Volcano
(m.p. attar)
How a volcano is formed
Powerful forces within the earth cause volcanoes. Scientists do not fully understand these forces. But they have developed theories on how the forces create volcanoes. This section describes how most scientists explain the beginning and eruption of a volcano.
The beginning of a volcano. A volcano begins as magma, melted rock inside the earth. magma results from the extreme heat of the earth''s interior. At certain depths, the heat is so great it partly melts the rock inside the earth. When the rock melts, it produces much gas, which becomes mixed with the magma. Most magma forms 80 to 160 kilometres beneath the surface. Some develops at depths of 25 to 50 kilometres.
The gas-filled magma gradually rises toward the earth''s surface because it is lighter than the solid rock around it. As the magma rises, it melts gaps in the surrounding rock. As more magma rises, it forms a large chamber as close as 3 kilometres to the surface. This magma chamber is the reservoir from which volcanic materials erupt.
The eruption of a volcano. The gas-filled magma in the reservoir is under great pressure from the weight of the solid rock around it. This pressure causes the magma to blast or melt a conduit (channel) in a fractured or weakened part of the rock. The magma moves up through the conduit to the surface. When the magma nears the surface, the gas in the magma is released. The gas and magma blast out an opening called the central vent. Most magma and other volcanic materials then erupt through this vent. The materials gradually pile up around the vent, forming a volcanic mountain, or volcano. After the eruption stops, a bowllike crater generally forms at the top of the volcano. The vent lies at the bottom of the crater.
Once a volcano has formed, not all the magma from later eruptions reaches the surface through the central vent. As the magma rises, some of it may break through the conduit wall and branch out into smaller channels. The magma in these channels may escape through a vent formed in the side of the volcano. Or it may remain below the surface.
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