GLG 101

Chapter 7

Metamorphic Rocks

Def.: rocks derived from preexisting rocks through the application of heat, pressure, and fluid activity; mineral composition and/or texture is changed; time is also a factor

Heat: increases rate of chemical reactions: source: magma, geothermal gradient

Pressure:

lithostatic pressure: weight of the overlying rocks applied equally in all directions

differential pressure: applied only in one direction (e.g. during mountain building)

Fluid activity: mostly water and carbon dioxide, with some ions in solution

Types of metamorphism:

Contact metamorphism

occurs when a body of magma alters the surrounding country rock; temperature and fluid content of magma are the most important factors

=> produces aureoles (concentric zones around the intrusion)

Dynamic metamorphism

mostly associated with fault zones where rocks are subjected to high differential pressures

=> produces mylonites (type of metamorphic rock)

Regional metamorphism

occurs over large areas; tremendous temperatures, pressures, and deformation within deeper portions of the crust; most obvious along convergent plate boundaries, also at divergent plate boundaries; mineral changes occur => index minerals for certain environments of heat and pressure

Classification of metamorphic rocks

Foliated metamorphic rocks

Texture: minerals are arranged in a parallel fashion; results from regional metamorphism 

-Slate: very fine grained with slaty cleavage, result of low-grade regional metamorphism

-Phyllite: coarser grained, glossy or lustrous sheen

-Schist: minerals visible, platy; exhibits schistosity or schistose foliation; wavy type of parting when split

-Gneiss: segmented bands of light and dark minerals; high-grade regional metamorphism; large grain size of minerals

-Migmatite: mix of igneous and metamorphic rocks due to partial melting of rock

Non-foliated (massive) metamorphic rocks

Texture: grains do not show preferred orientation; results from contact or regional metamorphism

-Marble: consists of calcite; is metamorphosed limestone

-Quartzite: consists of quartz; is metamorphosed quartz sandstone

-Greenstone: compact, dark-green, mafic rock formed under low- to high-grade metamorphic conditions; metamorphosed basalt

-Hornfels: fine grained, resulting from contact metamorphism