106Chap17a-time - Earth History Chapter 17: Geologic Time...

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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 • Unconformities – 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 • Unconformities – Types • Angular Unconformity: Erosional surface on tilted or folded strata over which younger strata are deposited. RELATIVE AGE DATING • Unconformities – Types • 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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