lava (läˈvə) [key], molten rock that erupts on the earth's
surface, either on land or under the ocean, by a volcano or
through a fissure. It solidifies into igneous rock that is also
called lava. Before reaching the earth's surface, the mixture of
solid and liquid rock, and gases, is known as magma. Lavas are
composed chiefly of silica and the oxides of aluminum, iron,
magnesium, calcium, sodium, and potassium. Silica, with soda and
potash, predominates in the light-colored, acid felsites; iron
oxides, lime, and magnesia, in the dark-colored, basic basalts.
Rock froth forms on the upper part of a lava flow if bubbles
solidify before the gas can escape. Light-colored, glassy froth
is pumice; dark, cindery or slaggy froth, of a coarser texture
than pumice, forms what is known as scoriae. Lava flows which
solidify as a mass of blocks and fragments with a rough surface
are called block lava, or aa; those which solidify with a
smooth, ropy, billowy surface are known as corded lava, or
pahoehoe. Lava can sometimes