Recognizing Rocks: Igneous, Sedimentary, and Metamorphic Types - Geology - page 101

The pressure and heat of two plates crashing into each other causes dynamo-
thermal metamorphism.
Both burial and dynamothermal metamorphism affect large areas of crustal
rocks and are considered regional metamorphism. Contact metamorphism, on the
other hand, is very local and affects only the rocks immediately surrounding the
heated materials.
Grades of metamorphism and index minerals
As parent rocks are exposed to heat and pressure, they begin to change. The degree of
change depends on the levels of heat and pressure they experience. The resulting meta-
morphic rocks are described by their degree of change, or metamorphic grade :
Low-grade metamorphic rocks retain characteristics of the parent rock. If they
are sedimentary rocks, they may still show signs of bedding planes or other struc-
tures. Low-grade metamorphic rocks have been exposed to relatively low temper-
atures and pressures.
High-grade metamorphic rocks look very different from their parent rock. Rocks
exposed to very high levels of heat and pressure change dramatically; their intern-
al structure no longer resembles the original rock.
In regional metamorphism, large areas of crustal rocks are being subducted or buried
and changed. The rocks deeper in the crust are subjected to higher temperatures and
pressures than the rocks closer to the surface. The result is that across a region you see
rocks of different metamorphic grades corresponding to the increasing degree of meta-
morphism.
Metamorphic grades are identified by the minerals in the rock because certain minerals
— called index minerals — form only under certain conditions of temperature and pres-
sure. Figure 7-14 illustrates the different minerals formed as the sedimentary rock shale
moves from low-grade metamorphism to high-grade metamorphism. At some point, high
enough levels of heat will cause the minerals to melt, resulting in magma and (eventu-
ally) an igneous rock instead of a metamorphic rock.