A volcano is an opening in the Earth's crust through which molten lava, ash, and gases erupt. In many cases, lava and ash form a mountain around the opening.
It used to be thought that volcanoes leaked molten rock and gases directly from the Earth's core. That is not the case. As hot, solid rock rises in the mantle, the pressure drops and a small part of the rock begins to melt. This liquefied rock, called magma, is less dense than solid rock. It squeezes out from the solid like water from a sponge. The rising magma creates wide channels in the crust as it forces its way to the surface. When it breaks through the surface, the pressure drops. Gases dissolved in the magma force it to erupt through the opening as lava.
TYPES OF VOLCANOES
The behavior of a volcano depends on the type of magma that fuels it. Volcanoes such as those near Hawaii and Iceland are sitting on top of a rising plume of hot mantle rock, called a hot spot. The lava that erupts from these volcanoes comes from great depths, sometimes more than 90 mi. (150km) into the mantle. Its
composition is not the same as the mantle, because only a tiny fraction of the mantle rock melts. This lava is runny when molten and sets as dense, black basalt. Because the lava is so runny, it can pour out through fissures at vast rates and flow across the land at speeds of up to 31 mph (50kph). Where this type of volcano erupts underwater, the lava cools quickly and
builds volcanic islands as it sets. Where gas bubbles through it, the runny lava erupts in spectacular fountains. Because this type of lava flows freely, eruptions are smooth rather than explosive.
A different type of volcano is found where ocean crust dives under the edge of a continent. The ocean crust partly melts to form a sticky lava that is rich in silica and contains some water. During an eruption, the sudden drop in pressure causes the water to turn to steam. This results in an explosion of fine ash and hot gases. This mixture, which can race down the sides of a volcano at 125 mph (200kph), is called a nuée ardente, French for "glowing avalanche."
LIVING WITH VOLCANOES
With their combinations of red-hot lava, toxic gases, and suffocating ash. volcanic eruptions can be deadly phenomena. But people continue to live on the sides of volcanoes despite the danger. This is because volcanic soil is often fertile, and eruptions can be few and far between, giving a false sense of security. The consequences can be disastrous. In 1902, when Mount Pelee on the Caribbean island of Martinique erupted, a nuee ardente raced down the mountain and engulfed the port of San Pierre- More than 29,000 people were killed. The only survivor was a prisoner in an underground cell. In A.D. 79, a similar eruption from Mount Vesuvius smothered the Roman towns of Boscoreale, Herculaneum, Pompeii, and Stabiae with mud and ash.
It is possible to predict at least some eruptions by monitoring volcanic gases and measuring changes in gravity as molten lava rises inside a volcano Sometimes, the whole mountain bulges. When Mount St. Helens, in Washington, started to bulge in 1980, most people were evacuated before the mountain blew its top. A huge landslide removed part of the volcano, exposing the pressurized molten lava. The lava then exploded sideways and upward. The blast hurled about half a cubic mile of rock into the air and flattened trees up to 19 mi. (30km) away.
Reference : The Kingfisher Science Encyclopedia De Charles Taylor
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