Extrusion
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Sometimes magma can force itself through a crack or fault in the rock at the Earth's surface. It pours out over the Earth's surface in a volcanic eruption. This process is called extrusion.
The rocks that form from extruded magma are called extrusive igneous rocks. Basalt and pumice are extrusive igneous rocks. The type of rock that forms depends on the magma it came from, but generally extrusive igneous rocks:
- are very fine grained - magma cools very quickly when it erupts onto the Earth's surface and the crystals in the rock don't have much time to grow
- may contain gas bubbles.
