (gray), and white blebs and streaks of dacitic pumice (white)dispersed within andesitic pyroclasts. Thin dotted lines show periodsof repose marked by non-eruptive accumulations of silty, organic-richreworked ash. Small white circles show glass compositions of ashgrains from Mount St. Helens, probably reworked from P tephra units
Table 5 Electron-microprobe major-oxide analyses of glasses from South Puyallup breadcrust bomb rinds
Sample Number SiO2 TiO2 Al2O3 FeOa MnO MgO CaO Na2O K2O P2O5 Cl SO3 Total
93 MW67 29 68.5 1.05 14.4 4.14 0.07 1.17 3.08 4.40 2.89 0.23 0.11 0.01 99.193 MW68 30 68.7 1.08 14.4 4.12 0.07 1.13 3.00 4.33 2.85 0.23 0.11 0.02 99.393 MW70 29 68.4 1.08 14.3 4.18 0.06 1.15 2.82 4.00 3.67 0.23 0.12 0.02 99.293 MW71 30 68.5 1.07 14.4 4.22 0.07 1.15 3.06 4.34 2.89 0.24 0.11 0.02 99.300SMN803 28 68.6 1.05 14.2 4.27 0.07 1.08 2.96 4.44 3.02 0.24 0.11 0.01 99.600SMN806 10 68.7 1.08 13.8 4.39 0.09 1.08 2.87 4.41 3.24 0.27 0.13 0.00 99.5
a Analyses are averaged glass compositions for individual bombs normalized to 100 wt.% with all Fe as FeO, total gives original total, numbergives number of analyses averaged
Bull Volcanol
tinuously stratified and burrowed ash. The absence of awell-developed silty organic-rich interval between the SL3and SL4 deposits is evidence for little or no hiatus betweendeposition of those tephras. The SL4 tephra consists ofmultiple strata (five or six at Summerland, individually0.75–2 cm thick) of fine-to-medium-grained brownish grayash with scattered outsized lithics (to 4 mm) interlayeredwith laterally discontinuous zones up to 1 cm thick of finer-grained and better-sorted ash, commonly with faint internallaminations. The internal complexity of the SL4 unit issuggestive of slight reworking, perhaps due to falling onsnow, followed by melting into place. The SL3 and SL4tephras are distinctive in that their glassy ash populationcontains common brown pumiceous grains (0.5–3 mm) thatare particularly abundant in a 1–2 cm thick zone two-thirdsheight above the base of the combined units. Isolated brownpumice lapilli (plagioclase–hypersthene–augite–Fe-Tioxides) are present in the SL4 deposit near Fan Lake onMount Rainier’s southeast flank (Fig. 1), but only ash-sizedbrown pumice have been found at the Summerland exposure.The SL3–SL4 unit is bounded above by a thin (3–4 mm),fine-grained, well-sorted, light grayish brown ash with faintinternal laminations and minor, outsized lithics and scoria-ceous glassy grains that marks the base of the SL5 tephra set.
Glasses from the SL3–SL4 tephra set are mainly dacitic(66–70 wt.% SiO2) and are distinctly less evolved thanglasses from the SL1 and SL2 deposits (Fig. 8; Table 3).Rhyolitic glasses are nearly absent. There is a possibletemporal trend of slightly increasing melt evolution in SL3–SL4 ashes, with values of 66–69 wt.% SiO2 in the SL3deposit, rising to 68–71 wt.% SiO2 in the upper SL4deposit. Glasses from SL4 pumice lapilli have uniformdacitic compositions (68 wt.% SiO2), similar to those ofglasses in many of the ash-sized grains; the pumiceouslapilli and ash appear to be the same magmatic componentdiffering only in grain size. Notably, glasses forming thedense rinds of breadcrust bombs from the South Puyallupblock-and-ash-flow deposit (68.4–68.7 wt.% SiO2) are aclose compositional match to SL4 ash and pumice-lapilliglasses (Table 5, Figs. 8 and 9), which is evidence that theSouth Puyallup block-and-ash flow was emplaced duringthe SL4 eruptions. A radiocarbon sample (twig) from themiddle of the SL4 unit gives an age of 2,540±180 cal yearBP, and the South Puyallup pyroclastic flow deposit hasan age of 2,580±150 cal year BP (outer portion ofcarbonized log); combining these yields a weighted meanage of 2,560±120 cal year BP for the SL3–SL4–SouthPuyallup episode (Table 2).
SL5 The SL5 tephras consist of faintly but regularlystratified fine-to-medium-grained brown and brownish grayashes (5–9 cm aggregate thickness at Summerland) thatoverlie the thin, light-colored ash stratum capping the SL3–
SL4 tephra deposits. Internal stratification (6–11 strata,individually 0.2–2 cm thick at Summerland) is definedchiefly by slight variations in grain size and color betweenfine-grained, well-sorted brown ash, and fine-to-mediumgrained, moderately sorted grayish brown ash, both withscattered outsized angular lithics (commonly to 2 mm,rarely to 6.5 mm). Many of the finer-grained brown strataalso contain scattered horizontally oriented plant needles.At the Summerland locality two of the finer-grained strataare closely spaced and appear as a prominent light-brownish gray doublet near the top of the SL5 deposit thataids its field recognition. As with other Summerland periodtephras, lithic grains predominate in the SL5 unit, followedin abundance by phenocryst fragments, but the glassy ashin the SL5 tephra is distinctive in consisting mainly ofrelatively coarse (to 2 mm) dark brown-to-black scoria-ceous grains that stand out against the overall brownish-gray deposit.
Glassy grains were sampled at four levels spacedthrough the SL5 deposit. The SL5 glasses differ from theunderlying SL3–SL4 glasses in that rhyolitic compositionsreappear in abundance, similar to SL1–SL2 compositions,accompanied by dacitic glasses similar to those of the SL3–SL4 units (Table 3, Fig. 8). It is unclear if those SL5 daciticglasses were reworked from earlier tephras, but the abruptreappearance of rhyolitic glasses, coupled with the distinc-tive scoriaceous habit of many grains, are evidence that theSL5 tephras were produced by an eruptive episodefollowing that of the SL3–SL4 deposits. This inference issupported by the compositions of plagioclase phenocrysts,as discussed in the section: Correlations of flow and falldeposits based on glass and mineral compositions. A single
11
12
13
14
15
16
17
58 60 62 64 66 68 70 72 74 76 78 80
SiO2 (wt.%)
Al 2
O3
(wt.
%)
C and SL2 pumices
SL1 pumice
C gray dacite
C white dacite blebs
SL4 pumice andS. Puyallup bomb rinds
MountSt Helensgrains
Fig. 9 Plot of glass SiO2 versus Al2O3 concentrations showingsimilarity of glasses between amphibole-bearing SL2 and C pumices,and between SL4 pumice and South Puyallup bomb-rinds glasses.Symbols as in Fig. 8
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radiocarbon sample (twig) from the SL5 deposit has an ageof 2,240±250 cal year BP (Table 2).
SL6 The SL6 tephras form a fine-to-medium grained ashdeposit (5–9 cm at the Summerland locality) that overlies theSL5 tephras and directly underlies the widespread MountRainier C pumice-and-scoria-fall deposit. The SL6 unit isdistinguished from earlier Summerland period tephras by amarkedly greater abundance of horizontally oriented plantneedles that impart a dark brownish hue and a laminatedappearance to the deposit, as well as by thin (to 0.5 cm),discontinuous strata of gray-to-light-gray gritty lithic ash(mostly 0.1–0.5 mm, rarely to 1 mm). Whitish hydrother-mally altered (silicified?) lithic grains are present in thesegritty laminae and increase in abundance in the upper quarterof the deposit. Minor post-depositional silicification of plantdebris as hard white coatings and infillings is also developedin uppermost SL6, apparently precipitated by componentsleached from the overlying C pumice-fall deposit.
Glassy grains are rare in the SL6 unit, but include bothpoorly vesicular blocky, and fewer scoriaceous-to-pumiceoustypes. Glassy ash grains were sampled from the lower, middle,and upper parts of the SL6 deposit (Table 3, Fig. 8). LowerSL6 glassy grains are wholly blocky and microlitic withrhyolitic glass compositions (72.0–74.8 wt.% SiO2). MiddleSL6 glassy grains are similar, including some with evenmore evolved glass compositions (72.2–76.5 wt.% SiO2), buttwo anomalous pumiceous ash grains were found that havedacitic glasses (69.3 wt.% SiO2), similar to pumiceous grainsfrom the earlier SL3–SL4 deposits. Upper SL6 glassy grainsare diverse, including blocky microlitic ash with rhyoliticglass (72.9–78.2 wt.% SiO2), pumiceous ash with evolveddacitic glass (69.1–69.5 wt.% SiO2) similar to some SL3–SL4 ejecta, and a single scoriaceous grain with less evolveddacitic glass (64.7 wt.% SiO2) that has abnormally lowAl2O3 (14.5 wt.%) compared with other Mount Rainierglasses at similar SiO2 values. The origin of this dacitic grainis uncertain, although some andesitic glasses from Mount St.Helens have similarly low Al2O3 concentrations (T. Sisson,unpublished analyses).
The abundant plant material in the SL6 deposit, thescarcity of glassy grains, and the similarities between SL6glasses and those of earlier Summerland tephras, suggestthat SL6 time was a mainly non-eruptive interval whenplants grew vigorously in sub-alpine meadows, trappingwind-blown sediments. However, the conspicuous hydro-thermally altered ash grains, scarce-to-absent in earlierSummerland tephras, is evidence that eruptions took placeand disrupted altered portions of the edifice. Possibly thesewere phreatic or weakly phreatomagmatic eruptions pre-cursory to the subplinian C tephra event. A singleradiocarbon sample (twig) from the SL6 unit gives an ageof 2,420±80 cal year BP (Table 2).
C tephra (SL7) The ∼2,200 cal year BP C tephra (Table 2)is the largest-volume single Holocene fall deposit fromMount Rainier (Mullineaux 1974; Swanson 1993) andforms a broad lobe extending to the east–northeast of thevolcano (0.2 km3 within 20 km of the volcano, or roughly0.1–0.15 km3 as dense rock). The deposit attains athickness slightly in excess of 30 cm along its maindepositional axis, including at the Summerland localitywhere it is measured at 30–35 cm (7.5 km from the vent).The C deposit consists mainly of brown porphyriticandesite pumice lapilli and ash (plagioclase–hypersthene–augite–amphibole–Fe-Ti oxides), but includes commondarker scoriaceous lapilli and bombs of slightly more maficandesite (Table 6). Both the pumice and the scoria containsmall (to 1 cm) but conspicuous white blebs and streaks ofpumiceous, phenocryst-poor high-Sr dacite that containsrhyolitic glass (Venezky and Rutherford 1997). The fluidalshapes of these white pumiceous blebs and streaks areevidence that the crystal-poor dacite was molten whenincorporated into the andesitic hosts. Some scoria pyro-clasts also contain minor angular lithics and (or) fragmentsof hydrothermal clay, probably entrained from shallowconduit walls. Additional components (<20%) of the Cdeposit are lapilli of a cement-gray, crystal-rich, poorlyinflated low-SiO2 dacite with moderately elevated Srconcentrations (Table 6), and dense prismatic lithics, manyof which are high-Sr andesite similar in composition to thehigh-Sr lava flows exposed through the Emmons andWinthrop Glaciers. These high-Sr components are probablyshallow conduit linings from that earlier magmatic event,exhumed by the subplinian C eruption.
Heterogeneity between and within the juvenile C ejectaresults from juxtaposition and mingling of magmas shortlybefore and during the eruption (Mullineaux 1974; Swanson1993; Venezky and Rutherford 1997). The dominant brownpumice lapilli have abundant strongly sieve-textured pla-gioclase and pyroxene phenocrysts containing wormy meltinclusions and embayments that are variably overgrown bynon-embayed rinds to as much as a few tens of micronsthick. These textures record an event of phenocrystresorption due to heating, mixing, or fluxing by introducedfluids, followed by a period of minor crystal regrowth.Small, irregularly shaped amphibole grains in the glassymatrix of the brown pumice appear to be stable post-resorption minerals, but they might also be partly resorbedrelicts of earlier amphibole phenocrysts, although they lackreaction-rim overgrowths common on unstable volcanicamphiboles. Microlites and microphenocrysts in the matrixof the brown andesitic pumice are also post-resorption-event crystalline phases, though their abundance is lowcompared with the non-pumiceous glassy ash grains thatdominate the earlier Summerland period tephras. Latemagma mingling is represented by co-eruption of pumice
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and scoria with distinct compositions, and by the late-stageentrainment of the crystal-poor white dacite magma intoboth andesite types.
Despite textural evidence for a major phenocryst resorp-tion event, analyzed C pumice lapilli have nearly homoge-neous dacitic glasses (64.3–64.7 wt.% SiO2) (Table 3).Venezky and Rutherford (1997) present a wider composi-tional range for glass in brown C pumice, attributed mainly
to local, syn-eruptive blending with rhyolitic melts from theentrained white dacite blebs. The homogeneous glasses inthe brown C pumice are indistinguishable in compositionfrom glasses in the hornblende-bearing pumice that eruptedduring the earlier SL2 event (Table 3, Figs. 8 and 9). This,and the presence of amphibole in both deposits (generallyminor at Mount Rainier), raises a speculative possibility thatthe dominant C andesite might have been remobilized SL2
Table 6 Whole-rock chemical compositions of ejecta from the 2,200 cal year BP C-tephra eruption of Mount Rainier
Sample Brown pumice Black scoria Cement-gray pumice Prismatic lithics
98RE692-P1
98RE692-P2
98RE692-P3
93RE41 93RE42 JV506C-G1
JV506C-G2
JV506C-L2
JV506C-L3
JV506C-L4
Latitudea 46.8646 46.8646 46.8646 46.8418 46.8418 46.8645 46.8645 46.8645 46.8645 46.8645Longitudea −121.6611 −121.6611 −121.6611 −121.7270 −121.7270 −121.6619 −121.6619 −121.6619 −121.6619 −121.6619SiO2 (wt.%) 60.32 60.34 60.37 58.98 59.41 63.12 63.17 62.16 60.70 60.66TiO2 0.95 0.95 0.96 1.13 0.99 0.78 0.76 0.78 0.87 0.89Al2O3 17.51 17.46 17.55 17.29 17.26 17.36 17.45 17.70 17.69 17.71FeOb 5.48 5.51 5.50 5.91 6.18 4.79 4.61 5.01 5.58 5.54MnO 0.09 0.09 0.09 0.10 0.10 0.09 0.09 0.10 0.10 0.10MgO 3.83 3.97 3.82 4.22 4.00 2.34 2.22 2.31 2.82 2.78CaO 6.02 6.00 5.97 6.40 6.48 5.27 5.28 5.71 6.17 6.17Na2O 3.97 3.88 3.96 4.03 3.84 4.00 4.08 4.09 4.11 4.13K2O 1.59 1.56 1.55 1.67 1.47 1.95 2.00 1.79 1.62 1.70P2O5 0.24 0.23 0.22 0.27 0.27 0.30 0.34 0.32 0.33 0.33Totalb 99.41 99.71 99.17 99.37 99.93 98.43 98.49 99.97 100.12 100.02Rb (XRF, ppm) 40 39 39 44 39 47 47 38 32 34Sr 496 478 488 475 475 660 680 820 865 864Y 19 16 17 20 20 14 15 16 16 17Zr 173 169 166 180 184 185 187 189 184 185Nb 12 12 11 14 15 10 11 10 10 9Ba 428 426 422 425 435 499 497 473 445 442Ni 54 59 58 50 46 14 12 5 9 8Cu 29 27 29 30 27 21 19 5 7 9Zn 68 69 68 73 67 70 69 67 69 68Cr 74 85 68 70 76 19 13 9 11 11V 120 110 119 83 74 79 85 93Rb (INAA, ppm) 43 43 42 42 43 45 33Cs 1.9 1.9 1.8 1.8 1.7 2.2 0.8Th 5.15 5.05 4.76 4.79 4.84 6.86 6.60U 2.0 2.0 1.9 1.5 1.5 2.3 2.1La 19.3 19.0 18.6 19.7 19.9 23.9 27.5Ce 43.7 42.8 39.6 38.9 39.5 48.0 61.1Nd 22.0 21.4 21.2 20.0 19.0 23.5 32.2Sm 4.56 4.63 4.44 4.44 4.39 4.38 5.69Eu 1.20 1.15 1.19 1.22 1.2 1.10 1.55Tb 0.58 0.60 0.58 0.57 0.59 0.46 0.62Yb 1.69 1.64 1.61 1.7 1.7 1.4 1.73Lu 0.24 0.23 0.22 0.24 0.24 0.21 0.26Hf 4.45 4.32 4.25 4.24 4.2 4.21 5.09Ta 0.84 0.86 0.82 0.98 1.02 0.69 0.68Sc 14.4 14.4 14.2 14.7 14.9 8.92 11.8Cr 82.9 87.5 77.6 81.3 83.2 14.7 10.0Co 19.4 19.9 19.9 21.2 21.1 11.4 15.8
a NAD27 CONUS geographic datumbAnalyses normalized to 100 wt.% with all Fe as FeO, total original total
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magma stored beneath the volcano. The sharply boundedwhite dacitic pumiceous blebs scattered in the C andesiticpumice and scoriae have microlite-free rhyolitic glasses(Table 3; Venezky and Rutherford 1997), as do the cement-gray, crystal-rich, poorly inflated dacite lapilli. Pervasivequench crystallites precluded glass analyses in C andesiticscoriae bombs.
SL8 The coarse, porous character of the C tephra makes it apoor substrate for subsequent deposits, with the result thatyounger tephras are disturbed or absent in many areasaround Mount Rainier; however, a thin (1–2 cm), fine-grained, gray ash deposit (SL8) overlies the C tephra atsome localities on the volcano’s east flank, commonlyseparated from the underlying C tephra by up to 1 cm ofbrown, probably non-eruptive silty ash (Fig. 7). The coarserfraction (to 0.5 mm) of the SL8 unit consists chiefly ofplagioclase and orthopyroxene phenocrysts and phenocrystfragments coated with clear, dense-to-moderately vesicularmicrolite-bearing glass. The dominant finer fraction con-sists mainly of gray, angular-to-fluidal shaped grains ofsimilar microlite-bearing clear glass. Brown blocky-to-scoriaceous glassy grains similar to dark-colored glassygrains in SL1 through SL6 deposits are also present, alongwith minor fine lithics, but are subordinate to the grayishgrains with clear glass. Glasses analyzed from the SL8 unitare rhyolitic with a restricted compositional range (72.7–73.4 wt.% SiO2) (Table 3, Fig. 8). A distinguishing featureof these glasses is that they have higher Al2O3 concen-trations (13.3–13.5 wt.%) than most Holocene MountRainier glasses with similar SiO2 values (commonly 12–13 wt.% Al2O3 at 73 wt.% SiO2), consistent with fastascent and limited degassing-driven growth of plagioclasemicrolites. The abundance, consistent texture, and appear-ance of the grayish glass-bearing grains, along with thelimited and distinctive compositional range of their glasses,are evidence that the SL8 deposit resulted from a magmaticeruption following the large C event. No ages have yet beenobtained from the SL8 deposit to determine if it belongswith the Summerland tephras, or if it erupted at a muchlater time.
∼1,500 year BP tephras and Twin Creek lahar episode
Two fine-grained Mount Rainier tephras (TC1, TC2)overlie the SL8 deposit at Summerland and elsewhere andunderlie the widespread Mount St. Helens W pumiceousash (Fig. 7). Bracketing radiocarbon samples (twigs)establish that these two post-SL8, pre-MSH-W ashes weredeposited at ∼1,500 cal year BP (Table 2 and Vallance etal., in prep). These ashes were deposited approximatelyconcurrent with the emplacement of clay-poor lahars of
Mount Rainier’s Twin Creek (TC) lahar episode (weightedmean age of 1,510±110 cal year BP from 11 Twin Creek14C samples, Zehfuss et al. 2003) whose runout sandsreached as far as the (present) Port of Seattle, 130 km ofriver distance from the volcano.
TC1 The TC1 tephra consists of a single stratum of fine-to-medium-grained, poorly sorted, brownish gray ash, 2–3 cmthick at the Summerland locality (Fig. 7). The mostabundant components of the deposit are angular gray lithicgrains (to 2.5 mm), and plagioclase and pyroxene pheno-cryst fragments, but dark-brown-to-black glassy grains arecommon. These glassy grains range from angular andblocky to fluidal and scoriaceous, and are rich in microliteswith distinctively acicular habits. Minor TC1 componentsare hydrothermally altered ash grains, and brown pumi-ceous ash that may be reworked from the voluminous Ctephra deposit. Glasses in the dark, blocky-to-scoriaceousgrains are wholly rhyolitic (73.9–77.2 wt.% SiO2), and aremore evolved than glasses from the preceding SL8 deposit(Table 3; Fig. 8). The absence of dense grains with daciticglasses, like those in many pre-C Summerland tephras, isevidence against a reworking origin for the TC1 glassygrains. Instead, the glassy grains are probably a minorjuvenile magmatic component to what may have been amainly phreatomagmatic eruption.
TC2 The TC2 tephra is a fine-grained, moderately sorted,dark gray ash stratum, ∼1 cm thick at the Summerlandlocality, that overlies the coarser and browner TC1 tephra(Fig. 7). The darker color of the TC2 deposit results from amuch higher abundance (20–30 vol.%) of dark-brown-to-black glassy ash grains, many with fluidal-scoriaceousshapes. The remainder of the TC2 deposit is made up ofdiverse angular lithics and phenocryst fragments. The darkglassy grains are microlite-rich with rhyolitic glasses (73.0–75.1 wt.% SiO2) that are generally less evolved than thoseof the underlying TC1 deposit (Table 3, Fig. 8). The contactbetween the TC1 and TC2 deposits is sharp withoutintergradation, consistent with two eruptions, but thecontact lacks incision or accumulations of organic-richnon-eruptive sediment, consistent with only brief reposebetween the TC1 and TC2 events.
∼1,000 year BP ash in the upper White River valley
Hoblitt et al. (1998) report a thin, fine-grained ash depositthat is exposed near the top of a terrace on the south side ofthe White River in Mount Rainier National Park, shortlywest of the road bridge to Sunrise, and date this ash to1,080±250 14C year BP (1,040±410 cal year BP) from firneedles separated from the deposit (R. Hoblitt, 2008,
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written communication). This deposit (1–2 cm) underliesthe 471 cal year BP (AD 1479) Mount St. Helens W tephra(Yamaguchi 1983; Fiacco et al. 1993) and overlies a thinlahar deposit (35 cm) that caps the Mount Rainier Cpumice-fall deposit in that area. The fine-grained ashdeposit is poorly sorted, containing common brown pumicein the ash fraction coarser than 1 mm, probably reworkedfrom the Mount Rainier C and SL3–SL4 deposits, but italso contains abundant dark glassy ash grains (<1 mm).Most of these glassy grains are dense, angular, and blocky,with few vesicles, but fluidal-scoriaceous glassy grains arealso present. All of the dark glassy grains are rich inmicrolites and have uniformly rhyolitic glass compositions(72.0–73.8 wt.% SiO2). Glasses are distinguished by theirlower SiO2 concentrations from the earlier TC1 and TC2tephra glasses (Fig. 8), and by their lower Al2O3 concen-trations from the similar-SiO2 SL8 tephra glasses (Table 3).A potentially correlative thin (∼1 cm) fine-grained ashydeposit is present on the opposite (north) side of the WhiteRiver and yields a calibrated radiocarbon age of 980±80 calyear BP (Vallance et al., in prep.), but glasses in thatsediment have not been analyzed.
The common dark glassy grains with their restricted glasscompositions are consistent with a magmatic eruptive originfor the ∼1,000 cal year BP tephra deposit. The fine grain-sizeand localization to a valley floor setting could result bydeposition as fallout from the dilute ash cloud from apyroclastic flow (Hoblitt et al. 1998). No contemporaneouspyroclastic flow deposits have been found in the area, butfar-traveled lahars and runout sands of the Fryingpan Creeklahar episode were emplaced down the White River systemat about this time (weighted mean age of 1,120±70 cal yearBP from four 14C samples from Fryingpan Creek deposits,Zehfuss et al. 2003). Possibly, pyroclastic flow(s) trans-formed directly to lahars during transit down the largeEmmons Glacier, leaving no primary deposits other than thefine-grained tephra.
∼500 year BP deposits
An enduring issue has been if an eruption triggered thevoluminous, clay-rich Electron Mudflow of ∼500 cal yearBP, or if the mudflow started by non-magmatic processessuch as spontaneous gravitational failure of an alterededifice flank, dislodgement by shaking from a tectonicearthquake, or perhaps by a heavy rainfall event (Scott2004). To investigate this we examined several ashydeposits immediately beneath and above the 471 cal yearBP MSH-W tephra, searching for evidence of contempora-neous eruptive activity from Mount Rainier. None of theashy deposits yielded compelling evidence for an eruptiveorigin, but a lahar deposit exposed at the confluence of theMain and West Forks of the White River, potentially age-
correlative with the Electron Mudflow (Crandell 1971,Scott et al. 1995), contains a texturally and compositionallyconsistent clast type that could represent juvenile eruptivematerial. Without a corroborating tephra deposit, theevidence is weak for an eruptive trigger for the ElectronMudflow. See Electronic Supplementary Materials fordescriptions of the lahar and of the non-eruptive ashysediments.
Mount Rainier X-tephra
Mullineaux (1974) interprets lapilli of brown pumice andscoria scattered on some neoglacial moraines as a depositfrom a minor pumice eruption of Mount Rainier, dated bytree ring counts to between AD 1820 and AD 1854. Thisdeposit was named the X tephra, not to be confused withthe similarly young X tephras erupted from Mount St.Helens and Glacier Peak, Washington (Mullineaux 1996,Beget 1982). At localities below the Emmons, Inter,Winthrop, and Ohanapecosh Glaciers (Fig. 1) we confirmedthat brown pumice lapilli are scattered locally on neoglacialmoraines, but in each case these lapilli are indistinguishablefrom Mount Rainier C-tephra pyroclasts based on presenceof resorbed and overgrown phenocrysts, minor amphibole,and distinctive dispersed blebs and streaks of whitepumiceous dacite. Below the Emmons Glacier the brownpumice lapilli are most abundant on young moraines belowa large avalanche chute descending from BurroughsMountain, which is carpeted with C tephra. Other localitieswhere Mount Rainier X tephra has been reported are alsosurrounded by highlands where Mount Rainier C tephra isabundant, and we interpret the brown pumice and scorialapilli scattered on most of the young moraines as MountRainier C tephra that was redeposited by snow avalanches,not by a mid nineteenth century pumice eruption. Thesituation below the Ohanapecosh Glacier is somewhatdifferent. There, Sigafoos and Hendricks (1972) describe aconcentration of presumed Mount Rainier X pumice withinthe area of neoglacial moraines. Reexamination of thatlocality shows the presence of a low ridge of disturbedHolocene tephras that survived erosion by thin neoglacialice advance. The pumiceous lapilli are Mount Rainier Ctephra, underlain by disturbed Mount St. Helens Ypumiceous ash, underlain by locally exposed MountRainier F tephra.
Other historic eruptions
Hopson et al. (1962) briefly summarize and generallydiscount reports of Mount Rainier eruptions on 14occasions between AD 1820 and AD 1894. No physicalevidence has been found in this study for these possibleeruptions, but accounts of the November–December 1894
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event, edited by Majors and McCollum (1981a,b), anddispatches by WM Sheffield and ES Ingraham (some byhoming pigeon) while on a winter expedition to MountRainier in December 1894 to investigate the event, includecredible descriptions, one at close range on 24 December1894, of small, dark eruption plumes rising from thesummit. Many other claims for the 1894 eruption, such asthat the profile of the mountain was changed or that therewere large rock avalanches, did not survive later scrutiny.Independent confirmation for an eruption is, however,lacking and a clear photograph of the volcano fromTacoma, dated 29 December 1894, shows neither eruptiveplumes nor dark tephra on the summit snowfields (Univer-sity of Washington Special Collections image WAT042).We consider it possible that phreatic eruptions took place inlate 1894, but were too small to leave preserved deposits.Likewise, some of the earlier discounted events may havebeen of similar scale, but the last magmatic eruption thatcan be amply documented was the ∼1,000 cal year BPevent that deposited ash in the White River and generatedthe Fryingpan Creek lahar assemblage (Hoblitt et al. 1998;Zehfuss et al. 2003).
Correlations of flow and fall deposits based on glassand mineral compositions
Compositions of glass and plagioclase phenocrysts can beused to refine the inferred sequence of events during theSummerland eruptive period. Previously it was noted thatglass compositions in the SL3–SL4 tephra units matchthose of glass in breadcrust bomb rinds from the SouthPuyallup pyroclastic flow deposit. This and the presence ofbrown pumiceous ash and rare brown pumice lapilli in theSL4 unit are evidence that the South Puyallup block-and-ash flow erupted as part of the SL3–SL4 episode, dated incombination at 2,560±120 cal year BP.
Paleomagnetic measurements establish that the SouthPuyallup block-and-ash flow and the high-Sr Emmons–Winthrop lava flows were erupted at close to the same time(Hagstrum and Champion 2002; Vallance et al., in prep),but cannot determine the order of those events. Strontium iscompatible-to-strongly compatible in plagioclase, relativeto melt, so we use electron-microprobe analyses of Sr inplagioclase phenocrysts (see Electronic SupplementaryMaterials for analytical methods) to search for tephrascorrelative to the high-Sr Emmons–Winthrop lava flows(Fig. 10, Table 7). Plagioclase phenocrysts from SL1through SL4 juvenile tephra grains and from the SouthPuyallup block-and-ash flow have similar ranges in Sr andanorthite concentration, with the exception of the distinctlyhigher-anorthite plagioclase phenocrysts in the SL2 amphi-bole-bearing pumice lapilli that are consistent with plagio-
clase grown from melt with higher dissolved H2Oconcentrations (Yoder et al. 1957; Sisson and Grove1993). High-Sr plagioclase phenocrysts appear abruptly inthe SL5 tephras and persist in the SL6 tephras. These highSr concentrations overlap those of phenocrysts from theEmmons–Winthrop high-Sr lava flows. These results leadto the interpretation that the SL5, and perhaps SL6, tephraswere deposited concurrent with effusion of the high-SrEmmons–Winthrop lavas, and therefore that the high-Srlavas effused shortly after eruption of the South Puyallupblock-and-ash flow and SL3–SL4 tephras.
SL1
SL2
SL3
SL4
SL1
SL2
SL3
SL4
SL6
SL5
summit lava
high-Sr EmmonsWinthrop lavas
SL4 pumiceS. Puyallup block & ash
SL2 pumice
SL1 pumice
C tephra
summit lava
C tephra
high-Sr EmmonsWinthrop lavas
SL6
SL5
SL4 pumice
S. Puyallup block & ash
SL2 pumice
SL1 pumice
young
old
young
old
35 45 55 65 75
500 1000 1500 2000 2500
average plagioclase An (mol %)
average Sr in plagioclase (ppm)
rela
tive
str
atig
rap
hic
leve
lre
lati
ve s
trat
igra
ph
ic le
vel
Fig. 10 Stratigraphic succession of average plagioclase phenocrystcompositions for late-Holocene Mount Rainier deposits, subdivided asSummerland eruptive period ash phenocrysts (blue diamonds), pumicelapilli (circles), South Puyallup breadcrust bombs (yellow square), C-tephra resorbed and overgrown phenocrysts (tan circle), non-resorbedphenocrysts (light blue circle), and phenocrysts in white dacitic blebs(white circle) within andesitic pyroclasts. Phenocryst compositionsfrom lava flows are shown at inferred stratigraphic levels as blue andred bars. Brackets show one standard deviation
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Relative timing of the Mount Rainier C tephra and theeast summit crater lava flows was also not resolved bypaleomagnetic and radiocarbon methods. The east summitcrater lava flows have normal Sr concentrations for MountRainier eruptives, as do their relatively simple-textured
plagioclase phenocrysts (Fig. 10, Table 7). In contrast, thegray dacite, white dacite blebs, and prismatic lithiccomponents in the C tephra have elevated whole-rock Srconcentrations. Plagioclase phenocrysts in the dominantbrown andesitic C pumice can be categorized into three
Table 7 Summary of electron-microprobe analyses of plagioclase phenocryst compositions from late-Holocene Mount Rainier eruptive products
Mean 1−σ Median Range Mean 1−σ Median Range Mean 1−σ Median Range
East summit crater lava (n=152)An (mol%) 50.6 6.7 49.5 66.3–32.8Ab 47.6 6.2 48.8 32.9–63.7Or 1.8 0.5 1.8 0.6–3.5Sr (ppm) 1,290 290 1,250 2,240–720
Brown C pumice, complex plag. (n=47) Brown C pumice, simple plag. (n=58) Brown C pumice, white streaks (n=28)An (mol%) 64.7 7.1 63.0 82.6–41.9 64.5 4.9 64.8 76.8–55.3 51.7 3.8 51.7 57.7–43.8Ab 34.5 6.8 36.2 17.0–56.2 34.7 4.8 34.5 22.9–43.4 46.5 3.5 46.6 41.0–53.8Or 0.8 0.3 0.9 0.3–1.9 0.8 0.2 0.7 0.3–1.3 1.7 0.3 1.7 1.3–2.4Sr (ppm) 1,200 210 1,230 1,670–750 1,270 280 1,250 1,990–590 1,910 230 1,940 2,320–1,330
SL6 (n=48)An (mol%) 51.0 6.0 49.6 65.7–41.2Ab 47.0 5.7 47.5 33.3–56.2Or 2.1 0.5 2.1 1.0–3.2Sr (ppm) 1,510 420 1,450 2,210–800
SL5 (n=124) Emmons-Winthrop lava (n=175)An (mol%) 52.6 3.1 52.3 66.9–47.5 53.5 10.6 50.9 86.0–31.8Ab 45.7 2.9 46.0 32.1–50.7 44.7 10.0 47.2 13.7–63.5Or 1.7 0.2 1.7 1.0–2.1 1.9 0.7 1.9 0.3–4.7Sr (ppm) 1,790 290 1,820 2,520–940 1,460 375 1,520 2,300–610
SL4 (n=103) SL4 pumice lapilli (n=151) S. Puyallup block & ash bomb (n=156)An (mol%) 52.5 6.7 53.3 69.4–38.6 52.0 9.5 50.5 82.9–36.6 52.0 9.5 50.5 82.9–36.6Ab 45.8 6.2 45.1 29.9–58.4 46.3 8.9 47.7 16.7–60.4 46.3 8.9 47.7 16.7–60.4Or 1.7 0.5 1.6 0.7–3.0 1.7 0.5 1.7 0.3–3.0 1.7 0.5 1.7 0.3–3.0Sr (ppm) 1,140 210 1,120 1,670–630 1,150 240 1,120 1,800–630 1,150 240 1,120 1,800–630
SL3 (n=151)An (mol%) 52.5 5.7 53.0 68.0–38.7Ab 45.9 5.3 45.5 31.3–58.6Or 1.6 0.4 1.5 0.7–3.5Sr (ppm) 1,120 210 1,150 1,540–520
SL2 (n=121) SL2 hbl pumice lapilli (n=72)An (mol%) 48.5 8.2 46.9 68.2–36.4 62.3 8.2 62.0 79.3–45.5Ab 49.5 7.6 51.0 31.0–60.3 36.6 7.7 37.2 20.3–52.0Or 2.0 0.7 2.0 0.8–3.6 1.1 0.6 0.9 0.4–2.7Sr (ppm) 1,050 220 1,040 1,600–550 1,180 260 1,180 1,780–370
SL1 (n=122) SL2 hbl pumice lapilli (n=107)An (mol%) 50.3 9.5 47.7 69.3–36.3 50.3 6.5 49.3 66.0–38.7Ab 47.8 8.8 50.2 29.9–60.8 47.8 6.1 48.7 33.1–58.3Or 1.9 0.7 2.0 0.7–3.0 1.9 0.5 1.9 0.8–3.0Sr (ppm) 1,040 220 1,030 1,520–410 1,240 230 1,250 1,700–630
Units arranged in approximate stratigraphic order (younger upward); n gives number of analyses per unit or subunit; Sr concentrations account forSi kβ X-ray interference=200 ppm (apparent)
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types. The highest Sr grains are water-clear idiomorphicphenocrysts within the blebs and streaks of white, pumi-ceous dacite dispersed in the dominant andesitic pumiceand scoriae. Phenocrysts directly in the groundmass of thebrown pumice can be subdivided into simple and stronglyresorbed-overgrown textural types. Neither type has notablyelevated Sr concentrations, but both have high anorthiteconcentrations that, along with the presence of amphibole,are consistent with higher magmatic H2O concentrationsthan for typical amphibole-free or -poor Mount Rainiereruptives. Fluidal shapes of the white dacite blebs andstreaks show that the crystal-poor dacite was molten whenentrained into the andesite, and Venezky and Rutherford(1997) interpret based on phase equilibrium results thatthe dacite was stored at a very shallow level prior toeruption.
Together, these results are consistent with the high-Srcomponents in the C tephra (prismatic lithics, crystal-richgray dacite lapilli, white dacite blebs) originating asvariably solidified conduit linings and residual meltsegregations left from the Emmons–Winthrop high-Sreffusive episode. Such high-Sr conduit fillings likely wouldhave been removed if the east summit crater magmas haderupted between the Emmons–Winthrop and C episodes.High-Sr plagioclase phenocrysts have also not been foundin east summit crater lava samples, consistent with high-Srmaterial having been removed by the time the east summit
crater magmas passed through the conduit system. Thesefactors lead to the interpretation that the C eruptionfollowed the Emmons–Winthrop–SL5–SL6 event(s) andexhumed residual high-Sr conduit fillings. The C eruptionwas then followed by effusions of the normal-Sr andesitemagmas now preserved as the rim of the east summit crater.Effusion of the east summit crater andesites may corre-spond with deposition of the fine-grained SL8 tephra.
Summary eruptive history of Mount Rainierover the last 2,600 years
Stratigraphic, compositional, and geochronologic resultsdocument ten–12 distinguishable eruptions of MountRainier over the last ∼2,600 years BP (Fig. 11, Table 4).Each eruption probably consisted of multiple explosiveevents spanning months to possibly years. Additionalsmaller eruptions may have taken place, but did not leavedeposits that have been recognized. Seven or eighteruptions took place over the period 2,600–2,200 cal yearBP and compose the Summerland eruptive period. Depositsfrom these eruptions are (1) SL1—magmatic and phreato-magmatic ash plus minor pumice lapilli (plagioclase–pyroxene), (2) SL2—magmatic and phreatomagmatic ashplus minor pumice lapilli (plagioclase–pyroxene–amphibole),(3) SL3–SL4—magmatic and phreatomagmatic ash, minor
0
500
1000
1500
2000
2500
3000
year
s be
fore
195
0
possible small phreatic explosions in 1894 (no physical evidence)
possible small magmatic eruption leading to lahar, no lava flows (link to Electron Mudflow??)
pyroclastic flow down the Emmons Glacier leading to ash in upper White River and lahars, no lava flows
explosive magmatic eruptions producing TC1 and TC2 tephras and lahars, no lava flows
one or two magmatic eruptions producing east summit crater lava flow and SL8 tephra
high-Sr Emmons-Winthrop lava flows and SL5 phreatomagmatic tephras
subplinian C tephra (SL7)SL6 phreatic and possibly phreatomagmatic tephras
SL3-SL4 phreatomagmatic tephras, minor pumice, South Puyallup pyroclastic flow, possible concealed lava flowsSL2 phreatomagmatic tephras, minor pumice, possible concealed lava flowsSL1 phreatomagmatic tephras, minor pumice, possible concealed lava flows
DORMANT TO ~4000 yr BP
late-Holocene Mount Rainier eruptionsFig. 11 Summary of late-Holo-cene eruptive events at MountRainier. Red circles designateeruptions with direct physicalevidence for ejection of juvenilemagma, orange circle representspossible eruption documentedonly by uniform clast type inWhite River confluence lahar,light blue circle representsreported 1894 eruption forwhich physical evidence islacking. Other historicallyreported small eruptions mighthave taken place but also lackcorroborating physical evidence.See Table 4 for approximateeruption magnitudes and corre-lations with lahars
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pumiceous ash and lapilli (plagioclase–pyroxene), contem-poraneous with the South Puyallup block-and-ash flow, (4)SL5—high-Sr magmatic and phreatomagmatic ash contem-poraneous with the high-Sr Emmons–Winthrop andesite lavaflows, (5) SL6—infrequent phreatomagmatic ash eruptionsprecursory to the C tephra, (6) the subplinian C pumice andscoria deposit, (7 or 8) east summit crater andesitic lava flowsaccompanied, preceded, or followed by the SL8 tephra. Lavasmay also have effused during the SL1, SL2, and SL3–SL4events, but any flows are concealed by subsequent lava flowsand by ice. Small-volume block-and-ash flows may also haveaccompanied those eruptions, but no primary deposits surviveother than in the South Puyallup drainage. Lahars of theSummerland lahar assemblage (Zehfuss et al. 2003) accom-panied the Summerland eruptions.
Three or four eruptions are documented following theSummerland eruptive period. Deposits from these eruptionsare (1, 2) the fine-grained TC1 and TC2 tephras that eruptednear 1,500 cal year BP and correlate with the Twin Creek laharassemblage (Zehfuss et al. 2003), (3) the ∼1,000 cal year BPfine-grained ash in the White River valley floor, andspeculatively, (4) the ∼500 year BP lahar at the confluenceof the main and west forks of the White River. The fine-grained character of the post-Summerland ashes, as well asthe contemporaneous formation of far-traveled lahars at∼1,500 and ∼1,000 cal year BP (Zehfuss et al. 2003) areconsistent with eruption of small-volume pyroclastic flows orsurges that transformed to lahars while transiting ice, or withdeposition of hot fallout tephras onto the upper mountain icecap, leading to melting and lahar generation.
The results presented here greatly increase the number ofphysically documented late-Holocene Mount Rainier erup-tions. Most of these eruptions were small-to-moderate(Table 4), and some left no known primary volcanicdeposits other than thin, fine-grained, dominantly lithictephras. Nevertheless, the eruptions were penecontempora-neous with far-traveled lahars, showing that even modestmagmatic or phreatomagmatic eruptions of Mount Rainiercan produce hazardous lahars, irrespective of lava effusionor dome growth. Despite considerable effort, however,evidence is weak for a magmatic eruption associated withthe ∼500 cal year BP Electron Mudflow, and we cannot ruleout that sizeable lahars may also form at Mount Rainierwithout a direct eruptive trigger.
Acknowledgements J Byman and C Harpell assisted JV in the field. DChampion provided advice on paleomagnetic results. R Oscarson expertlymaintained the USGSWestern Region electronmicroprobe facility.Whole-rock analyseswere performed by J Budahn (INAA) and by the late D Seims(XRF). J Fierstein, M-A Longpré, P Pringle, and K Wallace providedconstructive manuscript reviews, and J Stix edited the manuscript for theBulletin. P Pringle supplied key samples for the Zehfuss et al. (2003) study.This study was supported by the U.S. Department of the Interior,Geological Survey, Volcano Hazards Program.
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