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CHAPTER 18 VOLCANOES I. MAGMA A. Formation 1. Temperature increases with depth 2. Increased pressure increases melting point 3. Increased water content decreases melting point B. Types of Magma 1. Basaltic – rich in iron & magnesium, melts around 1000o C. Quiet eruptions Oceanic crust 2. Rhyolitic – high silica content; high water and gas content; explosive! Continental crust 3. Andesitic – mixture of basaltic & rhyolitic, found along continental margins C. Viscosity 1. Definition – resistance to flow 2. Basaltic=low viscosity (runny) 3. Rhyolitic=high viscosity (does NOT flow well) II. VOLCANOES A. Anatomy of a volcano 1. 2. 3. Vent- opening in crust; magma leaves earth and becomes lava Crater- as lava accumulates, mountain forms. The bowl-shaped depression at the top is the crater Caldera- once the magma chamber empties out, the mountain may collapse. B. Volcanic Landforms 1. Shield a. broad, gently sloping sides b. basaltic lava c. Example; Mauna Kea in Hawaii 2. Cinder a. steep sides, short b. rhyolitic lava; very explosive c. Example; Izalco in El Salvador 3. Composite a. steep top, gently sloping base b. andesitic lava c. Example; Mt. St Helens