Rock cycle called intrusive or plutonic and cools very slowly, producing a coarse-grained texture such as the rock granite. As a result of volcanic activity, magma (which is called lava when it reaches Earth's surface) may cool very rapidly while being on the Earth's surface exposed to the atmosphere and are called extrusive or volcanic rocks. These rocks are fine-grained and sometimes cool so rapidly that no crystals can form and result in a natural glass, such as obsidian, however the most common fine grained rock would be known as basalt. Any of the three main types of rocks (igneous, sedimentary, and
Pillow lava forming a lobe, until the pressure of the magma becomes sufficient to rupture the skin and start the formation of a new eruption point nearer the vent. This process produces a series of interconnecting lobate shapes that are pillow-like in cross-section. The skin cools much faster than the inside of the pillow, so it is very fine-grained, with a glassy texture. The magma inside the pillow cools slowly, so is slightly coarser grained than the skin, but nevertheless it is still classified as fine grained. Pillow lavas can be used as a way-up indicator in geology; that is, study of