by Janet Grischy

Created on: July 28, 2009

Igneous extrusion builds a volcano. A mound or peak forms, built of melted rock and associated materials. Rock formed by volcanic heat is called igneous, from Latin, meaning "of fire". The sides of a volcano can be steep, slope gently, or even form a volcanic plateau. Volcanoes are built around vents or fissures by material from deep within the earth.

Where the tectonic plates of the earth's crust come together, where one plate subsides under another, or where a hotspot forms within a plate, volcanoes are found. The Hawaiian volcanoes lie over a hotspot. Mount Lassen in the western United States formed where rock subsided beneath the continental plate. Magma, molten rock, forms deep beneath the earth's surface in areas like these. A pool of magma becomes the reservoir that powers a volcano. When magma comes out of the ground, it is called lava.

Volcanoes are divided into shield volcanoes, cinder cones, and composite or stratovolcanoes. Shape, size, and composition are different in each kind of volcano. Another volcanic feature is a caldera, a deep depression in a volcanic peak.

Shield volcanoes

Shield volcanoes are large, wide, and gently sloped. Lava emerges relatively slowly and quietly from fissures on a stratovolcano. They are usually built of basaltic lava, which tends to be runnier than other types. Because it is relatively fluid, it travels great distances before it cools. It may also carry less of the gasses that produce spectacular eruptions, though any eruption is spectacular enough. The Hawaiian Islands are built of immense shield volcanoes. The top of Mauna Loa is more than 28,000 feet above its base on the bottom of the ocean.

Cinder Cones

Cinder cones are the smallest volcanoes. They may be only a few hundred feet high and less than a mile in diameter at the base. They are steep. The crater is a relatively large part of the volcano. Cinder cones are formed entirely of pyroclastics, frothy bits of lava thrown out as blocks, bombs, cinders or ash. Ash may coat the ground for miles around a cinder cone, while lava may be thrown out as bombs up to 3 feet across. A steep sided cone is built of fragments too heavy to travel far.

Pyroclastics include pumice, which is full of cavities and will float on water. Scoria is a similar rock, but with coarser cavities. It does not float. Tephra is a general term for rock thrown through the air by volcanoes. It is classified by size as ash, cinders (lapilli), or volcanic blocks and

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