STUVIA 2024/2025
GEOL 101 - Chapter 9: Geologic Time
The Importance of a Time Scale
Rocks record geologic and evolutionary changes throughout Earth's history
Without a time perspective, these events have very little meaning
Numerical Dates
Specify the number of years that have passed since an event occured
Brainpower
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Relative Dates
Place rocks in a sequence of formation
Principle of Superposition
In an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger
than the one below
This principle also applies to surface features like lava flows and beds of ash
Principle of Original Horizontality
Layers of sediment are generally deposited in a horizontal position
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Rock layers that are flat have not been disturbed
Principle of Lateral Continuity
Beds originate as continuous layers that extend in all directions until they eventually thin out or grade
into a different sediment type
Principle of Cross-Cutting Relationships
Younger features cut across older features
Inclusions
Fragments of one rock unit that are enclosed within another rock unit
The rock containing this is younger
Conformable
Layers of rock that have been deposited without interruptions
Unconformity
A break in the rock record produced by nondeposition and erosion of rock units
Angular Unconformity
Tilted rocks are overlain by flat-lying rocks
Disconformity
Sedimentary strata on either side of the unconformity are parallel
Nonconformity
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Sedimentary strata overlay metamorphic or igneous rocks
Fossils
Traces or remains of prehistoric life preserved in rock
Paleontology
The study of fossils
Permineralization
Mineral-rich groundwater flows through porous tissue and precipitates minerals
Molds and casts
A mold is created when a shell is buried and then dissolved by underground water
A cast is created when the hollow spaces of a mold are filled
Carbonization
Happens when an organism is buried, followed by compression, which squeezes out gases and
liquids leaving a thin film of carbon
Impressions
Remain in the rock when the carbon film is lost
Amber
The hardened resin of ancient trees
Trace Fossils
Indirect evidence of prehistoric life
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Conditions favoring preservation
Most organisms are not preserved
Rapid burial and the possession of hard parts increases the chances of preservation.
Correlation
Involves matching of rocks of similar ages from different regions
Provides a more comprehensive view off the rock record
Correlation Within Limited Areas
Often accomplished by noting the position of the bed in a sequence of strata
Involves matching of rocks of similar ages from different regions
To correlate over larger areas, fossils are used for correlation
Principle of Fossil Succession
States that fossils are arranged according to their age
Index Fossils
Widespread geographically and limited to a short period of geologic time
Fossil Assemblage
A group of fossils used to determine a rock's age
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