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Volcanoes Main topics to be covered: • Magma • Intrusive Activity • Volcanoes Magma How does magma form? • Magma forms when temperatures are high enough to melt rock. • Rocks usually melt between 800°C and 1200°C • Pressure can also effect the formation of magma…greater pressure requires higher temperatures to melt rock. (p. 472 Fig. 18-1) • Presence of water allows rock to melt at a lower temperatures! Types of Magma • Basaltic Magma • Andesitic magma • Rhyolitic magma Do these magma types have familiar names? What makes these magma types different? Magma Types What makes magma different? 1. Viscosity – resistance to flow. (temp + comp) 2. Gas content – percent of trapped gasses in the magma. 3. Silica content – percent of silica in magma. 4. Explosiveness – how explosive the magma is. Magma Types viscosity Gas content Silica explosiveness Location of content magma Basaltic magma Andesitic magma low 1-2% ~50% least Oceanic and continental crust Intermediate 3-4% ~60% intermediate Subduction zone at continential margins Rhyolitic magma high ~70% greatest Continential crust 4-6% Intrusive Activity • Magma will rise upward in the Earth, coming in contact with or intruding into the overlying crust. Why does magma rise? Because it is less dense than surrounding rock! • Magma can cool inside the Earth forming plutons. • Plutons are large areas of coarse-grained igneous rock. Types of Plutons • Batholiths – largest type of pluton, irregular shaped mass of coarse-grained igneous rock. (greater than 100 km2) • Stocks – similar to batholiths, but smaller. • Laccoliths – mushroomed shaped pluton, cause by rocks bowing upward due to heat and pressure. (much smaller) • Sill – pluton that forms when magma intrudes into parallel layers of rock. (Palisades Sill) • Dike – pluton that cuts across existing rock. Intrusive Activity Diagram Palisades Sill Anatomy of a Volcano Vent – opening in the crust that allows magma to come out on the surface. Crater – bowl-shaped depression that connects the magma chamber to the vent, usually near the top of the volcano. Magma chamber – large pocket of magma below the surface. Caldera – large depressions that are formed when the summit of a volcano collapses into the magma chamber. (Figure 18-11) 3-Main Types of Volcanoes 1. Shield Volcano – a volcano with broad, gently sloping sides and a nearly circular base. 2. Cinder-Cone Volcano – Volcano with steep sides as a result of material being ejected and being piled up around the vent. 3. Composite Volcano – volcano with alternating layers of lava and volcanic fragments. •Numerous layers of basaltic lava build up. •Low viscosity and less explosive…lava flows for great distances. • Contains andesitic lava • Magma is more viscous and contains more water and gasses. • More explosive than shield volcanoes. •Contains mostly rhyolitic lava. •Lava contains large amounts of silica, water and gasses. •Violently explosive! Volcanic Material Tephra – rock fragments thrown into the air by a volcano, classified by size below. dust <0.25 mm dia. smallest ash 0.26-2.00 mm dia. larger lapilli 2.01-64.00 mm dia. larger blocks >64 mm dia. bombs >64 mm dia. biggest (angular) Biggest (rounded) Pyroclastic Flows • Hot material that is ejected during violent eruptions. • Contain poisonous gasses, dust, ash, and other material that move down hill at amazing speeds. • Can reach speeds of 200 km/h • Can reach temperatures of greater than 700°C Locations of Volcanoes • • • • Mostly occur at plate boundaries. 80% occur at convergent boundaries. 15% at divergent boundaries. ~5% at other locations not at boundaries, such as hot spots. • Most of Earth volcanoes occur in 2 main belts: 1. Circum-Pacific Belt 2. Mediterranean Belt. Hot Spots • Volcanoes that are far away from plate boundaries are usually over hot spots. • These are unusually hot area of the mantle, where molten material rises to the surface. • The heat melts rock of the crust, and the magma rises upward…forming volcanoes. • Hawaii is located over a mantle hot spot. • As the plates move over the hot spot, volcanoes are formed. http://www.geo.wvu.edu/~donovan/geol101/a nimations/55.swf Anatomy of a Volcano Video Clips:/www.cnn.com/interactive/nature/020 1/volcano/volcano.swf • http://dsc.discovery.com/convergence/pomp eii/videogallery/videogallery.html