How can molten rock become new solid rock?

A dark rock can carry the history of something that once flowed. At a volcano, molten material may spill across the ground, develop a dark crust and eventually become part of a new rocky landscape. The rock is not appearing from nothing: material that was molten is becoming solid.

Below Earth's surface, molten rock is called magma. Once it erupts at the surface, it is called lava. Lava transfers thermal energy to its cooler surroundings. Energy travels through the material and into nearby ground, while the surface also loses energy to air and by radiation. As cooling proceeds, minerals can begin to form solid crystals.

The outside cools before the deep interior. A solid crust can therefore cover a flow that is still partly molten underneath. That crust also slows further heat loss, rather like a cover that keeps the inside warm. Thick flows can take much longer to cool throughout than thin ones. A dark surface tells only part of the story.

Cooling also helps shape the rock's texture. When magma cools slowly underground, mineral crystals often have more time to grow large. Faster cooling near the surface commonly produces smaller crystals. Some lava cools into glass with very few crystals. Composition matters too, so cooling speed does not explain every difference between rocks.

Rock made by cooling molten material is called igneous rock. Basalt in a lava flow and granite formed below ground have different histories within that family. Their textures preserve clues to where and how they cooled. Even after becoming solid, a new rock can remain extremely hot: becoming rock and reaching the temperature of its surroundings are two different parts of the same long change.