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How Does Magma Reach the Surface? Tephra Volcanic Bomb Types of lava: • Pahoehoe lavas are thin. They flow smoothly and are often formed by small volumes of hot, fluid basalt. • Pahoehoe flows move forwards in tongues or lobes and are characterized by a glassy, plastic skin. • They may embrace obstacles at a rate of about 50m an hour. • When the pahoehoe lava flow cools, it often solidifies to a smooth surface. Pahoehoe lava flow, Kilauea volcano, Hawaii, November 1985. Basaltic lava flow Aa lava • Hawaiian term (pronounced 'ah-ah') for lava flows that have a rough rubble surface composed of broken lava blocks called clinkers. Lava flow in Hawaii – aa lava Aa lava, Etna Shield Volcano • Fluid lava flows quietly, spreading out in wide thin sheets • Flat wide cones are formed (looks like a shield) Mount Bachelor, Oregon Cinder Cones • Thick lava containing an abundance of dissolved gases • Does not flow readily through the vent so it clogs up and becomes blocked • Pressure builds up and a violent eruption is produced • Ejects solid pieces of tephra to form a narrow cone Lava Beds National Landmark Area in Mojave Natural Preserve Composite cone (Stratovolcano) • Alternating explosive and quiet eruptions • Results in alternating layers of solidified lava and volcanic rock particles Mount St. Helen live cam Mt. Shasta Pyroclastic Flow Volcanoes at Plate Boundaries • Most of the world’s volcanoes occur at plate boundaries called subduction zones • These eruptions are usually explosive and form cinder cones Subduction boundary Subduction Animation History of the Cascade Range Rift Eruptions • Usually located on the ocean floor, but can occur at any fracture in the earth’s crust • Lava oozes out of the fissures and cools quickly • This is how most of the ocean crust forms Rift Eruption Hot Spots • Volcanic activity in the middle of crustal plates • Form shield volcanoes • Hawaiian Islands • Hot Spot remains stationary as the plate moves over it Hawaiian Islands Hot Spot Hawaii Yellowstone Hot Spot Location of Major Volcanoes
