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Notes 17 – Volcanoes Mrs. Gordon Earth Science Tuesday, May 23, 2017 1. What is a volcano? • A. Volcano – a weak spot in Earth’s crust where magma can rise to the surface. Mount Etna in Sicily, Italy. Lava pools at Hawaii Volcanoes National Park. 10,200-foot Redoubt Volcano about 100 miles southwest of Anchorage B. Explosiveness: – Silica (quartz) content – • low silica = quiet eruptions = fluid and runny lava, LOW viscosity like water • high in silica = explosive eruptions = thick and pasty lava, HIGH viscosity like honey C. Volcanic Activity Terms: ~ Pyroclastic flow – Massive cloud of HOT ash, rocks, and gases ~ Lahar – Gigantic mud flow ~ Caldera – depression formed when a volcanic summit collapses into a partially emptied magma chamber. (Yellowstone, Crater Lake) 2. Where do volcanoes occur? • Most occur near plate boundaries. ~ Ring of Fire – a major volcanic belt that rims the Pacific Ocean. • A. Divergent boundaries - (ocean floor, Iceland) - two plates ripping apart and magma squeezes up / between Eruption in April 2010 of Iceland’s volcano Eyjafjallajokull. Laki is a volcanic system, belonging to Katla the central volcano in Iceland, and was at the origin of the biggest volcanic eruption on earth in historical times (1783), in the form of a flood basalt. • B. Convergent boundaries - (Cascades = Mt. St. Helens) - one plate is pushed under another, melts, and through convection, magma rises up. Picture by US Geological Survey scientist, Austin Post, on May 18, 1980. • C. Hot spots - (Hawaii, Yellowstone) – a large concentration of heat in one area, magma burns through the crust. 3. Types of Volcanoes: • Fissure Eruption Attach Diagram across from Notes 17 A. Shield Volcano – low silica = quiet eruptions Galapagos shield volcano B. Cinder Cone C. Composite Volcano (stratovolcano) – high silica content = violent eruptions • D. Fissure Eruption – very low silica – The greatest volumes of volcanic material come from fissure eruptions. Most occur on the ocean floor. Ex: Snake River flood basalts, along the Columbia River, seafloor spreading.