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Geologic Time
Earth’s History
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
Dating – 2 types
• Relative Dating
• Absolute Dating
– Know order of events but not dates
–Know exact age
Relative Dating – Key Concepts
• Law of Superposition
• Principle of Original Horizontality
• Principle of Cross-Cutting Relationships
• Principle of Inclusions
Law of Superposition
Each bed is younger than the bed below it.
Principle of Original Horizontality
Sedimentary layers are deposited horizontally
Principle of Cross-Cutting Relationships
• Faults and intrusions are younger than the rocks they cut through
Inclusions
• Fragments of one rock unit that have been enclosed within another are older
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
Principle of Inclusion
Using the Principle of Inclusions, which picture shows the granite intrusion to be the youngest event?
A B
B
Conformable Beds
• Beds that have not been disturbed
Uncomformities
– Angular
– Disconformity
– Nonconformity
• Substantial break or gap in the rock record
• 3 types
Angular Unconformity
• Older layers have been deformed, eroded, and younger layers deposited above
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
Nonconformity
Sedimentary layers overlie igneous or metamorphic layers
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:
Correlating Rocks
Matching rocks of the same age in different locations
How do we correlate?
Walking out the contact Matching fossils
Fossils
The remains or traces of prehistoric life
Types of Fossils
Body fossils– Bone, teeth, shells
Trace fossils– Record movement
Fossilization
Hard parts– Unaltered
Fossilization - Petrified
Turned into stone
Petrified Forest, Arizona
Fossilization - Replacement
Minerals replace organic matter
Fossilization – Molds & Casts
Mold – impression or cavity
Cast – deposited material that fills cavity
Mold Cast
Fossilization - Amber
Hardened Resin
Conditions favoring Preservation
Rapid burial Possession of hard parts
Dies Buried Eroded
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
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
Index Fossils
Widespread geographically Limited to a short span of geologic time
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
Geologic Time Scale