Lecture 6 Metamorphic Rocks What are metamorphic rocks? Metamorphic processes Texture of metamorphic rocks Types of metamorphic rocks Engineering considerations of metamorphic rocks Geologic rock cycle

Stephen Marshak

(a) (b) (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)

What are metamorphic rocks? Metamorphic rocks form from preexisting rocks (igneous, sedimentary, or other metamorphic rocks) through the action of heat and pressure. This process of the transformation of one rock type into another is called metamorphism (Greek: "changed form"). Metamorphism most often occurs deep within earth. Under increased temperature and pressure, the minerals of preexisting rocks become unstable and recrystallize in a solid state to become new minerals. Study of metamorphic rocks yields valuable information about metamorphic conditions on rock and about the geologic history of a region.

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

Metamorphic processes Heat and pressure (stress) are the primary agents of metamorphism. Heat: Heat provides the energy to drive the chemical changes that result in recrystallization of minerals. Where does heat come from to cause metamorphism? One way is the intrusion by hot magma. In effect, the surrounding rock is "baked" by the high temperature of the molten magma. This kind of metamorphism is called contact metamorphism. Another important way to get heat is deep burial. Temperature increases about 15 to 30 degrees for each kilometer of depth in the crust (geothermal gradient). Gradual burial in a sedimentary basin 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 preexisting minerals to form new minerals.

Metamorphic Processes (continued) Pressure: An increase in pressure reduces mineral space and drive chemical reactions that produced new minerals with closer atomic packing and higher density. Pressure increases with depth inside solid earth much like pressure increases with depth in water. Tectonic processes (such as subduction and continental collision) can bury rocks to tens of kilometers. In this case, metamorphism can occur over large areas and is called regional metamorphism. Regional metamorphism also occurs during mountain building when great volume of rocks are subjected to directed stress. The greatest volume of metamorphic rocks are best exposed in the deformed mountain belts and in ancient stable continental interiors known as shields, such as the Canadian Shield. Shields are assumed to be the remnants of ancient periods of mountain building. Contact metamorphism and regional metamorphism are the two main processes of metamorphism.

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

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

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

Textures of metamorphic rocks: How metamorphism changes rocks? Foliation During deformation where stresses are not uniformly oriented, many metamorphic rocks develop textures in which the mineral grains have strongly preferred orientations in the direction of least stress. The resulting mineral alignment often gives the rock a layered or banded texture called foliation (Latin: "splitting into leaf-like layers"). Depending on the metamorphism grade and parent rocks, the types of foliation include slaty cleavage, schistosity, and gneissic texture.

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

Foliated Metamorphic Rocks Names for foliated metamorphic rocks are typically based on their foliated textures. Slate: If the cleavage planes are very thin and the rock is fine grained, the cleavage is called slaty cleavage and the rock is called slate. Slate is usually produced by low-grade metamorphism of shale under directed pressure and low temperature. Schist: The grains in a schist are coarser than in slate and the surface of cleavage planes are relatively rough. Gneiss is a coarse-grained rock with coarse light- and dark-colored bands. Gneiss forms under high-grade metamorphism from granite or diorite and other rocks.

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

(a) Note the bedding plane is not necessarily parallel to the cleavage (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 is a strongly foliated rock with abundant platy and elongated minerals (muscovite, biotite, …) that can be readily split into thin flakes.

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

Nonfoliated metamorphic rocks Not all metamorphic rocks have a foliated texture. Metamorphic rocks composed of only one mineral having equidimensional crystals usually are not visibly foliated. Mineral composition forms the basis for naming nonfoliated metamorphic rocks. Marble is a coarse, crystalline metamorphic rock composed almost entirely of calcite or dolomite. Quartzite is a nonfoliated metamorphic rock formed from quartz sandstone.

Marbles are coarse, crystalline metamorphic products of heat and 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 is a nonfoliated metamorphic rock derived from quartz sandstone.

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

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

The rock cycle