Molten rock or magma that flows on the surface of
the Earth. Lavas have a wide range in composition, texture,
temperature, viscosity, and other physical properties, based
on the composition of the melt and the amount of volatiles
present. They tend to be very viscous (sticky, resistant to
flow) when they are rich in silica, and they form slow-moving
and steep-sided flows. If a large amount of volatiles are
added to silicic magma it may erupt explosively. Mafic, or
low-silica, lavas are less viscous and tend to flow more easily,
forming planar flows with gently sloping surfaces. Some
basaltic flood lavas have flowed over hundreds or thousands
of square kilometers, forming flat-lying layers of crystallized
lava. Other mafic and intermediate lavas form shield volcanoes
such as the Hawaiian Islands, with gently sloping sides
built by numerous eruptions. If mafic lavas are rich in
volatiles they tend to become abundant in empty gas bubbles
known as vesicles, forming pumice. Mafic lavas that flow on
the surface often form ropey lava flows known as pahoehoes,
or blocky flows known as aa lavas.
See also AA LAVA; IGNEOUS ROCKS; VOLCANO.
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