Molten rock originating as magma in the Earth's mantle that pours out onto the Earth's surface through volcanic vents (see volcano) at temperatures of about 1,300–2,200 °F (700–1,200 °C). Mafic lavas, such as basalt, form flows known by the Hawaiian names pahoehoe and aa. Pahoehoe is smooth and gently undulating; the lava moves through natural pipes known as lava tubes. Aa is very rough, covered with a layer of loose, irregular fragments called clinker, and flows in open channels. Lava that starts out as pahoehoe may turn into aa as it cools. Lavas of intermediate composition form a block lava flow, which also has a top consisting largely of loose rubble, but the fragments are fairly regular in shape, mostly polygons with relatively smooth sides. See also bomb; nuée ardente.

This entry comes from Encyclopædia Britannica Concise.
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