Rock Group Classification

Rocks are grouped into families (igneous, sedimentary, and metamorphic) based on their overall mode of formation. Rocks can also be grouped into subfamilies based on place and style of formation (intrusive, extrusive-as lava flows, and eruptive-as tephra clouds igneous), on precursor earth materials (clastic, chemical,and organic accumulation sedimentary), and on the presence or absence of a feature (foliated and nonfoliated metamorphic). The various groupings, then, give clues to some of the details of the formation process.