Diorite is a felsic igneous rock. Gabbro is a mafic igneous rock. Because it is heavier than the other mafic rocks, it is classed as an ultramafic rock . In general, the felsic rocks are less dense. Density refers to the quantity of matter contained in a unit volume , and is usually measured in weight per volume, like so many pounds per cubic foot.
Mafic rocks are denser, with ultramafic rocks being the most dense.
Magma that reaches the surface and solidifies forms extrusive igneous rock , and is called extrusion . Intrusive igneous rocks cool slowly and therefore more time is allowed for mineral grains to group together. Therefore, these intrusive rocks are usually coarse-textured , meaning they are composed of large grains of minerals within the rock which are visible to the eye. A good example is granite. Extrusive rocks cool very quickly, and therefore little time is allowed for silicate minerals to group or clump together.