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.