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USING THE ROCK RECORD TO INTERPRET GEOLOGIC HISTORY OBJECTIVE: To use rock layers to interpret the geologic history of an area. Research: Relative time: the order in which events occurred without actual dates. Principle of superposition: if a series of sedimentary rock layers has not been disturbed, the oldest layer is always on the bottom and the youngest layer on top. Fault: break or crack through rock layers along which rocks move. (Because faults can occur only after rock layers are formed, rock layers are always older than the faults they contain) unconformity: when a rock layer or part of one is missing from the layers. An unconformity means either the sediment was not being deposited or that rock was eroded away. intrusion: when magma forces its way into layers of rock. Magma creates igneous rock. (Intrusions are always younger than the sedimentary rock layers it passes through) extrusion: igneous rock that forms on earth’s surface (lava) (Extrusions are younger than the rock layers beneath them)
