Crystal CozartESC1000October 19, 2009Ch. 4 Homework1.) Scientists measure geologic time in two ways. Relative age measurement refers only to the order in which events occurred. Determination of relative age is based on a simple principle: In order for an event to affect a rock, the rock must exist first. Thus, the rock must be older than the event. This principle seems obvious, yet it is the basis of much geological work. Absolute age is age in years. Absolute age tells us both the order in which events occurred and the amount of time that passed since they occurred. 3.) The principle of superposition states that sedimentary rocks become younger from bottom to top (as long as tectonic forces have not turned them upside down or thrust an older layer over a younger one). This is because younger layers of sediment always accumulate on top of older layers. If the rocks formed over a long time, different fossils appear and then vanish from bottom to top in the same order in which the organisms evolved and then became extinct.
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