Final answer:
The three main classes of rock are igneous, sedimentary, and metamorphic. Primitive rock is not one of these classes and is not found on Earth.
Step-by-step explanation:
The three main classes of rock in the Earth's crust include igneous, sedimentary, and metamorphic rocks. Igneous rocks, like basalt and granite, form from the cooling and solidifying of molten material. Sedimentary rocks, such as sandstone and shale, are created by the accumulation and compaction of sediments or by chemical precipitation. Metamorphic rocks, exemplified by schist and marble, are formed through the alteration of other rock types under heat and pressure.
A fourth type, mentioned as primitive rock, is not present on Earth and therefore does not constitute one of the three main rock classes. Earth's geology is characterized by dynamics such as plate tectonics, which leads to the formation of these rocks through various processes like continental drift, mountain building, and volcanic eruptions.