1. The Geologist as Detective Chapter 6 : Interpreting Geologic History

2. How does a Geologist tell how old a rock is? They look for clues in the rocks They follow a set of rules or principles that helps them determine the sequence of events leading to the formation of a particular rock outcrop Think of it as rock detective work!!!

3. Fundamental Principles of Geologic History Uniformitarianism Superposition Original Horizontality or Cross Cutting

4. Uniformitarianism The idea that the processes that shaped the Earth today are the same processes that occurred in the geologic past “ The Present is the key to the past ”.

5. Superposition States the oldest rock layer is on the bottom, the youngest on top This occurs in an undisturbed sequence of rock layers only

6. Original Horizontality or Cross Cutting States the rock is always older than the processes that changed it. Rocks are laid down in horizontal layers. After rocks form, they fold or fault. After rocks form, magma will well up between the layers or onto the surface (igneous intrusions or extrusions).

7. Sequencing Events in Rocks: Terms to use Unconformity: a buried erosional surface or gap in rock record Emergence/uplift Submergence/sinking Deposition Tilting Faulting Intrusion/extrusion Melting and solidification

8. Keep the three basic principles in mind when deciding the order of events in rock formation!!!