The most common type of rock found on Earth's surface is igneous rock, formed from the cooling and solidification of molten rock. These rocks can form either below the surface as intrusive igneous rocks, which cool slowly and have large crystals, or above ground as extrusive igneous rocks, which cool quickly and have smaller crystals. Examples include basalt and granite. Igneous rocks play a crucial role in understanding Earth's geological history and are fundamental in the rock cycle.