William E. Ferguson. Geologic Time A major difference between geologists and most other scientists...
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Transcript of William E. Ferguson. Geologic Time A major difference between geologists and most other scientists...
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