Resumo de Volcano - s.k. attar

Kinds of volcanic materials

Three basic kinds of materials may erupt from a volcano. They are (1) lava, (2) rock fragments, and (3) gas. The material that erupts depends chiefly on how sticky or fluid a volcano''s magma is.

Lava is the name for magma that has escaped onto the earth''s surface. When lava comes to the surface, it is red hot and may have a temperature of more than 1100 °C. Highly fluid lava flows rapidly down a volcano''s slopes. Sticky lava flows more slowly. As the lava cools, it hardens into many different formations. Highly fluid lava hardens into smooth, folded sheets of rock called pahoehoe. Stickier lava cools into rough, jagged sheets of rock called aa. Pahoehoe and aa cover large areas of Hawaii, where the terms originated. The stickiest lava forms flows of boulders and rubble called block flows. It may also form mounds of lava called domes.

Other lava formations include spatter cones and lava tubes. Spatter cones are steep hills up 30 metres high. They build up from the spatter of fountainlike eruptions of thick lava. Lava tubes are tunnels formed from fluid lava. As the lava flows, its outer surface cools and hardens. But the lava underneath continues to flow. After the flowing lava drains away, it leaves a tunnel.

Rock fragments, generally called tephra, are formed from sticky magma. Such magma is so sticky that its gas cannot easily escape when the magma approaches the surface or central vent. Finally, the trapped gas builds up so much pressure that it blasts the magma into fragments. Tephra includes, from smallest to largest, volcanic dust, volcanic ash, and volcanic bombs.

Volcanic dust consists of particles less than 0.25 millimetre in diameter. Volcanic dust can be carried great distances. In 1883, the eruption of Krakatoa in Indonesia shot dust 27 kilometres into the air. The dust was carried around the earth several times and produced brilliant red sunsets in many parts of the world. Some scientists believe that large quantities of volcanic dust can affect the climate by reducing the amount of sunlight that reaches the earth.

Volcanic ash is made up of fragments less than 0.5 centimetre in diameter. Most volcanic ash falls to the surface and becomes welded together as rock called volcanic tuff. Sometimes, volcanic ash combines with water in a stream and forms a boiling mudflow. Mudflows may reach speeds of 100 kilometres per hour and can be highly destructive.

Volcanic bombs are large fragments. Most of them range from the size of a rugby ball to that of a basketball. The largest bombs may measure more than 1 metre across and weigh up to 90 metric tons. Small volcanic bombs are generally called cinders.Write your abstract here.

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