Section 3 Metamorphic Rocks

Abstract

Metamorphic rocks are any rocks that have been derived from pre-existing rocks by mineralogical, chemical, and/or structural changes in response to marked changes in temperature, pressure, shearing stress, and chemical environment, generally at depth in the earth’s crust (Dictionary of Geological Terms, AGS). In contrast to intrusive magmas metamorphic rocks are foliated or massive but contain no strong joint systems (Fig. 45). Five Units and eight Variants are ordered in contexts of two Groups: R – Cratonic Units that occur in a continental area that has been little deformed since Precambrian time (Figs. 30 to 32) J – Non-cratonic Units that occur in orogenic (mountain chains) belts, around the boundaries of intrusive igneous rocks, and in other tectonic terrains (Figs. 33, 34).