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How and Where Volcanoes Form Quick Review • What is magma? • Why does magma rise? • At what kind of plate tectonic boundary/ boundaries do we find volcanoes? • Why do volcanoes form here? Definition of Volcano • A. Volcano: Both an opening in Earth’s crust through which molten rock, gases and ash erupt and the landform that develops around this opening B. Magma Formation 1. Decreasing pressure can lower the melting temperatures of materials in the asthenosphere (ex. Rift valley/mid-ocean ridge) 2. Increasing temperatures can cause materials in the asthenosphere to melt. (ex. Hot spot) 3. Increasing water in the asthenosphere can lower melting temperatures (ex. Subduction boundaries) C. Rise to the Top 1. Magma tends to rise to the surface because it is less dense than the solid material surrounding it 2. The traits of the magma and the rate at which is rises depends on the amount of silica 3. Higher silica, thicker magma, more explosive Volcano Location Reason Volcano Formation Special Notes Examples Row 1 Volcano Location Subduction Boundary Divergent Boundary Hot Spot Reason Volcano Formation Special Notes Examples Row 2 Volcano Location Reason Subduction Boundary Collision between Oceanic/Continental Or Oceanic/Oceanic High temperature + low pressure = lots of magma Hot molten magma from deep within the Earth rises up through the crust and reaches the surface Divergent Boundary Hot Spot Volcano Formati on Special Note s Examples Row 3 Row 4 Row 5 Special Notes Examples Volcano Location Reason Volcano Formation Subduction Boundary Collision between Oceanic/Contine ntal Or Oceanic/Oceanic Divergent Boundary High temperature + low pressure = lots of magma Hot Spot Hot molten magma from deep within the Earth rises up through the crust and reaches the surface On plate that does not sink Where the plates separate In the middle of plates away from plate boundaries Row 4 Volcano Location Reason Volcano Formation Special Notes Subduction Boundary Collision between Oceanic/Cont inental Or Oceanic/Oce anic On plate that does not sink Divergent Boundary High temperature + low pressure = lots of magma Where the plates separate Hot Spot Hot molten magma from deep within the Earth rises up through the crust and reaches the surface In the middle of plates away from plate boundaries oceanic/oceanic, forms “VOLCANIC ARC” Most of these volcanoes are underwater Hot spots do not move, but the crust does Examples Row 5 Volcano Location Reason Volcano Formation Special Notes Examples Subduction Boundary Collision between Oceanic/Continental Or Oceanic/Oceanic On plate that does not sink oceanic/ocean ic, forms “VOLCA NIC ARC” O/C: Cascades in California O/O: Aleutians in Alaska Divergent Boundary High temperature + low pressure = lots of magma Where the plates separate Most of these volcanoes are underwat er Mid-ocean ridge, Iceland Hot Spot Hot molten magma from deep within the Earth rises up through the crust and reaches the surface In the middle of plates away from plate boundaries Hot spots do not move, but the crust does Hawaiian Islands, Canary Islands, Yellowstone