As we commonly know, rocks are classified as igneous rocks, metamorphic rocks, and sedimentary rocks. Igneous rocks are rocks that have been melted, and are formed when a hot molten rock cools, from magma. These rocks melt in the upper mantle of the Earth and below the crust. The igneous rocks are rich in minerals mainly silicates, like feldspars, that are silicates of aluminum, or potassium (alkaline feldspars) or rich in sodium and calcium like in the plagioclase feldspars. This magma that moves upwards towards the surface of the Earth in the mantle could cool underground, forming intrusive rocks, and when magna, like after expelled by a volcano, is lava that will form extrusive rocks. The molten rock will solidify in different ways, depending on the rate of loss of heat. The magma in the mantle and deep in the crust, will cool down much slower that the lava that comes out from a volcano; this rate of cooling will give rocks different forms and structure. If the cooling is very fast, it can produce rocks that look like glass with no crystal forms, like obsidian.
Metamorphic rocks are modified by high temperature and pressure do to the forces acting on the crust, and the interior of the Earth. These rocks are chemically combined with many ingredients that are then given the names to these rocks accordingly, like marble, gneiss and slate, for some examples.
Sedimentary rocks are the result of the erosion by water (rain, ice) and air (winds). These can only happen in planets that have an atmosphere were winds can form, and flowing water that can transport the rocks over the surface. Shale and sandstone are the most abundant sedimentary rocks on the surface of the Earth, in a proportion of 70% shale and 20% sandstone.
But what about primitive rocks? These rocks were not transformed inside planets by pressure and temperature changes as we just read for the igneous and metamorphic rocks. There primitive or original rocks are common in asteroids and what we can see in the meteorites.
Geologists also teach us about breccias, rocks that are broken that show sharp edges glued together by high temperature and pressure, but not from the interior of the Earth, but from impacts that can be discover in the interior of craters.
AstroDidactics 