geologic time earth’s history. uniformitarianism the present is the key to the past processes that...
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Geologic Time
Earth’s History
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Uniformitarianism
The present is the key to the past
Processes that happen today are the same processes that happened in ancient times
Modern RipplesFossilized Ripples Modern MudcracksFossilized Mudcracks
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Dating – 2 types
• Relative Dating
• Absolute Dating
– Know order of events but not dates
–Know exact age
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Relative Dating – Key Concepts
• Law of Superposition
• Principle of Original Horizontality
• Principle of Cross-Cutting Relationships
• Principle of Inclusions
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Law of Superposition
Each bed is younger than the bed below it.
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Principle of Original Horizontality
Sedimentary layers are deposited horizontally
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Principle of Cross-Cutting Relationships
• Faults and intrusions are younger than the rocks they cut through
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Inclusions
• Fragments of one rock unit that have been enclosed within another are older
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Relative Dating
Youngest G A
BCFD
Oldest E
E is younger than D. Which Principle used?
Law of Superposition
G is younger than F. Which Principle is used?
Principle of Cross-Cutting Relationships
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Principle of Inclusion
Using the Principle of Inclusions, which picture shows the granite intrusion to be the youngest event?
A B
B
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Conformable Beds
• Beds that have not been disturbed
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Uncomformities
– Angular
– Disconformity
– Nonconformity
• Substantial break or gap in the rock record
• 3 types
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Angular Unconformity
• Older layers have been deformed, eroded, and younger layers deposited above
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Disconformity
• Beds are parallel relative to each other but there is a break in sequence
• Implies a cessation of sedimentation or erosion, but no tilting
• Hard to recognize
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Nonconformity
Sedimentary layers overlie igneous or metamorphic layers
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Unconformities
1. Surface 1 is an example of ? Nonconformity2. Surface 2 is an example of ? Angular unconformity3. Surface 3, 4 and 5? Disconformity4. Granite is _________ than the Schist. Younger
Question: Answer:
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Correlating Rocks
Matching rocks of the same age in different locations
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How do we correlate?
Walking out the contact Matching fossils
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Fossils
The remains or traces of prehistoric life
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Types of Fossils
Body fossils– Bone, teeth, shells
Trace fossils– Record movement
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Fossilization
Hard parts– Unaltered
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Fossilization - Petrified
Turned into stone
Petrified Forest, Arizona
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Fossilization - Replacement
Minerals replace organic matter
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Fossilization – Molds & Casts
Mold – impression or cavity
Cast – deposited material that fills cavity
Mold Cast
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Fossilization - Amber
Hardened Resin
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Conditions favoring Preservation
Rapid burial Possession of hard parts
Dies Buried Eroded
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Fossils and Correlation
William Smith
– Each rock formation contained fossils unlike the ones above & below
– Sedimentary strata in widely separated areas could be identified & correlated by their fossil content
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Principle of Fossil Succession
Fossils occur in a definite, invariable sequence in the geologic record
– Apply with Law of Superposition and can document evolution of life through time
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Index Fossils
Widespread geographically Limited to a short span of geologic time
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Correlating Rocks & Fossils
Igneous rocks are dated radiometrically
Age in another area determined indirectly
Ages of fossils can be bracketed
Fossils in another area can be correlated containing same fossils
Doing this repeatedly the geologic time scale established
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Geologic Time Scale