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Lec 7 - W 4 Th (Week 5 Tues) - Geoltime - Lecture 7:The...

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1/25/2013 1 (DUWK 6FLHQFHV ±²³±$´ 5HVRXUFHVµ (QYLURQPHQW DQG 6XVWDLQDELOLW\ LQ D 0DWHULDO 6RFLHW\ Lecture 7:The context of geologic time The overwhelming majority of Earth’s history is recorded in BBBBBB . Note that written human history only extends back about 5,000 years. Geologic materials record 4.6 billion years of hi t ! Rocks as Time Machines history ! Knowing how specific rocks are formed is great, but they become even more useful when we can interpreted when they formed and the implications of their formation in the context of Earth as a whole. 7\SHV RI 7LPH Two concepts of time are used in geology: Relative time: based on the BBBBBBBB timing of geological events (i.e. the order in which events occurred) occurred). Absolute time: the determination of age in years before present, based on the decay of radioactive BBBBBBBB .
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1/25/2013 2 Relative time All events occur in a BBBBBBB , regardless of their duration or specific time of occurrence. In relative time, we view geologic events in terms of their relative age (did something occur before or after another event?) No specific dates involved :KDW GR ZH PDNH RI VRPHWKLQJ OLNH WKLV" /DZ RI 2ULJLQDO +RUL]RQWDOLW\ /DZ RI 6XSHUSRVLWLRQ 5HODWLYH 'DWLQJ DQG 6RPH /DZV /DZ RI &URVV·&XWWLQJ 5HODWLRQVKLSV /DZ RI )RVVLO 6XFFHVVLRQ 8QFRQIRUPLWLHV Original Horizontality Sediments and some bodies of extrusive igneous rocks (e.g. lava flows) are deposited as horizontal layers or strata . Based on this principle, the law of original horizontality states that strata are laid down horizontally. This is important because if strata are not BBBBBBBBBB , it means that they were disturbed later by tectonic forces.
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