Transcript Volcanoes
Volcanoes Chapter 7 Volcanoes and Plate Tectonics • Volcano – weak spot in the crust where a mountain forms when layers of lava and volcanic ash erupt and build up and push through • Magma - less dense than the rock around it so it is very slowly forced upward toward Earth’s surface made up of gases, water molten rock forming stuff • Lava – magma that reaches the earths surface Where Volcanoes Occur? • Volcanic Belts form along boundaries of Earth’s crust, one major is the Pacific Ring of Fire • 1. Divergent Boundary – plates moving apart – seafloor spreading • Example: N. Atlantic Ocean. The North American plate is moving away from Eurasia and African plates. • Iceland formed by the separation of these plates at the rift • 2. Convergent Boundary – 2 plates collide, rumple up and cause mountains instead of volcano • Juan de Fuca Plate is converging with the N. • Convergent Plates (cont) • Magma that is forced upward forms volcanoes of the Cascade Mountains • Pacific Ring of Fire – area around the Pacific Plate where there are earthquakes and volcano’s • Hot Spots – middle of a plate, area’s in Earth’s mantle that are hotter than neighboring areas • Magma forms and rises towards the crust • Example: Hawaiian islands, plates move and form islands when it gets over the hot spot Types of Eruptions – depend on the pressure build up and material • 2 factors: • 1. Water Vapor and other gases trapped within the magma, • if they escape easily – quiet eruptions • Gas trapped under high pressure eventually escapes – explosive eruptions • 2. Magma is either basaltic or granitic • Basaltic magma – contains less silica, is very fluid and produces quiet, non-explosive eruptions magma runs down the side like in Hawaii • Granitic magma – thick and contains lots of silica and has a high water content, it gets trapped in vents, causing pressure to build up so when it erupts the gases expand rapidly carrying pieces of lava 3 Forms of Volcanoes • 1. Shield Volcano – Quiet eruptions that spread out basaltic lava in broad, flat layers Hawaiian Islands (pg 399) • 2. Cinder Cone Volcano – Explosive eruptions that throw lava into the air forms steep sided consolidated volcano ( pg 400) Paricutin • Tephra – various sizes of this lava after it has cooled • Ash – fine • Bombs – large heavy rocks • 3. Composite Volcano – formed at convergent plate boundaries above subduction zone, this type of volcano alternates between quiet and explosive tephra layer alternating with a lava layer Mt. St. Helens (pg 400) • Subduction Zone – area where an ocean plate descends into the upper mantle Volcano Features • Magma that cools underground forms igneus rock (most magma does not reach the surface) • Balholith – largest igneous rock bodies, hundreds of km wide and thick Yosemete has exposed one • Dike – magma squeezed into a vertical crack and hardened • Sill – magma squeezed into a horizontal crack • Laccolith – dome of rock • Volcanic Neck – cone of the volcano erodes and leaves the igneous rock Ship Rock New Mexico • Caldera – formed after eruption when the top collapses down into magma chamber Crater Lake Vent – opening in the surface which magma reaches the earths surface through (1000 yrs) Crater – opening at the top of a volcano’s vent Geothermal Energy from Volcanoes • Geothermal energy – utilizing the heat from magma and converting it to electricity by heating water which produces steam that turns the turbines that run the generators to make electricity (fig 15-5) • Geothermal energy used in 20 countries and US • Hawaii wants to heat by geothermal energy of Kilauea • Many people are against this because some of the rain forest must be cleared for roads, drilling etc, they say it will cut down cultural trees