Igneous rocks are solid materials formed from the cooling and crystallization of molten material, known as magma when beneath the Earth's surface, or lava when erupted onto the surface. These rocks are fundamentally classified by the source and rate of their cooling. Intrusive igneous rocks form when magma cools slowly beneath the surface, often resulting in large crystals due to the prolonged cooling period. Conversely, extrusive igneous rocks form when lava cools quickly at the surface, which often yields smaller crystals or a glassy texture. The composition of the original molten material, determined by the relative amounts of silica and other elements, dictates the chemical makeup and physical properties of the resulting rock type.