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RELATIVE DATING: WHICH CAME FIRST? » » RELATIVE DATING is any method of determining whether an object or event is older or younger than other objects and events. PRINCIPLE OF SUPERPOSITION states that younger rocks lie above older rocks in an undisturbed sequence. http://comp.uark.edu/~sboss/geol1113study04s09.htm GEOLOGIC COLUMN » » » Is an IDEAL sequence of rock layers that contains all of the rock layers and fossils on Earth arranged from oldest to youngest. This information comes from geologic data from all around the world. Is used to interpret rock sequences and identify layers in puzzling rock sequences. ROCK LAYERS » » » » Are arranged from oldest to youngest Are in horizontal layers HOWEVER, sometimes rock layers are disturbed by processes within the Earth Geologists use the relationships between rock layers and the disturbance to determine the relative age of the rock layers and the events that disturbed them. DISTURBED ROCK LAYERS » There are four types of disturbances: Faults - breaks in the Earth’s crust. Intrusions - molten rock flows into existing rock. Folding - layers bend due to pressure. » Tilting- internal forces slant the rock layer. » » » IN WHAT ORDER DID THE ROCK LAYERS FORM? WHICH CAME FIRST, EARTHQUAKE OR INTRUSION? UNCONFORMITIES » » » » Are gaps in the geologic column. Geologists find that certain rock layers you expect to be present in a certain place are not there. Erosion: when an area is worn down by water, wind, or other elements. Erosion and nondeposition are the major causes of unconformities. TYPES OF UNCONFORMITIES » Disconformities- part of a parallel rock layer is missing » » » Disconformities are the most common type of unconformity! Nonconformities- sedimentary rock layers lie on top of an eroded igneous surface. Angular unconformities-exists between horizontal rock layers and layers that are tilted and folded. Types of Unconformities Animation