Volcanoes

Volcanoes are the solid structure created when lava, gases, and hot particles escape to the Earth's surface through vents. Volcanoes are usually conical. A volcano is "active" when it is erupting or has erupted recently. Volcanoes that have not erupted recently but are considered likely to erupt in the future are said to be "dormant." A volcano that has not erupted for a long time and is not expected to erupt in the future is "extinct."

The word volcano comes from Vulcan, the Roman god of fire and blacksmiths. Once, people believed the lava and ash spewed from Vulcan's forge.

There are many types of volcano ( see kinds of volcano ) . The most distinctive are the cone-shaped composite volcanoes, which build up from alternating layers of ash and lava in successive eruptions.

fig. 6 A cone-shaped volcano

Beneath a composite volcano there is typically a large reservoir of magma called a magma chamber. Magma collects in the chamber before an eruption. From the magma chamber a narrow chimney, or vent, leads up to the surface. It passes through the cone of debris from previous eruptions.

Volcanic gases are poisonous and deadly. The gases that rise from magma often smell like rotten eggs, can sting the eyes and throat, and can eat through clothing—or even human flesh!

When a volcano erupts, the magma is driven up the vent by the gases within it. As the magma nears the surface, the pressure drops, allowing the gases dissolved in the magma to boild out. The expanding gases - mostly carbon dioxide and steam - push the molten rock upwards and out of the vent.

Not all lava is the same: A pahoehoe (pa-hoy-hoy) lava flow travels quickly and often forms smooth, ropy patterns, while an aa (ah-ah) lava flow moves slowly and hardens to form sharp chunks.

If the level of magma in the magma chamber drops, the top of the volcano's cone may collapse into it, forming a giant crater called a caldera. Caldera is Spanish for " boiling pot " . The world's largest caldera is Toba on Sumatra, Indonesia, which is 1775 sq km.

When a volcano with caldera subsides, the whole cone may collapse into the old magma chamber. The caldera may fill with water to form a crater lake, such as Crater Lake in Oregon, USA.

All the magma does not gush up the central vent. Some exits through branching side vents, often forming their own small "parasitic" cones on the side of the main one.