One of the three basic types of rock that also include sedimentary and metamorphic. Igneous rocks are formed by the solidification of magma and comprised predominately of silicate minerals. Based on bulk chemical analysis, igneous rocks can be grouped into four major groups based on their SiO2 content: 1. Felsic: composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling Molten silicate (rock) beneath the surface of a planetary body or moon. When it reaches the surface, magma is called lava. chamber.