Geologic time
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Transcript of Geologic time
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Geologic timeGeologic time
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Geologic Time
Geologic time scaleRelative dating principlesAbsolute dating w/ radioactivity Correlation of rock layers
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• The relative geologic time scale has a sequence of eons eras periods epochs – but no numbers
indicating how long ago each of these times occurred
Relative Geologic Time Scale
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Geologic time scale
Eons
Phanerozoic - last 570 million years
Cenozoic - recent life
Mesozoic - age of middle life
Paleozoic - ancient life
Precambrian - from birth of Earth up to before complex life forms developed
Eras
0-66.4 m.y.a
66.4-245 m.y.a
245-570 m.y.a
(mammals, humans)(mammals, humans)
(dinosaurs, 1st scrawny mammals)(dinosaurs, 1st scrawny mammals)
(fish, trilobites, clams, corals, ferns )(fish, trilobites, clams, corals, ferns )
(algae, bacteria, some fossils without shells like jellyfish)(algae, bacteria, some fossils without shells like jellyfish)
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“Telling time geologically”
Earth’s history concealed in rocks
Goal of geology: unraveling Earth’s history
Principle time keeping devices:
Relative dating - putting rocks/events in proper order
Absolute dating - determining event’s actual time
Relative Dating: Key principles
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Relative Dating: Key principles
Law of superpositionLaw of superposition
Principle of original horizontalityPrinciple of original horizontality
Principle of cross-cutting relationshipsPrinciple of cross-cutting relationships
InclusionsInclusions
UnconformitiesUnconformities
A bed of rock is older than that above, younger than that belowA bed of rock is older than that above, younger than that below
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Law of superpositionLaw of superposition
Principle of original horizontalityPrinciple of original horizontality
Principle of cross-cutting relationshipsPrinciple of cross-cutting relationships
InclusionsInclusions
UnconformitiesUnconformities
A bed of rock is older than that above, younger than that belowA bed of rock is older than that above, younger than that below
Relative Dating: Key principles
Sedimentary layers deposited flatSedimentary layers deposited flat
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Law of superpositionLaw of superposition
Principle of original horizontalityPrinciple of original horizontality
Principle of cross-cutting relationshipsPrinciple of cross-cutting relationships
InclusionsInclusions
UnconformitiesUnconformities
A bed of rock is older than that above, younger than that belowA bed of rock is older than that above, younger than that below
Sedimentary layers deposited flatSedimentary layers deposited flat
Faults & dikes are younger than beds they crossFaults & dikes are younger than beds they cross
Relative Dating: Key principles
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InclusionsInclusions
UnconformitiesUnconformities
Fragments of one rock enclosed in anotherFragments of one rock enclosed in another
Relative Dating: Key principles
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Fragments of one rock Fragments of one rock enclosed in anotherenclosed in another
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InclusionsInclusions
UnconformitiesUnconformities
Fragments of one rock enclosed in anotherFragments of one rock enclosed in another
Break in time in depositionBreak in time in deposition
(Conformable layers: deposited w/out interruption)
Relative Dating: Key principles
• An unconformity represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed.
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Relative Dating: Key principles UnconformitiesUnconformities
3 types Angular unconformity
Disconformity
Nonconformity
• A disconformity is when two sedimentary rock layers are separated by an erosional surface.
• A nonconformity is when the erosional surface separates older metamorphic or intrusive igneous rocks from younger sedimentary rocks.
• An angular unconformity indicates that during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred.
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Relative Dating: Key principles UnconformitiesUnconformities
3 types
Angular unconformity
Disconformity
Nonconformity
Tilted/folded sedimentary rocks overlain by younger, more flat layers
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Deposition
Folding/Uplift
Erosion
Subsidence/more erosion
Recipe for an angular unconformity
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Relative Dating: Key principles UnconformitiesUnconformities
3 types
Angular unconformity
Disconformity
Nonconformity
Layers on either side of unconformity are parallel
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Relative Dating: Key principles UnconformitiesUnconformities
3 types
Angular unconformity
Disconformity
NonconformityInterface between sedimentary layers and metamorphic or igneous rock
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Unconformity Types
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Relative Dating Principles
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Correlation of rock layers
Matching up rocks from different places that are similar in age
Physical criteria
Fossils
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Correlation of rock layers
Physical criteria (OK for short distances)
position of a bed in sequence of beds
distinct/uncommon minerals
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Correlation of rock layers
Fossils
fossil organisms succeed each other in systematic fashion
any time period can be determined by fossil content
“principle of fossil succession”
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Correlation of rock layers
“Index fossils”
geographically wide-spread
short span of time
T&L Figure 8.10
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Overlapping Ranges of Fossils
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corr
elat
ed s
eque
nce
Mostlynonmarinelimestone
1Kansas
2Indiana
3Ohio
4Pennsylvania
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Absolute dating w/ radioactivity
Atommodel
Chemistry review
nucleusele
ctro
ns
protronsneutrons
# protrons = atomic #,defines the element
# neutrons can vary:“isotopes”
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Absolute dating w/ radioactivity
Parent DaughterNow 1000 0 Next year 500 500 2 yrs from now 250 750 3 yrs from now 125 875 10 yrs from now 1 999
Example: an isotope with a half life of 1 year
# of atoms
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Radioactive Decay
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Absolute dating w/ radioactivity