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1. UNIFORMITARIANISM: -The processes that shape the Earth today are the same processes that occurred in the geologic past “The present is the key to the past” 2. ORIGINAL HORIZONTALITY -Sedimentary rock layers are ALWAYS laid down in horizontal layers, until some other process alters them Original Horizontality sedimentary rocks form in horizontal layers parallel to Earth’s surface and will remain that way unless disturbed SILT SAND CLAY SILT CLAY Horizontal Layers of the Grand Canyon 3. SUPERPOSITION -The bottom layer of horizontal sedimentary rock layers is always the oldest, UNLESS the layers have been overturned or have had older rock formed on top of it *The lower layers must first be in place before the next layer can be deposited YOUNGEST OLDEST For undisturbed rocks, the oldest layer is on the bottom and the youngest is on top Supai is oldest – WHY? Law of Superposition In undisturbedsedimentary sedimentary rock or lava flows flows: In undisturbed rock or lava Top = youngest layer Bottom = oldest layer Unconformity Gaps or missing layers in the rock record due to erosio Angular unconformity 1 2 3 Extrusion: When molten rock flows on the Earth’s surface forming an igneous rock Intrusion: Occurs when magma squeezes between layers of pre-existing rock beneath the Earth’s surface -Causes surrounding layers to metamorphose (Contact metamorphism) Igneous intrusions/extrusions that “cut across” or enter other rocks are younger than the rocks they cross Law of Cross-Cutting Relationships Any feature that cuts across a rock or layer of sediment is younger than the rock or layer it cuts Example of CrossCutting Intrusion Intrusion- igneous - igneousrock rockthat thatforms formswhen whenmagma magma squeezes squeezesbetween betweenexisting existingrock rockand andhardens hardens Examples of Cross-Cutting Example of Cross-cutting (extrusive) Extrusion - igneous rock that forms when lava flows on Earth’s surface and hardens North shore of Lake Superior, Ontario Canada A dark-colored dike has intruded into older light colored granite. • The dike is younger than the granite. Templin Highway, Castaic, California A small fault displaces tilted beds. • The fault is younger than the beds. Key to Rock Types Fossils of different Plants and Animals are found in sedimentary rock layers. When different fossils are found in the same layer, they existed during the same time period. Rock particles/fragments that are found in rock layers are older than the rock layer it is found in. Igneous intrusions that “cut across” or enter other rocks are younger than the rocks they cross. FOLDS AND FAULTS Fold: Folds and Faults are always younger than the original rock layers -bends in rock layers produced by movements of the earth’s crust Faults: -breaks in the rock where shifting of rock layers has occurred, often associated with earthquakes Principles of Geology: Big Ideas (you should know) 7 Laws of Geology Relative aging of rock using Superposition Definitions of intrusion, geologic dike, unconformity Matching rock layers using fossils Definitions: folding, fault, uplifting