How is volcanic rock defined in terms of its formation?

Volcanic rock is defined as being formed from magma that cools and solidifies on or very near the Earth’s surface. This process occurs when volcanic eruptions bring molten rock (magma) to the surface, where it quickly cools upon exposure to the atmosphere or water. This rapid cooling often leads to the formation of fine-grained textures, as the crystals do not have much time to grow.

The formation environment is crucial in distinguishing volcanic rock from other types of igneous rock, particularly intrusive or plutonic rocks, which cool slowly beneath the Earth's surface, allowing for larger crystal formation. The characteristics and mineral composition of volcanic rocks can vary widely based on the composition of the magma and the cooling environment, but their defining trait remains their surface formation.