Rock microstructure intrusions are a result of uninterrupted slow cooling. When a rock cools too quickly the liquid freezes into a solid glass, or crystalline groundmass. Often vapor loss from a magma chamber will cause a porphyritic texture. "Embayments" or 'corroded' margins to phenocrysts infer that they were being resorbed by the magma and may imply addition of fresh, hotter magma. Ostwald ripening is also used to explain some porphyritic igneous textures, especially orthoclase megacrystic granites. A crystal growing in a magma adopts a habit (see crystallography) which best reflects its environment and cooling rate. The usual phenocryst habit is the ones
Igneous rock or without suspended crystals and gas bubbles, is called magma. It rises because it is less dense than the rock from which it was created. When magma reaches the surface from beneath water or air, it is called lava. Eruptions of volcanoes into air are termed "subaerial", whereas those occurring underneath the ocean are termed "submarine". Black smokers and mid-ocean ridge basalt are examples of submarine volcanic activity. The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock is produced in the following proportions: Magma that erupts from a volcano behaves according to its