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Any place on thesurface of the earth, which spews magma, is called volcano. In a more defined sense, it means any mountain spewing fire, which originates from the magma. Magma is a molten rock from the lower layers of the earth's crust that penetrates to the earth's surface in the form of lava. An eruption, which is the hurling of the magma and gases, passes through a volcanic vent or fissure.
Deep underneath the volcanic vent, in the depth of 100 to 300 kilometres, we find the magma reservoir, which is connected to the upper earth's mantle. The material rising from great depths collects there and melts. Neighbouring rock is also melting. When thepressure in the reservoir is higher than the pressure of the rock above it, an eruption will occur. A new volcano may form, or a dormant volcano becomes active.
At the top of the volcanic vent is the crater in the form of a bowl or a funnel. Released gases, magma in the form of molten lava, or loose products push upwards and are hurled into the atmosphere in an enormous cloud. Volcanic bombs (boulders of lava, which can reach the size of a building), cinders, lapilli (small, barely several centimetres large fragments of lava), sand, and ash are the loose volcanic products.
The products of the eruptions form a wall, steep inside and sloping outside, which grows upwards into a cone. Lava products are called the pyroclastic rock (from Greek: pyros = fire, klasis = to break).
In addition to eruptions through the volcanic vent, we may encounter eruptions alongside fissures. In general, these are primary eruptions, often accompanied byearthquakes. When a fissure forms, tremendous amounts of lava and loose products may be expelled. Analogously to the vent eruptions, the material is expelled to one place. This may lead to the formation of a lined-up volcanic crest.
The type of eruption depends on theaggregate state of the magma at the exit. There are three states: volcanic gases, flowing lava, and solid material. When the magma cools, it releases gases which are explosively expelled. It is not possible to obtain appropriate samples, the composition can be determined only by smell, the colour of the flames, and similar factors, or by taking samples from lava lakes or fumaroles. The composition of the gases is different depending on the type of eruption. However, they do contain almost always water, carbon dioxide, hydrogen, hydrogen chloride, hydrogen sulphide, hydrogen fluoride, nitrogen, ammoniac, and various rare gases.
The lava flow depends on its viscosity. When a mass cools down by 50 degrees Celsius, the viscosity at constant composition will increase tenfold. There is a ropey lava, which flows as a narrow fluid stream, and lava bombs, which form when the upper layer of the lava is congealing and congealed, while the lower layer Is still liquid. The layer opens and the flowing lava penetrates the fissure. Following additional thickening, the lava crust breaks into blocks in the course of its slow progress, and these blocks fall down. Very viscous lava does not flow and forms a plug in the vent of the crater. This sometimes leads to fierce explosions, which sometimes tear apart and hurl part of this plug. When gases and hot slabs of lava cannot escape vertically through the plug, they will push through the sides of the volcano and tumble down as clouds of molten mass. They reach high speeds and may cover and so destroy large areas.
Mixed eruptions and composite volcanoes (also called stratovolcanoes) are quite common. They owe their name to their structure: the alternating layers of tuff and lava form a symmetrical conic mountain. When the volcano grows over a certain height limit, its walls cannot support the pressure of the magma in the volcanic vent and the side wall is torn apart. In cases, where thetop of the mountain is blown up, it is replaced by a caldera, a large funnel-shaped crater. New eruptions form a new cone, which is the case of Vesuvius, for example, or a volcano in the Bay of Naples.
During a central eruption the lava rises together with other material through the volcanic vent, which reached great depths. Deposits of coarser material around the volcanic vent form thecone, which may grow several metres in a few days. Central eruptions may occur in any of the regions that have active volcanoes.
In the case of an existing volcano, repeated opening of the volcanic vent may often result in explosive ejection of boulders filling the vent, together with ash and lava. During this type of eruptions, large part of the volcano may be blown away.
The characteristics of the various eruptions maybe determined in accordance with the location type. These types are Icelandic, Hawaiian, Stromboli, volcanic, and Pelee.
The so-called linear volcanoes are quite common in Iceland. Their main characteristic is the outflow of thin, fluid lava, which rises from deep fissures. It does not form volcanic cones. An example of the Icelandic type is the eruption of the fissure Laki in Iceland in 1783 (25 kilometres long).
The Hawaiian type corresponds to the shield volcano. These volcanoes produce few gases, with lava pouring slowly out of the volcano. There are no explosions and no pyroclastic clouds. Mauna Loa (4170) and Mauna Kea (4206) in Hawaii are good examples.
The characteristics of the Stromboli type is an almost endless, moderate activity, with low explosiveness, during which hot chunks of lava are accompanied by clouds of vapour and small amounts of ash.
Volcanic type is the type with mixed volcanic eruptions. The type was named in 1889 by Mercalli, following his studies of the island volcano Vulcano (Aeolian Islands). Today, Vesuvius provides a better example of that type. The characteristics of this type are violent explosions, accompanied by dark, pine-shaped clouds of ash.
The Pelee type is characterised mainly by high-viscosity lava, accompanied by dense, hot clouds and a considerate amount of volcanic ash. This type was named after the Mount Pelee on Martinique, where in 1902 a violent eruption, followed by a gigantic explosion, killed thousands of people.
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