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Dating Rock Layers How to tell how old the layers of rock are. What is Age? Relative Age – The age compared to the ages of other rocks. (older than this rock, younger than that rock.) Absolute Age – The number of years since the rock formed. (150 million years old, 10 thousand years old.) What are the ways to tell RELATIVE AGE. What is the Law of Superposition? In horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it. Extrusion vs Intrusion EXTRUSION INTRUSION • When magma cools on the surface of the Earth it’s called an EXTRUSION. • When magma cools beneath the surface of the Earth it’s called an INTRUSION. • Extrusions are always YOUNGER than the rock it is sitting on. • Intrusions are always YOUNGER than the rocks around it. Extrusions? Intrusions? G First of all, What whichisrock this layer called? is the oldest? F EXTRUSION E D More rock layers are deposited on top of the extrusion. INTRUSION C B A Your Turn To Label In your notes, draw a similar structure and label the following: Intrusion Extrusion Youngest Oldest What is a Fault? • A BREAK in the Earth’s crust. • A fault is always YOUNGER than the rock it cuts through. This crack in the rock layers is called a fault. What is an Unconformity? • The surface where new rock layers meet a much older rock surface beneath them. • An unconformity shows where some rock layers have been lost because of erosion. Become a Geologist • Put the layers in order from oldest to youngest for the picture below: Become a Geologist • Put the layers in order from oldest to youngest for the picture below: