and basalt, a volcanic igneous rock is the protolith of amphibolite. but geologists have found many more types of metamorphic rocks than protoliths, so factors other than original composition must also play a role in creating these rocks. study of geologic structures, such as folds and faults, suggests that there is a wide range of pressures and temperatures inside growing mountain belts. quite likely, this plays a critical role in explaining variations in metamorphic rocks. laboratory experiments have helped geologists understand metamorphic conditions. the condition under which metamorphism occurs is beneath the level of weathering and sedimentation to form the sedimentary rocks, generally at temperatures greater than 200 degrees and at conditions that do not produce a melt such as goes into igneous rocks. so the range in temperatures are roughly about 200 degrees "c" to about 800 degrees "c." they occur-- the process and formation of the rocks occur at depths generally from 2 to several 10s of kilometers in depth beneath the earth's surface. at the