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become solid rock. As the lava cools, it often changes colour. Basalt, for example, changes from red when liquid to black when solid.
Basalt is the most common form of lava. There are two contrasting kinds of basaltic lava. Lava that flows slowly
and develops a thick skin and a rough, broken surface is called aa lava (a term borrowed from the Hawaiian language and pronounced ah-ah). Lava can also flow in thin sheets and develop a smooth surface. It is called pahoehoe lava (pronounced pa-howie-howie). In general, the stickier the lava, the rougher the surface it produces.
Although basalt is the most common form of lava, andesite and rhyolite are also found in more acid volcanoes. All of these rocks contain a mixture of silica
and iron-rich minerals such as the pyroxenes. When the proportion of silica to pyroxenes is low, the rock is very dark, develops very small crystals, and when molten, flows like water.
Basalt is the rock containing the least silica (usually less than half), and it flows out to make sheets of lava which may
(Below) This diagram shows
a side view through a lava flow. The aa lava is thick and develops a crust that breaks up as the lava below moves; the more runny pahoehoe lava moves forwards in thin sheets which cool one by one.
The edge of an aa lava flow
Boulders of lava
Lava moving very slowly
Thick crust forms
Pahoehoe lava
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Aa lava
Lava moving fairly quickly
Thin crust forms