William E. Ferguson

Geologic TimeGeologic TimeA major difference between

geologists and most other scientists is their attitude about time.

A "long" time may not be important unless it is > 1 million years.

Amount of Time Amount of Time Required for Required for

Some Geologic Some Geologic Processes and Processes and

EventsEvents

Some geologic processes can be

documented using historical

records(brown area is new

land from 1887-1988)

UniformitarianismUniformitarianism

The present is the key to the past.

— — James HuttonJames Hutton

Natural laws do not change—

however, rates and intensity of

processes may.

Two ways to date Two ways to date geologic eventsgeologic events

1 RELATIVE DATING (relative position of fossils, structure, geomagnetics)

2 ABSOLUTE DATING (isotopic, tree rings, varves, etc.)

RELATIVE GEOLOGIC RELATIVE GEOLOGIC TIMETIMESteno Laws (1669) developed to

arrange rock units in time-order

• Principle of Superposition

• Principle of Original Horizontality

• Law of Cross -Cutting Relationships

• Law of InclusionsLaws apply to both sedimentary and volcanic rocks.

Principle of SuperpositionPrinciple of Superposition

In a sequence of undisturbed

layered rocks, the oldest rocks

are on the bottom.

Jim Steinberg/Photo Researchers

Oldest rocksOldest rocks

Youngest rocksYoungest rocks

Principle of Superposition

Principle of Original Principle of Original HorizontalityHorizontality

Layered strata are

deposited horizontal or

nearly horizontal or nearly

parallel to the Earth’s

surface.

Principles of original Principles of original horizontality and superpositionhorizontality and superposition

Law of Cross-Cutting Law of Cross-Cutting RelationshipsRelationships

• A rock or feature is younger than any rock or feature it cuts across.

Law of Law of Cross-cutting RelationshipsCross-cutting Relationships

LAW OF INCLUSIONSLAW OF INCLUSIONS

• Included rocks are older than surrounding rocks.

PRINCIPLE OF FAUNAL PRINCIPLE OF FAUNAL SUCCESSIONSUCCESSION

Principle of Faunal Succession - groups of fossil plants & animals have followed one another in a definite & discernable order so certain fossil assemblages characterize a specific time.

INDEX FOSSILS - fossils used to correlate a specific time periodBased on distinct preservable parts, lived a short time , in a specific environment with wide distribution - MICROFOSSILS

Ammonite Fossils Petrified Wood

Chip Clark Tom Bean

CORRELATIONCORRELATION

• Process used to tie separated strata together

• Based on matching physical or fossil features such as– Physical continuity - trace of rock unit– Similar rock types - marker beds, coal

seams, rare minerals, odd color– Fossils

Using Fossils to Correlate RocksUsing Fossils to Correlate Rocks

Correlating beds using Correlating beds using index fossilsindex fossils

South rim of the Grand Canyon

Generalized Generalized Stratigraphic Stratigraphic

Section of Rocks Section of Rocks Exposed in the Exposed in the

Grand CanyonGrand Canyon

after: Beus & Moral (1990)after: Beus & Moral (1990)

Some of the Geologic Units Exposed in the Grand Canyon

Michael Collier

UnconformityUnconformityA buried surface of erosion

Separates much older, eroded strata from younger ones

Hiatus - the time gap or the time lost in the record

Unconformitites - 3 kindsUnconformitites - 3 kinds

• Disconformity - undeformed beds

• Nonconformity - sedimentary over igneous or metamorphic rx.

• Angular Unconformity - flat sediments overly tilted beds

Formation of a DisconformityFormation of a Disconformity

South rim of the Grand Canyon250 million years old250 million years old

550 million years old550 million years old1.7 billion years old1.7 billion years old

Paleozoic StrataPaleozoic Strata

PrecambrianPrecambrian

South rim of the Grand Canyon250 million years old250 million years old

550 million years old550 million years old

Nonconformity1.7 billion years old1.7 billion years old

Nonconformity in the Grand Canyon

Nonconformity in the Grand Canyon

Vishnu SchistVishnu Schist(~1700 million years old)(~1700 million years old)

Tapeats SandstoneTapeats Sandstone(~550 million years old)(~550 million years old)

Angular unconformity, Grand Canyon

The Great Unconformity of the Grand CanyonThe Great Unconformity of the Grand Canyon

Geoscience Features Picture Libraryc

Formation of an Formation of an Angular UnconformityAngular Unconformity

The Geologic Time ScaleThe Geologic Time Scale

• Divisions in the worldwide stratigraphic column based on variations in preserved fossils

• Built using a combination of stratigraphic relationships, cross-cutting relationships, and absolute (isotopic) ages

The The Geologic Geologic Column Column

and and Time ScaleTime Scale

dinosaurs in

plants, fish

animals with skeletons

dinosaurs out

ice ages

oldest Emperor seamount,

~80 Ma

Kaua‘i

Absolute geochronologyAbsolute geochronology

• Adds numbers to the stratigraphic column based on fossils.

• Based on the regular radioactive decay of some chemical elements.

Isotopic datingIsotopic dating• Radioactive elements (parents) decay to

nonradioactive (stable) elements (daughters).

• The rate at which this decay occurs is constant and knowable.

• Therefore, if we know the rate of decay and the amount present of parent and daughter, we can calculate how long this reaction has been proceeding.

IsotopesIsotopesDifferent forms of the same

element containing the same

number of protons, but varying

numbers of neutrons.

i.e.i.e.:

235235U, U, 238238UU 8787Sr, Sr, 8686SrSr 1414C, C, 1212CC

Naturally Occurring Naturally Occurring Isotopes of CarbonIsotopes of Carbon

Electron Capture

Beta Decay

Alpha Decay

Production and Decay Production and Decay of Radiocarbonof Radiocarbon

Radioactive Radioactive Decay of Decay of

Rubidium to Rubidium to StrontiumStrontium

Half-lifeHalf-life

The half-life of a radioactive isotope is defined as the time required for half of it to decay.

Proportion of Proportion of Parent Atoms Parent Atoms

Remaining as a Remaining as a Function of Function of

TimeTime

Geologically Useful Decay Geologically Useful Decay SchemesSchemes

Parent Daughter Half-life (years)235U 207Pb 4.5 x 109

238U 206Pb 0.71 x 109

40K 40Ar 1.25 x 109

87Rb 87Sr 47 x 109

14C 14N 5730

PROBLEMSPROBLEMS

• NEED A CLOSED SYSTEM!!!

– MINERAL MAY LEAK PARENT OR DAUGHTER

– MINERAL MAY BE CONTAMINATED WITH EITHER PARENT OR DAUGHTER

Another Clock Another Clock PaleomagnetismPaleomagnetism

• Earth’s magnetic field reverses every half million years

• Reversals are recorded in rocks that are forming at that time - seafloor

• Time scale calibrated by both relative & absolute time methods

Earth’s Magnetic

Field

Lavas Lavas record record

magnetic magnetic reversalsreversals

magnetically polarized layers in a volcano