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Igneous Rocks

Igneous Rocks. Igneous rocks – formed from the crystallization of magma Two main types – extrusive and intrusive Extrusive – fine-grained igneous rocks that are cool quickly (ex. – rhyolite) Intrusive – coarse-grained igneous rocks that are cooled quickly (ex. – granite).

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Igneous Rocks

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  1. Igneous Rocks • Igneous rocks – formed from the crystallization of magma • Two main types – extrusive and intrusive • Extrusive – fine-grained igneous rocks that are cool quickly (ex. – rhyolite) • Intrusive – coarse-grained igneous rocks that are cooled quickly (ex. – granite)

  2. Composition of magma • Magma is composed of: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), potassium (K), and sodium (Na) • Silica (SiO2) is the most abundant compound in magma

  3. Where does magma come from? • Magma (“melted rock”) from the upper mantle and lower crust, which is believed to be heated from by remaining energy from Earth’s molten formation and heat generated from decay of radioactive elements

  4. Factors that affect magma formation • Main factors that affect magma formation are: temperature, pressure, water content, and mineral composition • Temperature increases with depth of crust • Pressure increase with depth crust

  5. Factors that affect magma formation • Increased water content decreases melting temperature • Mineral composition - different minerals have different melting points

  6. How do rocks melt? • Because rocks are made up of different minerals and minerals have different melting points, not all parts of a rock melt at the same time • Partial melting – process whereby some minerals melt at low temperatures while other minerals remain solid • As each group of minerals melts they get mixed into the magma and change its composition

  7. How do rocks form? • When magma cools, it crystallizes in the reverse order of partial melting – first minerals to crystallize are first ones that melted • Fractional crystallization – process wherein different minerals form at different temperatures – again this changes magma composition but in reverse to partial melting

  8. - illustrates relationship between cooling magma and mineral formation Bowen’s Reaction Series

  9. Quartz

  10. Layered intrusions • In some magma the minerals form into distinct bands in the order shown in Bowen’s reaction series – this is called a layered intrusion • Layered intrusions are good sources of rare metals such as nickel and gold

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