1911 Encyclopædia Britannica/Lava

LAVA, an Italian word (from Lat. lavare, to wash) applied to the liquid products of volcanic activity. Streams of rain-water, formed by condensation of exhaled steam often mingled with volcanic ashes so as to produce mud, are known as lava d’acqua, whilst the streams of molten matter are called lava di fuoco. The term lava is applied by geologists to all matter of volcanic origin, which is, or has been, in a molten state. The magma, or molten lava in the interior of the earth, may be regarded as a mutual solution of various mineral silicates, charged with highly-heated vapour, sometimes to the extent of super-saturation. According to the proportion of silica, the lava is distinguished as “acid” or “basic.” The basic lavas are usually darker and denser than lavas of acid type, and when fused they tend to flow to great distances, and may thus form far-spreading sheets, whilst the acid lavas, being more viscous, rapidly consolidate after extrusion. The lava is emitted from the volcanic vent at a high temperature, but on exposure to the air it rapidly consolidates superficially, forming a crust which in many cases is soon broken up by the continued flow of the subjacent liquid lava, so that the surface becomes rugged with clinkers. J. D. Dana introduced the term “aa” for this rough kind of lava-stream, whilst he applied the term “pahoehoe” to those flows which have a smooth surface, or are simply wrinkled and ropy; these terms being used in this sense in Hawaii, in relation to the local lavas. The different kinds of lava are more fully described in the article Volcano.