What is an igneous rock?

An igneous rock is defined as a type of rock that is formed from the cooling and solidification of molten rock, known as magma when it is beneath the Earth's surface and lava when it is above ground. This process can occur either underground, where the magma cools slowly, leading to the formation of coarse-grained igneous rocks like granite, or at the surface during volcanic eruptions, resulting in fine-grained rocks like basalt.

The characteristics of igneous rocks, including their mineral composition and texture, are directly influenced by the rate at which the molten rock cools. Rapid cooling tends to produce small crystals, while slower cooling allows for larger crystals to form. This fundamental process distinguishes igneous rocks from other types, such as sedimentary rocks, which are formed from the accumulation and compaction of sediments, and metamorphic rocks, which arise from the alteration of existing rocks due to heat and pressure.