Chapter 8
Metamorphism and Metamorphic Rocks
Metamorphism
 The change of one rock type into another by heat and/or pressure
 Metamorphic rocks are produced from igneous and sedimentary protoliths
 Most common rock type in the continental crust, but often not seen at the surface
 During metamorphism the rock must remain essentially solid. If it progresses to the point of
being largely melted, it is considered igneous.
 Metamorphic settings
 Contact or thermal metamorphism – driven by a rise in temperature within the
host rock
 Hydrothermal metamorphism – chemical alterations from hot water
 Regional metamorphism
 Occurs during mountain building
 Produces the huge volumes of metamorphic rock
Agents of metamorphism
 Heat
• Most important agent
• Recrystallization results in new, stable minerals
• Two sources of heat
• Contact metamorphism – heat from magma
• An increase in temperature with depth due to the geothermal gradient
 Pressure and differential stress
• Increases with depth
• Confining pressure applies forces equally in all directions
• Rocks may also be subjected to differential stress which is unequal in different
directions
 Chemically active fluids
• Mainly water with other volatile components
• Enhances migration of ions
• Aids in recrystallization of existing mineral
• Sources of fluids
• Hydrated minerals such as clays and micas
• Fluids purged from crystallizing melts
• Metasomatism
• Dissolved ions migrate with fluids and react
 The importance of parent rock
• Most metamorphic rocks have the same overall chemical composition as the
parent rock from which they formed
Metamorphic textures
 Foliation
• Examples of foliation
 Parallel alignment of flattened mineral grains and pebbles
 Compositional banding
 Slaty cleavage where rocks can be easily split into thin, tabular sheets
 Foliated textures
• slaty cleavage (slates): fissile, aphanitic
•
Schistosity (schists): fissile, micaceous or parallel-aligned elongated minerals,
phaneritic
• Gneissic (gneiss): not fissile, phaneritic
 Other metamorphic textures
• Those metamorphic rocks that lack foliation are referred to as nonfoliated
 Quartzite
 Marble
• Porphyroblastic textures
 Large grains, called porphyroblasts, surrounded by a fine-grained matrix
of other minerals
 Foliated rocks
• Slate – dull, aphanitic, from shale or mudstone
• Phyllite – shiney, aphanitic, from shale or mudstone
• Schist – phaneritic, from shale or mudstone, and sometime other protoliths
• Gneiss – phaneritic, generally high in grade, phaneritic, from many protoliths.
Can be banded, but isn’t always.
 Common metamorphic rockNonfoliated rocks
• Marble
• Coarse, crystalline
• Parent rock was limestone or dolostone
• Composed mainly of calcite or dolomite crystals
• Used as a decorative and monument stone
• Exhibits a variety of colors
 Nonfoliated rocks
• Quartzite
• Formed from a parent rock of quartz-rich sandstone
• Quartz grains are fused together
Hornfels
• Non-foliated rock from shale or mudstone that forms in contact aureoles
Metamorphic environments
 Contact or thermal metamorphism
• Results from a rise in temperature when magma intrudes country rock
• A zone of metamorphism called an aureole forms in the rock surrounding the
magma
• Most easily recognized when it occurs at the surface, or in a near-surface
environment
 Regional metamorphism
• Produces the greatest quantity of metamorphic rock
• Associated with mountain building
Metamorphic zones
 Index minerals and metamorphic grade
• Certain minerals, called index minerals, are good indicators of the metamorphic
conditions in which they form
• Migmatites
• Highest grades of metamorphism that is transitional to igneous rocks
• Contain light bands of igneous components along with areas of unmelted
metamorphic rock
•
Figure 8.25
Metamorphic facies
 Based on metamorphism of basalts: greenschist, blueschist, amphibolite, granulite, eclogite
Metamorphism and plate tectonics
 Most metamorphism occurs along convergent plate boundaries
• Faulting thickens the continental crust and deeply buries sediments
• Formation of Earth’s major mountain belts including the Alps, Himalayas, and
Appalachians
 Other metamorphic environments
•
Burial metamorphism
 Associated with very thick sedimentary strata
• Deformation along shear zones
 Occurs at depth and high temperatures
 Pre-existing minerals deform by ductile flow
 Other metamorphic environments
• Impact metamorphism
 Occurs when meteorites strike Earth’s surface
 Products are called impactites