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Unformatted text preview: Earth History
Chapter 17: Geologic Time Early scientific methods of dating
• Sedimentary rock thickness
– 1 million to 2 billion years • Ocean salinity
GEOL 106 – 90 million years • Heat loss of cooling earth
– 75,000 years James Hutton and Uniformitarianism
•The present is the key to the past. •Charles Lyell, Principles of Geology.
•(1830) How are rocks dated today?
• Relative Dating • Absolute Dating 1 Geologic Ages
• Age of the Earth
– 4.6 billion years How are relative ages determined?
• Principle of Uniformitarianism
– Geologic processes occur today operated similarly in the past
Mudcracks today Mudcracks in the past • Age of the oldest rocks on Earth
– 4.03 billion years
• NW Territories in Canada • Age of the oldest fossil on Earth
– 3.77 billion years
• NW Australia How are relative ages determined?
• Principles of stratigraphy • Stratigraphy = study of strata (layers) in the Earth’s crust
– The branch of geology that deals with the origin, composition, distribution and succession of sedimentary rocks. How are relative ages determined?
• • • • • • Principle of superposition Principle of original horizontality Principle of lateral continuity Principle of cross-cutting relationships Principle of inclusions Principle of fossil succession 2 Determining Relative Ages
• Principle of Original Horizontality
– Sediments and extrusive igneous rocks are deposited in horizontal beds Determining Relative Ages
• Principle of Superposition
– Younger rocks are on top of older rocks Determining Relative Ages
• Principle of crosscutting relationships
– Intrusions and faults are younger than the intruded or faulted rocks Baked contact 3 Determining Relative Ages • Principle of Fossil Succession
• • • • Life has varied through time Fossil assemblages are easy to distinguish Relative age can be determined (superposition) Correlation Principle of inclusions: inclusions are older than including rock Relative Age Dating
– Surfaces which represent geologic time not represented in the rock record – Hiatus: Interval of time not represented – Types
• Disconformity: Surface of erosion or nondeposition between beds which are parallel to one another Unconformities
Disconformity: Gap in the rock record between 2 parallel layers of sedimentary rock 4 Relative age dating
• Angular Unconformity: Erosional surface on tilted or folded strata over which younger strata are deposited. RELATIVE AGE DATING
• Noncomformity: Erosional surface cut into igneous or metamorphic rocks, overlain by sedimentary rocks. ? ? granite RELATIVE AGE DATING
• Stratigraphic correlation
– Tracing rock units of the same age to different areas – Use surface correlation, well logs, fossils, etc. – Key beds (coal, ash) – Index fossils Stratigraphic correlation 5 ...
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- Spring '08