Case Study
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Case Study
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Structure of a volcano:
Volcanoes draw their magma from deep below the solid crust but it is often stored prior to an eruption in a magma chamber. Great pressures build up and molten magma is forced up the narrow vent to the crater at the surface. Here it may be ejected with great force to form a huge cloud of gas, steam, ash and volcanic bombs or it may just flow much more gently down the sides of the volcano to form a lava flow. Sometimes hot clouds of gases and ash (pyroclastic flows) may race down the sides of the volcano. Some of the magma may find its way though smaller vents to form secondary cones on the sides of the main volcano, or it may simply be intruded (forced) between the layers of pre-existing rock to form dykes, sills and laccoliths.
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Types of volcano:
Ash and cinder cones are formed by fragments of solid material that explode upwards and then accumulate around the vent to form a cone. The sides are usually steep (30-40 degrees) and concave as material spreads out near the base. Example: Paracutin.
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Lava cones come in two varieties. Both build up when there is no explosive activity. The first are called shield volcanoes and they are formed from very fluid lava which flows long distances. The slopes are very gentle (5-10 degrees) and the volcanoes have huge circumferences. Example: Mauna Loa, Hawaii. The second are called dome volcanoes and they are formed from very sticky lava which flows only short distances and cools very quickly. Example: Mount Pelee, Martinique
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The third and most common type are called composite cones or stratovolcanoes. They are formed when a volcano experiences more than one type of eruption. Part of the volcano is formed by gentle lava flows and part by explosive ash eruptions. Example: Mount Etna, Italy.
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There is also the possibility that magma may rise to the surface along cracks or fissures and spread out across the land rather than forming volcanoes. In this way, thick lava plateaus are eventually formed. Example: Deccan Plateau, India.
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Other volcanic landforms:
In areas of current volcanic activity and also in areas where volcanoes are no longer active, there may be much other evidence of the heat and pressure at work underground. Geysers are fountains of steam and water which shoot high up into the air at regular or irregular intervals. Solfatara are formed when gases escape to the surface. Most are sulphurous and can be smelled from a long way away. Mud pools may form where hot water mixes with surface mud. Bubbles of gas may escape noisily through the mud. |