Earth's crust, or outermost rocky layer, sits on top of a deeper layer called the mantle, which stores heat from two sources: the formation of the Earth 4.65 billion years ago and the radioactive decay of uranium, thorium, and potassium. When cracks between huge crustal plates open up, the gap causes the underlying mantle to rise up. The upwelling partially melts that region of the mantle; scientists call that decompression melting. The molten rock, or magma, is less dense than solid rock, so it moves upward, the way a cork bobs to the surface of water. As the magma reaches the upper layers of the crust or even Earth's surface, it cools and hardens into a solid known as igneous rock. Scientists categorize igneous rocks according to their chemical composition, the method of their formation, and their degree of crystallization.

The bubbles in a bottle of carbonated beverage do not exist until someone removes the sealed cap, thus releasing the external pressure on the liquid. Likewise, when magma erupts from beneath the Earth's surface, the sudden release of pressure causes dissolved gases to form bubbles within the molten rock. Geologists call these bubbles "vesicles." As the bubbles grow larger, due to decreasing pressure and more gas coming out of the magma, the magma becomes less dense, so it may rise faster and spew out of the ground. Rapid cooling of the lava may solidify it into a porous, cavity-filled rock before all the bubbles have a chance to reach the surface of the liquid and burst. Pumice and scoria are two types of rock rich in vesicles. In some cases, heated groundwater may deposit other minerals such as calcite into empty vesicles after the erupted magma has solidified, so the resulting rock may resemble a cookie filled with nuts. Scientists call this texture "amygdaloidal."