The speed that molten rock cools depends on the type of eruption and many environmental factors, and the varying speeds can form different kinds of rock. For example, pumice is lava that has hardened as it is still flying through the air or frothing from an eruption, and all the air in and around it makes it porous. When lava hits the air and quickly cools, it forms the glassy rock obsidian. Sometimes magma can cool and harden inside the earth without ever erupting; this takes a long time and the result is a very hard rock called granite.
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