n.
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 1,3002,200 °F (7001,200 °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 .