Human earth shapers/ETHS101/Rock Record/Field trip

=Field Trip= Greg and Karin head into the field to practice their rock reading skills on a beach in Tasmania.

Summary

 * If we assume that the same processes operated in the past as now operate then we can read the rocks. This is the principle of uniformitarianism.


 * Sediments are laid down, one on top of the other in sub-horizontal layers.


 * In sedimentary rocks, the oldest rocks are at the bottom and get younger as you go up unless the rocks have later been turned over by folding or some other mechanism.


 * Fossils can be preserved in sedimentary rocks and, because they are the same age, they can help us work out how old the rocks are.


 * The size of grains in sedimentary rocks tell us about the environment in which they were laid down – small grains means that the sediments were laid down in calmer waters than large grains.


 * Some special features tell us more – like dropstones as evidence of icebergs (and therefore cold climates).


 * Features that cross-cut sedimentary rocks must have been formed after the sediments had turned to rock.


 * Rocks formed from magma (molten rock) often cross cut sedimentary rocks, and represent the intrusion of the magma in dykes through the sedimentary rocks.


 * The crystal size in igneous rocks (those formed from magma) tell us about conditions – small crystals indicate that the magma solidified quickly (for example basalt that solidified on or near the earth’s surface); large crystals occur in rocks that solidified deep in the earth.


 * This is just the beginning, if you look in more detail, then you can work out much more from rocks.