Extrusive rocks cool rapidly because they solidify at Earth's surface. Igneous rocks are also classified by their chemical compositions. Darker than felsic, lighter than mafic. Usually dark-colored, but high olivine content can lend green colors. Other rare colors can be found.
Magma forms from melting rock.
Since rocks are made of different minerals, and different minerals melt at different temperatures, rocks do not melt uniformly. Instead, those minerals whose melting points are below a given temperature will be molten at that temperature, whereas those whose melting points are higher will still be solid. This phenomenon is called partial melting . Melting temperatures can be affected by such environmental conditions as pressure and the presence of water. Generally speaking, higher pressures increase the melting temperature and melting temperatures are lower in the presence of water than in dry conditions.