Lecture 6 Metamorphic Lecture 6 Metamorphic RocksRocks

What are metamorphic rocks?What are metamorphic rocks? Metamorphic processesMetamorphic processes Texture of metamorphic rocksTexture of metamorphic rocks Types of metamorphic rocksTypes of metamorphic rocks Engineering considerations of metamorphic Engineering considerations of metamorphic

rocksrocks Geologic rock cycleGeologic rock cycle

Stephen Marshak

(a) This thin section of a limestone shows small fossil shells distributed in a matrix of lime mud. (b) This thin section of marble (same composition as limestone) shows interlocking grains. Atoms have been completely re-arranged. (Yong II Lee)

(a)

(b)

What are metamorphic rocks?What are metamorphic rocks? Metamorphic rocksMetamorphic rocks form from preexisting rocks (igneous, sedim form from preexisting rocks (igneous, sedim

entary, or other metamorphic rocks) through the action of heat aentary, or other metamorphic rocks) through the action of heat and pressure. This process of the transformation of one rock type ind pressure. This process of the transformation of one rock type into another is called nto another is called metamorphismmetamorphism (Greek: "changed form"). (Greek: "changed form").

Metamorphism most often occurs deep within earth. Under increMetamorphism most often occurs deep within earth. Under increased temperature and pressure, the minerals of preexisting rockased temperature and pressure, the minerals of preexisting rocks become unstable and recrystallize in a solid state to become nes become unstable and recrystallize in a solid state to become new minerals.w minerals.

Study of metamorphic rocks yields valuable information about Study of metamorphic rocks yields valuable information about metamorphic conditions on rock and about the geologic history metamorphic conditions on rock and about the geologic history of a region.of a region.

This metamorphic rock, California exhibits a slaty cleavage. The typThis metamorphic rock, California exhibits a slaty cleavage. The type of rock cleavage allows it to split easily in the flat plates visible in te of rock cleavage allows it to split easily in the flat plates visible in this photo. Photo by E.J. Tarbuck.his photo. Photo by E.J. Tarbuck.

Metamorphic processesMetamorphic processes HeatHeat and and pressure (stresspressure (stress) are the primary agents of metamorp) are the primary agents of metamorp

hism.hism.

HeatHeat: Heat provides the energy to drive the chemical changes th: Heat provides the energy to drive the chemical changes that result in recrystallization of minerals.at result in recrystallization of minerals.

Where does heat come from to cause metamorphism? One way iWhere does heat come from to cause metamorphism? One way is the intrusion by s the intrusion by hot magmahot magma. In effect, the surrounding rock is. In effect, the surrounding rock is "baked" by the high temperature of the molten magma. This kin "baked" by the high temperature of the molten magma. This kind of metamorphism is called d of metamorphism is called contact metamorphismcontact metamorphism..

Another important way to get heat is Another important way to get heat is deep burialdeep burial. Temperature i. Temperature increases about 15 to 30 degrees for each kilometer of depth in thncreases about 15 to 30 degrees for each kilometer of depth in the crust (e crust (geothermal gradientgeothermal gradient). Gradual burial in a sedimentary b). Gradual burial in a sedimentary basin can bury rocks formed at the surface to several kilometers.asin can bury rocks formed at the surface to several kilometers.

Contact metamorphism occurs around hot magma intrusions. Contact metamorphism occurs around hot magma intrusions. Increases in temperature and inclusion of pore fluids cause Increases in temperature and inclusion of pore fluids cause preexisting minerals to form new minerals.preexisting minerals to form new minerals.

Metamorphic Processes (continued)Metamorphic Processes (continued) PressurePressure: An increase in pressure reduces mineral space and drive c: An increase in pressure reduces mineral space and drive c

hemical reactions that produced new minerals with closer atomic phemical reactions that produced new minerals with closer atomic packing and higher density.acking and higher density.

Pressure increases with depth inside solid earth much like pressure Pressure increases with depth inside solid earth much like pressure increases with depth in water. Tectonic processes (such as subductiincreases with depth in water. Tectonic processes (such as subduction and continental collision) can bury rocks to tens of kilometers. In on and continental collision) can bury rocks to tens of kilometers. In this case, metamorphism can occur over large areas and is called this case, metamorphism can occur over large areas and is called reregional metamorphism.gional metamorphism.

Regional metamorphism also occurs during Regional metamorphism also occurs during mountain buildingmountain building wh when great volume of rocks are subjected to directed stress. The greaten great volume of rocks are subjected to directed stress. The greatest volume of metamorphic rocks are best exposed in the deformed est volume of metamorphic rocks are best exposed in the deformed mountain belts and in ancient stable continental interiors known as mountain belts and in ancient stable continental interiors known as shieldsshields, such as the Canadian Shield. Shields are assumed to be the , such as the Canadian Shield. Shields are assumed to be the remnants of ancient periods of mountain building.remnants of ancient periods of mountain building.

Contact metamorphism and regional metamorphism are the two mContact metamorphism and regional metamorphism are the two main processes of metamorphism.ain processes of metamorphism.

A. Buried rocks are subject to pressure from the load A. Buried rocks are subject to pressure from the load above. B. During mountain building, rocks are subject to above. B. During mountain building, rocks are subject to directional stress that shortens and deforms rock strata. directional stress that shortens and deforms rock strata. In these cases, metamorphism can occur over large In these cases, metamorphism can occur over large areas and is called areas and is called regional metamorphism.regional metamorphism.

Metamorphic rocks are widely distributed in continental Metamorphic rocks are widely distributed in continental shields (such as the Canadian Shield) and in the cores of shields (such as the Canadian Shield) and in the cores of folded mountain belts.folded mountain belts.

Metamorphic gradeMetamorphic grade

Metamorphism occurs incrementally, from slight Metamorphism occurs incrementally, from slight change (low grade) to dramatic change (high change (low grade) to dramatic change (high grade) as the intensity of heat and pressure grade) as the intensity of heat and pressure

increasesincreases..

Textures of metamorphic rocks: How Textures of metamorphic rocks: How metamorphism changes rocks?metamorphism changes rocks?

FoliationFoliation

During deformation where stresses are not uniformly oriented, mDuring deformation where stresses are not uniformly oriented, many metamorphic rocks develop textures in which the mineral grany metamorphic rocks develop textures in which the mineral grains have strongly preferred orientations in the direction of least ains have strongly preferred orientations in the direction of least stress. The resulting mineral alignment often gives the rock a laystress. The resulting mineral alignment often gives the rock a layered or banded texture called ered or banded texture called foliationfoliation (Latin: "splitting into lea (Latin: "splitting into leaf-like layers").f-like layers").

Depending on the metamorphism grade and parent rocks, the tyDepending on the metamorphism grade and parent rocks, the types of foliation include pes of foliation include slatyslaty cleavage, cleavage, schistosityschistosity, and , and gneissicgneissic texture.texture.

Foliation. Under directed stress, elongated minerals become reorienteFoliation. Under directed stress, elongated minerals become reoriented or recrystallized resulting in alignment along the direction perpendicd or recrystallized resulting in alignment along the direction perpendicular to the stress. Under intense metamorphism, a granite (left) could tular to the stress. Under intense metamorphism, a granite (left) could transform to gneiss with a foliation of alternating light- and dark-coloreransform to gneiss with a foliation of alternating light- and dark-colored bands known as gneissic texture.d bands known as gneissic texture.

Foliated Metamorphic RocksFoliated Metamorphic Rocks Names for foliated metamorphic rocks are typically Names for foliated metamorphic rocks are typically based on thbased on th

eir foliated textureseir foliated textures..

SlateSlate: If the cleavage planes are very thin and the rock is fine grai: If the cleavage planes are very thin and the rock is fine grained, the cleavage is called slaty cleavage and the rock is called slned, the cleavage is called slaty cleavage and the rock is called slate. Slate is usually produced by low-grade metamorphism of shate. Slate is usually produced by low-grade metamorphism of shale under directed pressure and low temperature.ale under directed pressure and low temperature.

SchistSchist: The grains in a schist are coarser than in slate and the sur: The grains in a schist are coarser than in slate and the surface of foliation planes are relatively rough.face of foliation planes are relatively rough.

GneissGneiss is a coarse-grained rock with coarse light- and dark-color is a coarse-grained rock with coarse light- and dark-colored bands. Gneiss forms under high-grade metamorphism from gred bands. Gneiss forms under high-grade metamorphism from granite or diorite and other rocks.anite or diorite and other rocks.

Slate Slate is a fine-grained foliated metamorphic rock with slaty (very is a fine-grained foliated metamorphic rock with slaty (very thin) rock cleavage. Slate is usually produced by low-grade metamothin) rock cleavage. Slate is usually produced by low-grade metamorphism of shale under directed pressure and low temperature.rphism of shale under directed pressure and low temperature.

(a) Note the bedding plane is not necessarily parallel to the cleavage. (b) Slate easily splits into thin sheets, which have been used as roof shingles for millennium. (S. Marshak)

Compression of a bed end-on to create slaty cleavage perpendicular to the bedding. (W.W. Norton)

Schist Schist is a strongly foliated rock with abundant platy and elongais a strongly foliated rock with abundant platy and elongated minerals (muscovite, biotite, …) that can be readily split into thited minerals (muscovite, biotite, …) that can be readily split into thin flakes.n flakes.

Gneiss Gneiss is a coarse-grained rock with coarse light- and is a coarse-grained rock with coarse light- and dark-colored bands. Gneiss forms under high-grade dark-colored bands. Gneiss forms under high-grade metamorphism from granite or diorite and other rocks.metamorphism from granite or diorite and other rocks.

Nonfoliated metamorphic rocksNonfoliated metamorphic rocks Not all metamorphic rocks have a foliated texture. Metamorphic Not all metamorphic rocks have a foliated texture. Metamorphic

rocks composed of only one mineral having equidimensional cryrocks composed of only one mineral having equidimensional crystals usually are not visibly foliated. Mineral composition forms tstals usually are not visibly foliated. Mineral composition forms the basis for naming nonfoliated metamorphic rocks.he basis for naming nonfoliated metamorphic rocks.

MarbleMarble is a coarse, crystalline metamorphic rock composed alm is a coarse, crystalline metamorphic rock composed almost entirely of calcite or dolomite.ost entirely of calcite or dolomite.

QuartziteQuartzite is a nonfoliated metamorphic rock formed from quart is a nonfoliated metamorphic rock formed from quartz sandstone.z sandstone.

MarblesMarbles are coarse, crystalline metamorphic products of heat a are coarse, crystalline metamorphic products of heat and pressure acting on limestones and dolomites. nd pressure acting on limestones and dolomites.

The marble in this unfinished sculpture by Michelangelo is fairly soft and easy to carve, but does not crumble. (S. Marshak)

Quartzite Quartzite is a nonfoliated metamorphic rock derived from quais a nonfoliated metamorphic rock derived from quartz sandstone.rtz sandstone.

Transition from quartz sandstone to quartzite. In a quartzite, the grains have grown. If the quartzite cracks, the crack ignores the grain boundaries. (W.W. Norton)

Engineering considerations of Engineering considerations of metamorphic rocksmetamorphic rocks

Foliated rocks possess prominent Foliated rocks possess prominent directional directional propertiesproperties. The strength is much weaker in the . The strength is much weaker in the direction of the foliation than in other directions. direction of the foliation than in other directions. Care should be taken that loads (bridges, dams, Care should be taken that loads (bridges, dams, buildings) are not transferred to foliated buildings) are not transferred to foliated directions. In tunnel construction, foliated directions. In tunnel construction, foliated metamorphic rocks are generally more costly metamorphic rocks are generally more costly because of more steel supports needed.because of more steel supports needed.

Geologic Rock CycleGeologic Rock Cycle

The types of rocks we talked about can be The types of rocks we talked about can be transformed from one type to another. The transformed from one type to another. The various processes of the rock cycle provide an various processes of the rock cycle provide an excellent demonstration of the Earth as a excellent demonstration of the Earth as a dynamic system.dynamic system.

The rock cycleThe rock cycle