If the lava is very hot and has a low viscosity (runny with a low gas and silica content) the lava flow is called Pahoehoe. If, on the other hand, the lava has a high viscosity (thick and pasty with a high gas and silica content) it is called Aa.

Silica is a white or colorless crystal that is present in sand and quartz. It is one of the most abundant compounds in the Earth's crust.

The photograph shows a pahoehoe flow on the left and an aa flow on the right.

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