IGNEOUS ROCKS     I. IGNEOUS ROCKS A. Oceanic crust 1. Ferromags 2. Fairly dense (~3.3 gm/ml) 3. 0-10 Km thick B. Continental crust 1. Nonferromags 2. Density (2.7 gm/ml) 3. 0-70 Km thick (35 Km average) II. PLATE TECTONIC SYSTEM A. Lithosphere--rigid, rocky portion of the crust and uppermost mantle B. Asthenosphere--relatively soft, flowing zone beneath the lithosphere 1. Plastic Convection--Mechanism by which material moves due to differences in density, usually due to heat 2. Magmas are generated at relatively shallow levels in the Earth's crust and mantle 3. Magmas sources are active plate tectonic regions a. Mid-ocean ridges--oceanic crust is pulled apart e.g. 3 Km high, Mid-Atlantic ridge b. Continental rifts--continental crust is pulled apart, forming a huge valley called a rift valley e.g. East African Rift System c. Subduction zones--one tectonic plate is subducted, or dives, beneath another e.g. west coast of South America, Andes Mountains d. Hot spots--a localized zone of melting beneath the lithosphere 4. Sources of heat a. Frictional heat--along subduction zone boundaries e.g. Andes 6 cm/year b. Radioactive decay--spontaneous decay of radioactive elements produces heat, e.g. K, U, Th c. Latent heat--heat from formation of Earth Geothermal gradient--rate at which heat increases with depth, oC/Km. Average continental ~ 30oC/Km 5. How lavas reach the Earth's surface a. Crack propagation--forces are concentrated along the tips of fractures - Density = mass / volume - Mach 2 dikes - Diatremes b. Stoping--process by which large pieces of the roof of a magma chamber drop off into the chamber c. Assimilation--country rock reacts chemically with the magma xenolith--alien blocks of country rock within a magma chamber