Igneous Rocks
Formed from magma or lava, igneous rocks can be classified according to their composition. This classification specially takes into account: the relative proportion of silica, magnesium, and iron minerals found in these type of rocks; their grain size (which reveals how fast they cooled); and their color. Rocks that contain silica, along with much quartz and feldspar, tend to have pale colors; those with low silica content have dark colors created by iron and magnesium-containing minerals, such as olivine, pyroxene, and amphiboles. A rock's texture is determined by the configuration of its crystal grains.
Granite - This rock is formed by big grains of feldspar, quartz, and mica. Its lightcolored components indicate an abundance of silicon and that the rock is acidic. Because of its great resistance to wear, granite is often used as a construction material. THE MINIMUM DEPTH AT WHICH GRANITE FORMS - 1 mile(1.6 km)
Gabro - This rock contains ferromagnesian minerals, such as olivine, pyroxene and augite, which form dark-colored crystallizations, and feldspars, which give a white coloring to some of its parts. Gabbro generally solidifies slowly, leaving it with thick grains.
Basalt - This rock forms most of the oceanic crust. Its low silicon content gives it its characteristic dark color (between blue and black). Its rapid cooling and solidification gives it a very fine grain. Because of its hardness, it is used to build roads; it is not, however, used to make paving stones because it is too slippery.
Granodiorite - This rock is often confused with granite, but it is grayer since it contains larger numbers of quartz and sodic plagioclase crystals than it does feldspar. It has thick grains and contains dark crystals called nodules.