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.

You can tell a lot about the history of igneous rocks by looking at the size of crystals within them. Rocks that cool quickly contain small crystals, while slow-cooled rocks are filled with large crystals. When magma erupts at the Earth's surface, heat radiates out from the lava allowing it to cool rapidly and the atoms and molecules do not have time to grow into large crystals before the lava solidifies. The resulting rock has such small crystal grains that humans have a difficult time distinguishing them, even with a handheld lens. Geologists describe the texture of these fine-grained igneous rocks as "aphanitic," from the Greek word meaning "unseen." Deep inside the Earth's crust, the magma cools much more slowly because the surrounding rocks insulate the magma from rapid heat loss. This allows the crystals to grow into mineral grains that are easier for humans to see. Geologists describe the resulting coarse-grained rock texture as phaneritic, from the Greek word meaning "visible." Some igneous rocks contain crystals that are much larger than the crystals in the matrix surrounding them. Scientists call these specimens, which resemble a chocolate-chip cookie, porphyritic rocks, and the larger crystals are called phenocrysts. The phenocrysts had started to form within the magma before it later cooled rapidly, probably due to that magma erupting at a volcano.