Post on 15-Aug-2015
Geologic Dating
Relative Dating…
…your Cousins
Tim Martin Earth Science Teacher Greensboro Day School
tmartin@greensboroday.org
Determine the Sequence
Card A Burn Glue Green Paint Brown Paint Impact Cut
A B C D E F G H I J K L M N O
BURN IMPACT SAND GREEN BURN BROWN GREEN FOLD CUT BROWN IMPACT GREEN CUT FLOLD SAND
GLUE IMPACT GREEN BURN BROWN GLUE CUT GREEN BROWN CUT GLUE BURN GLUE GLUE GLUE
GREEN BROWN CUT IMPACT PLANT SAND GLUE CUT GREEN BURN CUT GLUE SAND IMPACT IMPACT
BROWN* BURN PLANT BROWN GLUE FOLD BURN SAND GLUE GREEN BURN SAND PLANT PLANT FOLD
IMPACT* FOLD BROWN GLUE CUT PLANT PLANT GLUE IMPACT PLANT PLANT PLANT BROWN GREEN PLANT
CUT* GREEN BURN CUT GREEN GREEN BROWN BURN BURN GLUE BROWN BROWN IMPACT BROWN GREEN
Explanation of analysis
Explain how you came up with the sequence.
What observations are helpful?
Can you come up with any rules?
Determining the Sequence -what students said…
Layer is Younger if… Has the least amount of
alteration, Is on top If it overlaps another
feature The one under everything is
oldest Cut is younger than the object
it cuts Glue is younger than the cut it
runs into Erosion comes after
deposition Need to have something to
make the sand stick
Older things changed by newer If paint was first, no paint in cut Younger (cut, burn, impact) will
not have paint on sides If layer is burnt it came before
burn If something changed a layer,
that layer came before altering event
Folds mess stuff up.
Uniformitarianism
The geologic processes at work today are the same processes that were at work in the past
Volcanoes, Earthquakes, Weathering and Erosion, Sedimentation, Deposition, etc…
4 Principles of Geologic Dating
Principle of Superposition Any layer on top will be younger…
(unless the layers are overturned)
Younger
Older
Principle of Superposition Any layer on top will be younger.
Younger
Older
Kaibab Limestone
Toroweap Formation
Coconino Sandstone
Hermit Shale
Supai Group
Principle of Original Horizontality
Sedimentary layers will be laid down horizontally. Later they may be folded or tilted by tectonics
Uplifted layers Death Valley CA
Folds Wild Rose – West of Death Valley CA
Principle of Cross cutting relationships
If a feature (fault, dike or sill) cuts across sedimentary layers, it must be younger
A
B
C
D
Fault Death Valley CACMC Dike Mt. Moran Grand Teton National Park WY
Normal Fault Bishop Tuff Bishop CA
Principle of Inclusions
If a layer contains fragments of another layer, it must be younger than the particles it includes.
Inclusions in conglomerate Mosaic Canyon Death Valley CA
Devonian Trilobite (Phacops rana) Sylvania OH
Inclusions may happen from the bottom
Xenoliths and intrusions Whitney Portal Independence CA
Unconformities
A distinct change in rock layers that indicates missing time in the geologic record caused by surface erosion.
Disconformity Nonconformity Angular Unconformity
The Great Unconformity GCNP
1.7 b.y.o.
500 m.y.o.
Nonconformity Panguitch UT
What is the Sequence?
Determine the Geologic Sequence
H E C J G I D A K B F
What is the Sequence?
K J M E F L D C B A I H G
What is the Sequence?
VCOMXKEBSZJGLFTADPNHR
Laboratory Manual in Physical Geology Busch /Tasa AGI NAGT
Absolute Geologic Dating
Absolute Geologic Dating Varve count
Seasonal sediment layers formed in glacial lakes. (Count layers like tree rings)
State line outcrop NM 200ka Permian varves
Absolute Geologic Dating Geomagnetic Reversals
Absolute Geologic Dating
Radiometric Dating Measure the natural rate of decay of naturally
radio-active minerals.
Radiometric Dating
Radioactive decay Radioactive isotopes decay from one
isotope to another or from one element to another by emitting energy.
Radiometric dating By measuring the % of parent and
daughter isotopes, age may be calculated.
Radiometric Dating
Half-life The time it takes for half of the mass of a
radioactive isotope to change into the daughter isotope
0 1 2 3 4
Radiometric DatingRADIOACTIVE DECAY
0
20
40
60
80
100
120
0 2 4 6 8 10 12
HALF LIFE
ISO
TO
PE
PE
RC
EN
T
Parent Isotope
Daughter Isotope
Radiometric Dating
Common Radioactive Isotopes used in dating
Parent Daughter Half-life
C14 N14 5,730 yrs
K40 N40 1.25 b.y.
U235 Pb207 704 m.y.
U238 Pb206 14 b.y.