Relative Ages of Rocks Relative Dating * early geologists tried to determine relative age of earth * relative dating = placing rocks in their correct sequence of formation * relative dating tells whether one rock is older than another but not how old each is Relative dating just puts these in order from oldest to youngest but does not give a numerical age for any of them Rules and Principles of Relative Dating Number One: Law of Superposition - The oldest rocks in undisturbed sedimentary rock layers are on the bottom - Sediment is carried by water or wind. When the wind or water is not moving fast enough, the sediment falls out in even horizontal layers. So which of these layers is the oldest? Does this still work if your rocks are tilted or if they are volcanic? Rules and Principles of Relative Dating Number Two: Principle of Original Horizontality - sedimentary rocks are layed down horizontally * However, over time, they can tilt as plates collide or diverge. So if your rocks aren't horizontal, they must have been moved by plate forces. Example of Principle of Original Horizontality Rules and Principles of Relative Dating Number Three: Principle of crosscutting relationships - younger features (fault lines or igneous rocks) cut through older features - basically this says that you can't have an earthquake crack a rock that did not exist yet Examples of Cross-Cutting Features (Dikes) Which is older, the dike or the sedimentary layers? When a fault goes through a rock layer, it shifts one side up or down (depending on the type of fault) A is a dike (igneous intrusion) and B is a fault line. Put the following events in order from oldest to youngest: Dike A, Fault B, Layer D, Layer C The correct order from oldest to youngest is Layer D, Layer C, Fault B, Dike A Unconformities Unconformity– long period when deposition stops, then erodes, and then deposition starts again - (Basically a time break in the rock record) where a layer is missing - These develop when running water or glaciers wash or wear away rock. Unconformities There are 3 main types of unconformities. One of them is very difficult to identify in the field. Type 1: Angular Unconformity: tilted rocks overlain by flat strata (beds); tops of tilted rocks eroded * These are pretty easy to identify. Look for tilted rocks with flat rocks on top of them The yellow line is the angular unconformity. Unconformities Type Two: Disconformity: - not easily identifiable - (usually shown with a wavy line) - deposition ceased then started again much later - this gives a large age gap between 2 layers - Geologists often need to use absolute dating to find out if there is a disconformity Sediments deposited on the sea bottom, plates moved the sea bottom up, deposition stopped, erosion began then land sank and sediments accumulated again forming a disconformity Unconformities Type Three: Nonconformity: - new (igneous) rocks intrude from below and displace older one - These can include the following igneous intrusions: A. Dike: ~Vertical intrusion of magma B. Sill: ~ horizontal intrusion of magma C. Batholith: Very large magma feature underground that never reaches surface Putting it all together: Put these in order from oldest to youngest. Disconformity A, Dike B, Layer C, Layer D, Layer E, Fault F The correct order from oldest to youngest is: Layer E (E is older than D because it is above it), Layer D, Fault F (the fault is younger than A because it does not go through it), Disconformity A, Layer C, Dike B (B is youngest because it goes through all other layers)