Lava, molten rock ejected from volcanoes and fissures (cracks) in the earth's surface. The term is also used to refer to the rock formed when lava cools and solidifies. High mountains, chains of islands, and extensive plateaus made from lava are found in many parts of the world. In many places, weathered lava has produced highly fertile soil.

Geologists refer to molten rock within the earth as magma. They believe magma forms at depths where rocks are under great pressure and extremely hot. After slowly rising through the earth's crust, the molten rock is ejected as lava, which, in some cases, can have a temperature of more than 2,000 F. (1,200 C.).

Lava consists mainly of silica (silicon dioxide). Mafic lava, sometimes called basic lava, is around 50 per cent silica. Felsic lava, sometimes called acidic lava, is around 70 per cent silica. Lava with a silicon content between that of mafic and felsic lava is called intermediate lava. Lava contains various amounts of dissolved gases. Most of these gases escape from the lava as it is ejected, producing phenomena ranging from small lava fountains to extremely violent explosions.

Mafic lava is usually very fluid, so that volcanic gases are released before they become explosive, and commonly produces large flows. Mafic lava has been known to flow as fast as 20 miles an hour (32 kmh) and as far as 60 miles (97 km). It typically forms low, dome-shaped volcanoes, such Mauna Loa in Hawaii. Mafic lava commonly solidifies into basalt, a black, gray, or dark-green rock that typically forms smooth, billowy masses or tall, hexagonal (six-sided) columns. Basalt has formed some huge plateaus, such as the Deccan in India and the Columbia Plateau in Washington, Oregon, and Idaho. In both these regions, basalt covers an area of more than 200,000 square miles (518,000 km2), with an average thickness of 500 feet (152 m) in the Columbia Plateau and 2,000 feet (600 m) in the Deccan.

Because felsic lava is less fluid, the pressure of gases and steam tends to build up until the lava is ejected explosively. Felsic lava typically forms steep-sided volcanoes, such as Mont Pelee on the island of Martinique. This type of lava usually solidifies into rhyolite, a fine-grained rock resembling granite.

Intermediate lava is usually less fluid than mafic and more fluid than felsic lava. It may flow or be ejected explosively, depending on its temperature and gas content. It usually solidifies into fine-grained, grayish sheets of andesite.

When lava cools too fast to form crystals, it becomes a glassy rock called obsidian. A glassy form of lava known as pumice is porous and, when dry, light enough to float in water.