Types of Volcanoes.ppt

Types of Volcanoes.ppt

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  Types of Volcanoes Composite Volcanoes  Shield Volcanoes  Cinder Cones  Spatter Cones
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  Composite Volcanoes  Compositevolcanoes, also called strato volcanoes, are formed by alternating layers of lava and rock fragments. This is the reason they are called composite.  Strato-volcanoes often form impressive, snow-capped peaks which are often exceeding 2500m in height, 1000sq.km in surface, and 400km3 in volume.  Although strato-volcanoes are usually large and conical, we can distinguish different shapes of them: concave (like Agua), pyramidal (like Stromboli), convex-concave (like Vesuvius), helmet-shaped (like Mount Rainier), collapse caldera (like Graciosa), nested (like El Piton in Teide), multiple summits (like Shasta), elongated along a fissure (like Hekla).
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  Shield Volcanoes  Shieldvolcanoes are huge in size. They are built by many layers of runny lava flows. Lava spills out of a central vent or group of vents. A broad shaped, gently sloping cone is formed. This is caused by the very fluid, basaltic lava which can't be piled up into steep mounds  Shield volcanoes may be produced by hot spots which lay far away from the edges of tectonic plates. Shields also occur along the mid-oceanic ridge, where sea-floor spreading is in progress and along subduction related volcanic arcs.  The eruptions of shield volcanoes are characterized by low-explosivity lava- fountaining that forms cinder cones and spatter cones at the vent. Famous shield volcanoes can be found for example in Hawaii (e.g. Mauna Loa and Kilauea).
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  Cinder Cones  Acinder cone is a steep conical hill formed above a vent. Cinder cones are among the most common volcanic landforms found in the world. They aren't famous as their eruptions usually don't cause any loss of life. Cinder cones are chiefly formed by Strombolian eruptions. The cones usually grow up in groups and they often occur on the flanks of strato volcanoes and shield volcanoes.  Cinder cones are built from lava fragments called cinders. The lava fragments are ejected from a single vent and accumulate around the vent when they fall back to earth.  Cinder cones grow rapidly and soon approach their maximum size. They rarely exceed 250m in height and 500m in diameter.  The shape of a cinder cone can be modified during its (short) life. When the position of the vent alters, aligned, twin or secant cones develop. Nested,buried or breached cones are formed when the power of the eruption varies.  A great example of a cinder cone is Paricutín in Mexico. It was born in February 20, 1943 in a corn field and grew to 300 feet in 5 days.
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  Spatter Cones  Whenhot erupting lava contains just enough explosive gas to prevent the formation of a lava flow, but not enough to shatter it into small fragments the lava is torn by expanding gases into fluid hot clots, ranging in size from 1cm to 50cm across, called spatter.  When the spatter falls back to Earth the clots weld themselves together and solidify forming steep-sided accumulations. These accumulations focused on an individual vent are called spatter cones.
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  Different Types ofVolcanic Eruptions 1. low water, low silica - runny lava flows (not viscous) 2. low water, high silica (very viscous) - pasty lava - often building domes 3. high water, low silica(not viscous) - fountain of runny lava 4. high water, high silica (very viscous) - explosion
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  Impacts and Effectsof Volcanoes  The sudden violence of volcanic eruptions causes catastrophe and devastation. Various notorious eruptions of volcanoes in the past, such as Mount Pelée, Tambora, Krakatau and Pinatubo have demonstrated the devastating impact of volcanic activity on nearby landscapes and communities. Many people got killed. A big number of people had to abandon their homes and land forever. Even the whole world's climate was changed for a while as a result of an eruption!  These effects of volcanic eruptions are mostly the result of certain hazards. Volcanoes provide different hazards during an eruption. Each hazard poses different risks affecting different areas.
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  How Volcanoes Erupt An eruption begins when pressure on a magma chamber forces magma up through the conduit and out the volcano's vents. When the magma chamber has been completely filled, the type of eruption partly depends on the amount of gases and silica in the magma. The amount of silica determines how sticky (level of viscosity) the magma is and water provides the explosive potential of steam.  Obstacles also influence the type of eruption. When the pipe is blocked by a stopple or an accumulation of pumice, the pressure in the pipe will build up very high resulting in an explosion.  When magma reaches earth's surface it is called lava. It may pour out in gentle streams called lava flows or erupt violently into the air. Rocks ripped loose from the inside of the volcano or torn apart by the gas may be shot into the air with the lava. These rocks blown out of a volcano are called pyroclastic rocks. The rock fragments fall back to earth in many different shapes and sizes: – Dust - particles less than 1/100 inch in diameter Dust particles may be carried great distances. In a powerful eruption they may be carried around the earth several times. – Ash - fragments less than 1/5 inch in diameter Most volcanic ash falls to the surface and cemented together by water to form a rock called volcanic tuff. – Bomb - A rounded piece of newly hardened lava which takes shape while flying through the air. – Block - A piece of lava that has sharp corners. – Cinder - Bubbly rock formed by liquid lava cooling in the air. – Pumice - Cinder so bubbly that it floats in water.  Volcanic activity is classified by how often a volcano erupts. A volcano may be active, intermittent, dormant, or extinct. Active volcanoes erupt constantly. Intermittent volcanoes erupt fairly regularly. Dormant volcanoes are inactive, but not long enough to determine whether they will erupt again or not. Extinct volcanoes have been inactive since the beginning of recorded history.
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  Active Volcanoes inthe last 10,000 years