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Grade 9 Academic Geography - Strand 3 Lesson 5 Earth Formation About 4.6 MYA, the Earth formed from cosmic debris Cut in half, the Earth has four distinct parts: Crust Mantle Outer Core Inner Core Crust Crust is the outermost layer It is between 6 and 100 kilometres thick. The thicker layers are typically mountains, while the thinner areas are located at the bottom of the ocean. The crust constantly changes due to erosion, deposition, glacial action and plate tectonics. There are two types of crust. Continental - Granite-like rock rich in silicon, oxygen and aluminum. Oceanic - Igneous, basaltic rock containing greater amounts of heavier elements such as magnesium and iron Lithosphere The lithosphere is composed of the crust and the upper part of the mantle. The lithosphere is divided into several layers of slow moving plates. New crustal rock forms at the “diverging” lithospheric plate boundaries. Old crustal rock melts at a subduction zone (...area where one plate plunges under another plate). A subducted plate moves down to an environment of higher temperature and pressure where the rocks melt into magma. The magma is released by volcanoes. A plate can consist of both crust types.