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Volcanoes volcano --a vent in the surface of Earth, through which magma and gases are expelled. Volcanism: any activity that includes moving magma onto Earth’s surface. Mantle and crust melt to form liquid rock under Earth’s surface (magma). As magma rises, it enlarges because it melts surrounding rock and intrudes into other rocks, causing them to fall into the magma “body.” Magma that gets to the surface is called lava. When this magma/lava cools, it is called igneous rock, if it cools INSIDE the crust, it is called intrusive igneous rock or a pluton. Volcanoes are likely to form in: 1. Tectonic plate boundaries a. Convergent—plates move toward each other b. Divergent—plates move away from each other 2. Hot Spots—areas not on the edge of plates a. Mantle plumes rise/reach lithosphere, forms volcano b. Plate continues to move, volcano goes with it and becomes inactive, new one forms c. Hawaii was formed this way---by a hot spot. 3. Mid Ocean Ridges—forms from divergent plates a. Volcanoes on these ridges contain pillow lava (lava rapidly cooled by water) i. Iceland is split in ½ between two tectonic plates. Fissures exist in the middle of Iceland where lava flows to the surface. 4. Subduction zones—one plate moves under another a. Oceanic plate moves under continental plate b. Island arc—oceanic lithosphere subducts under oceanic lithosphere. Most “major” zone is the Pacific Ring of Fire Volcanic eruptions can be signaled by a change in EQ activity and are either: a) quiet: produced by mafic magma, runny with low viscosity and low volume of trapped gases (ex. Hawaiian volcanoes) b) explosive: produced by felsic magma, a sticky lava with a high viscosity and a high volume of trapped gases (ex. Mt. St. Helens in Washington) Fragments of rock that form from a volcanic eruption are called pyroclastics. a) dust b) ash c) bombs-red hot lava cools as it spins in the air after ejection from the dome. d) lapilli: rocks less than 64 mm diameter. e) blocks: can be as large as a house Types of volcanoes: 1. shield: wide area…from quiet eruptions. 2. cinder: small area…from explosive eruptions. 3. composite: alternating layers from explosive and quiet eruptions. A caldera is a depression that forms when the top of the volcanic cone collapses inward. Newberry Caldera, Oregon