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« Volcanoes - geography. Iceland Beneath Iceland, in the North Atlantic Ocean, plumes of molten rock, or magma, rise from the mantle, the layer below Earth's crust.

This magma produces lava that cools toform basalt and related rocks.

Basaltic lava flows fairly smoothly, so it tends to form broad volcanoes with gentle slopes, known as shield volcanoes.

Shield volcanoestypically do not erupt violently.

These volcanoes often develop at oceanic hot spots (see Island Hot Spots Map Trek), as with the island of Hawaii in the Pacific and theAzores in the Atlantic. Japan A different kind of volcano, called a stratovolcano, has shaped the islands of Japan.

Stratovolcanoes form when blocks of crust melt deep below the surface and rise asmagma.

This kind of magma solidifies at the surface, forming andesite and related rocks.

Andesitic lava forms steep, cone-shaped volcanoes because it is thick, gummy,and resistant to flow.

Because andesitic lava tends to surround pockets of hot, pressurized gas, these volcanoes often have violent eruptions.

Stratovolcanoes, includingMount Fuji in Japan, are common in subduction zones such as the Map Trek around the Pacific Ocean (see Subduction and Ring of Fire Map Treks). Mount Fuji, Japan The steep slopes of Mount Fuji rise to an elevation of 3,776 m (12,387 ft), even though the mountain lies less than 35 kilometers (22 miles) from the sea coast. Mid-Atlantic Ridge Earth's most common type of volcano, the fissure vent, is probably the least well known, because these volcanoes usually occur deep beneath the ocean surface alongdivergent boundaries (see Divergent Boundaries Map Trek).

Any break in Earth’s surface through which molten rock emerges is called a volcano, so volcanoes need notbuild sloping, mountainous landforms.

Fissure vents emit basaltic lava that tends to form ridges, such as the Mid-Atlantic Ridge, on the ocean floor. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation.

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