Lava may seep slowly out and cool quickly, or it may burst forth violently and pour at the rate of 40 miles an hour over fields, forests and villages.
The lava hardens in swirls and ripples like petrified molasses. It may pour over a cliff and harden into a solid waterfall. If the surface of lava cools and hardens while that beneath continues to flow, lava caves are created. Lava stalactites and stalagmites, twisted, corded and very fragile, may be found within the caves.
Rocks formed from lava are called extrusive igneous rocks. Obsidian is a lava that has cooled so rapidly that it has not crystallized but has become a glassy substance called "volcanic glass."
When lava that is filled with vapor or gas hardens rapidly, many tiny pores remain in the rock after the disengagement of the vapors. This rock is a kind of frothy obsi-dian that will float on water and is called pumice.
Lavas that contain less silica form a fine grained black rock called basalt, and light colored fine grained volcanic rock is called felsite.
