Volcanic Rocks: Eruption Style & Lava Flows Lava Fountains.
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Transcript of Volcanic Rocks: Eruption Style & Lava Flows Lava Fountains.
Volcanic Rocks: Eruption Style & Lava Flows
Lava Fountains
The Causes of Volcanic Eruptions
How do magmas rise?
If only due to bouyancy most basaltic lavas could not erupt through continental crust
The Causes of Volcanic Eruptions: Volatiles
Decrease in Pressure
H2O is the most important volatile in controlling volcanic eruptions
DissolvedH2O
Increase inTemperature
Crystallisation(decrease in temp)
Depolymerisation + confining pressure
Depolymerisation
Fractionation
The Closed Magma Chamber
Magma rises until it is trapped within a closed magma chamber
The Closed Magma ChamberCooling and crystallisation causes the exsolution of volatiles
Magma chamber expands due to an over-pressure
The Closed Magma Chamber
Expansion of the magma chamber causes tensile failure of country rocks
Magma chamber expands due to an over-pressure
Rise of less dense vesicular magma
Conduit formed due to fracturing
An Open Conduit
Exsolution of volatiles due to decreasing pressure
Increasing acceleration
An Open Conduit
Exsolution of volatiles due to decreasing pressure
Break up of magma
Foam reaches 77 vol% vesicles
Magma/gas aerosol 600 m/s
Interactions with Water
Hydrovolcanic Eruptions
Hydrovolcanic Eruptions
Hyaloclastite
Hydrovolcanic Eruptions
Efficient cooling of magma
Phreatic EruptionsPhreatomagmatic
0.3
Hydrovolcanic Eruptions
Efficient cooling of magma
Phreatic EruptionsPhreatomagmaticPhreatic Eruptions: Maars
Mainly fragmented country rock, sediments can deform in-situ
Hydrovolcanic Eruptions
Efficient cooling of magma
Phreatic EruptionsPhreatomagmaticPhreatic Eruptions: Maars
Mainly fragmented country rock, sediments can deform in-situ
Contact between magma and an acquiferHigh abundance of lithic materials
Hydrovolcanic Eruptions
Surtseyan
Phreatomagmatic Eruptions
Hydrovolcanic Eruptions
Kilauea
Explains the presence of ashes on kilauea
Magma-Mixing
Acid Magma
Basaltic Magma
Extra volatiles Extra heat
Expansion due to vesicle formation, temp increase and/or volume of the injected magma
Extrusive Activity
Loss of volatiles allows extrusive activity
• Low viscosity magmas• Low H2O magmas
Basaltic magmas
Intermediate/acid magmas after explosive activity (i.e. loss of volatiles)
Size and Shape of Lava Flows
Andesite lava flow
Rhyolite DomeBasaltic pahoehoe flows
Rhyolites
Andesites, Dacites
Basalts
Size and Shape of Lava Flows
Rhyolites
Andesites, Dacites
Basalts
Gradient
ViscosityLow viscosity High viscosity
High temperatureLow crystal contentHigh water content
Low-Si (basaltic)
Low temperatureHigh crystal contentLow water contentHigh-Si
Effusion RateHighLow
Shield Volcanoes
Mauna Kea
Mauna Loa
Rift Zone
Low viscosity basaltic magmas usually lead to shield volcanoes. Eruptions occur mainly from vents.
Fissure Eruptions
Splatter Ramparts and Splatter Cones
Lava Fountains
Mauna Loa
Fissure Eruptions
Splatter Ramparts and Splatter Cones
Lava Fountains
Mauna Loa
Splatter forms agglutinate scoria
Low viscosity, high extrusion rate lavas expand as single flow units
Basaltic Flows: Pahoehoe & Aa
Pahoehoe (ropey) surface formed by folding of the crust
Low viscosity
Basaltic Flows: Pahoehoe & Aa
Flow expands by extrusion of tongues and toes of lavaHigher viscosity/lower extrusion rates
Basaltic Flows: Pahoehoe & Aa
Lava blisters and hornitos can occur on the upper surface of the flow
Basaltic Flows: Pahoehoe & Aa
Insulated flow
Basaltic Flows: Pahoehoe & Aa
Aa flows roll forwards over their breccia
Higher viscosity (lower T)
Basaltic Flows: Channels
Flows tend to form channels
Basaltic Flows: Channels
Flood Basalts
Snake River Flood Basalts
Flood Basalts
Flood Basalts
Giant’s Causeway Shrinkage cracks due to contraction on cooling.Intermediate cooling rates.
Pillow Lavas
Submarine Volcanics
Intermediate Lavas
• Channelled by block levees• Ridges due to compression of magma• Extensive basal and flow top breccias.• Associated with stratovolcanoes or calderas
Dacite & Rhyolite Lavas
Mono Coulee, California
Can form lava flows where viscosity and extrusion rate are sufficiently high
Dacite & Rhyolite Lavas
Mono Coulee, California
Obsidian
Banding
Dacite & Rhyolite Domes
More commonly form lava domes, often within summit crater.
Collapse of lava domes is a major cause of pyroclastic eruptions.
Extrusive Igneous Rocks: Ultrabasic
KomatiiteMg-rich Ultrabasic lavas
Pyroxenite lavas are also known.
Archean lavas
Extrusive Igneous Rocks: Basic Rocks
Intermediate Rocks
Dacite, latite and Andesite often containhornblende
Extrusive Igneous Rocks: Basic Rocks
Intermediate Rocks
Dacite, latite and Andesite often containhornblende
BasaltAnorthite-rich plag + pxOften contains olivine
Extrusive Igneous Rocks: Basic Rocks
Intermediate Rocks
Dacite, latite and Andesite often containhornblende
BasaltAnorthite-rich plag + pxOften contains olivine
Andesite
More Na-rich plagHornblende
Extrusive Igneous Rocks: Basic Rocks
Intermediate Rocks
Dacite, latite and Andesite often containhornblende
BasaltAnorthite-rich plag + pxOften contains olivine
GabbroAnorthite-rich plag + pxOften with olivine
Dolerite = medium-grained equivalent
Extrusive Igneous Rocks: Basic Rocks
Basalts are split into Tholeiites – these have noolivine in their groundmass and can contain quartz.Alkali Basalts – these have olivine in their groundmass.
Olivine-rich basalts (i.e. Mg-rich) are known as picrites.
Tholeiite – can contain phenocrysts of olivine but not in the groundmass
Extrusive Igneous Rocks: Basic Rocks
Basalts are split into Tholeiites – these have noolivine in their groundmass and can contain quartz.Alkali Basalts – these have olivine in their groundmass.
Alkaline basalts are the trachybasalts and include varieties such as hawaiite, mugearite and benmoreite.
Trachyte – alkali feldspar + px, often has aligned phenocrysts = trachytic texture.
Extrusive Igneous Rocks: Alkaline Rocks
Rocks rich in alkali metals (Na and K) within feldspathoids
Phonolites: sanidine, anorthoclase (alkali feld) phenocrysts, nepheline, px in groundmass.
Tephrite: plagioclase, px and nepheline. If olivine is present it is a basanite.
Nephelinite: nepheline and cpx.
Leucitite: leucite and cpx. If olivine and pyroxene dominate it is a ugandite.
Feldspathoids