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Zircon (Zirconium Silicate) Extremely stable and resistant to change Occurs in small amounts in granite and felsic rock. Australian zircon may be residue from crust that no longer exists. Earth`s Early Heat Sources 1. Asteroid activity 1. During the Proterozoic and Archean Eons asteroids (silica and metal rich rocks) were hypothesized to have been consistently hitting Earth.These impacts transferred energy in the form of heat. Earth’s Early Heat Sources 2. Gravitational contraction 1. As more and more asteroids strike Earth, its overall mass starts to change and therefore Earth`s gravitational constant changed. This increase of compressional energy resulted in excessive thermal energy. Earth`s Early Heat Sources 3. Abundance of radioactive isotopes 1. 2. There were more radioactive isotopes during the Proterozoic and Archean Eons than there are now Radioactive decay produces heat as a by-product Try these questions Read section 22.1 and answer questions 2, 5 and 6 The Early Earth Earth’s Differentiation and formation of crust Earth`s asthenosphere differentiated over time into layers based on density Iron sank; oxygen and silicon floated up to the surface *THIS IS CALLED DIFFERENTIATION The Early Earth Early igneous crust was most probably basaltic in nature (low silica, low in dissolved gases) Crust was rapidly recycled during this time Upon the first crusts’ subduction, continental crust was created Primary basaltic crust brought water into the mantle during subduction and created low density continental crust. Early continental crust were called microcontinents The Early Earth The Precambrian Shield is an example of persistent Precambrian rock (craton: oldest and most stable part of a continent). In North America, most of this craton now exists in Canada, hence Canadian Shield The Early Earth At the end of the Proterozoic, (~1 bya) Earth’s first supercontinent formed (RODINIA) Try these questions: P. 627 Q: 1-6