LAVA

magma (molten rock) poured out onto the Earth's surface at temperatures from about 700 to 1,200 C (1,300 to 2,200 F). The viscosity ranges from about 100,000 times that of water to the point that the lava can scarcely flow at all. Mafic (ferromagnesian, dark-coloured) lavas, such as basalt, characteristically form flows known by the Hawaiian names pahoehoe and aa. Pahoehoe lava flows are characterized by smooth, gently undulating, or broadly hummocky surfaces. The liquid lava flowing beneath a thin, still-plastic crust drags and wrinkles it into tapestry-like folds and rolls resembling twisted rope. Pahoehoe lava flows are fed almost wholly internally by streams of liquid lava flowing through natural pipes known as lava tubes. Typically, the margin of a pahoehoe flow advances by protruding one small toe after another. In contrast to pahoehoe, the surface of aa lava is exceedingly rough, covered with a layer of partly loose, very irregular fragments commonly called clinker. Aa lava flows are fed principally by rivers of liquid lava flowing in open channels. Typically, such a feeding river forms a narrow band, 815 metres (2550 feet) wide, along the centre line of the flow, with broad fields of less actively moving clinker on each side of it. At the front of the flow, clinker from the top rolls down and is overridden by the pasty layer. Thin basaltic lava flows generally contain many holes, or vesicles, left by bubbles of gas frozen into the congealing liquid. Thick flows, which remain hot for long periods, may lose most of their gas before the lava congeals, and the resulting rock may be dense with few vesicles. Pahoehoe and aa flows may be identical in chemical composition. In fact, it is common for a flow that leaves the vent as pahoehoe to change to aa as it progresses downslope. The greater the viscosity, and the greater the stirring of the liquid (as by rapid flow down a steep slope), the greater the tendency for the material to change from pahoehoe to aa. The reverse change does not occur. Lavas of andesitic or intermediate composition commonly form a somewhat different type of flow, known as a block lava flow. These resemble aa in having tops consisting largely of loose rubble, but the fragments are more regular in shape, most of them polygons with fairly smooth sides. Flows of more siliceous lava tend to be even more fragmental than block flows. Apparently the escape of gas from the cooling and crystallizing magma causes a series of minute explosions all through the flow, thoroughly shattering the lava into a mass of blocks that are little separated from each other.