Geology The Rock Cycle Igneous Rocks Molten magma, produced by plate tectonics, has flowed up from deep within the crust of the Earth. Sometimes it solidifies beneath the crust, in which case it's called intrusive, or plutonic rock. Obsidian is an example of magma that cools very quickly. An example of such a rock is vesicular basalt, show here.
Cavities are the result of gas bubbles in the magma.
If the rock is underground, material surrounding the rock, because of its weight, exerts a force on the rock, producing stress. But at this point in the rock cycle, the igneous rock is buried.