Transcript Volcanoes
Volcanoes Volcano Definition - A vent at which lava, pyroclastic debris (ash and fragments of previously solidified rock), and gas erupt. - Eruption may build a mountain around the vent. (Mountain is also called a volcano). - Anatomy of a volcano: - magma chamber at depth - a vent to the surface Word bank: conduit, vent, crater, side vent, ash cloud, crust, magma chamber, lava Where do Volcanoes Occur? Divergent volcanism: 1. Mid-ocean ridges • Where plates separate under water • Creates new sea floor 2) Rifts Where plates separate on land • • e.g., East African rift (next slide) Iceland (this slide) Great African Rift Valley • Bbb Mid Ocean Ridge It is thought that the Great Rift Valley will eventually become an ocean. Where do Volcanoes Occur? Convergent Boundaries - the majority of major volcanoes - e.g., the Andes, Japan, Aleutians (ring of fire) Where do Volcanoes Occur? Hot spots - e.g., Hawaii; Yellowstone Park Nature of lava • Characteristics of lava depend on its temperature and viscosity. – viscosity is a measure of the ability of a fluid to resist flow • high viscosity means thick • low viscosity means watery and fast • The higher the temperature of a lava, the lower its viscosity (i.e., the easier it flows) • The greater the gas content, the lower the viscosity. Frothy lava flows more easily. Types of Eruptions • Lava dominated eruptions: – Characteristic of low viscosity eruptions. – If lava has low viscosity, it fountains out easily. You will see lava fountains, lakes and rivers, that flow long distances. Low Viscosity flow fountains, lakes & rivers of lava Types of Eruptions • Explosive eruptions: – Sticky, very viscous lava clogs up the vent. Gas (CO2 and H2O) that come out of solution and make bubbles. Gas pressure builds up and the volcano explodes. – Result is a cloud of ash that blankets countryside or an ash flow that rushes down the volcano side at speeds up to 100 km/h, incinerating everything in its path. • Explosions can also occur when water gets into magma chamber and turns to steam or when part of the mountain slips away and pressure is released. Lava Vocab • Tephra – materials that explode along with lava – Cooled chunks of lava – Rocks, ash, parts of crust, etc. • Pyroclastic flow – Rapidly moving clouds of tephra mixed with hot, suffocating gases – http://www.youtube.com/watch?v=WbkPo0Hws 0M Types of Volcanoes • The fissure volcano: is a long crack in the earth's surface through which magma erupts. These cracks may form as two tectonic plates pull apart. • The shield volcano: is a broad, shallow volcanic cone, which arises because the running lava, which is fluid and hot, cools slowly. Types of Volcanoes • The dome volcano: has a steep, convex slope from thick, fast-cooling lava. • The ash-cinder volcano: throws out (in addition to lava) much ash into the air. Through this the volcanic cone is built up from alternate layers of ash and cinder. Types of Volcanoes • The composite volcano: are also built up from alternate layers of lava and ash but, besides its main crater, it has many little craters on its slope. • The caldera volcano: is an older volcano with a large crater which can be 62 miles (100km) wide. In this crater many little new craters are formed. Intrusive Activity • Plutons – – Intrusive igneous rock bodies formed by cooled magma that forced its way into the crust and cooled before hitting the surface • 4 types – Batholiths and stocks – Laccoliths – Sills – Dikes Batholiths • Large, irregularly shaped • Coarse-grained igneous rock • Common in interior of many mountain chains Laccoliths • Lens-shaped • Smaller than batholith • Stone Mountain, GA Sills • Magma intrudes parallel to layers of rock Dike • Cuts across preexisting rocks Pluton Review