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Lava streams that plunge over cliffs or the steep walls of craters form
impressive lava cascades or lava falls. Where cascades spill into preexisting
craters, lava lakes may be formed. Such lakes are considered inactive and
generally form a solid crust within hours or a few days. The still molten lava
beneath this crust then takes weeks to years, depending on lake size, to cool and
solidify completely. Lava lakes formed at the site of, and sustained by, active
eruptive vents are considered active. The crust formed on these lakes is not
permanent and breaks up in response to circulation and sloshing of the underlying
molten lava. Repeated overflows from active lava lakes raise their level by
rampart construction similar to overflowing lava streams. By this process of
levee growth, lakes may become perched many feet above their surroundings.
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