Metamorphic Rocks

Dr. Pamela Gore
Georgia Perimeter College

Objectives

Identify metamorphic rocks by their physical and chemical properties.
Explain the formation of metamorphic rocks in terms of the rock cycle.
Discuss several ways in which metamorphic rocks form.
Explain what is meant by the term "parent rock" and be able to give examples of metamorphic rocks and their parent rocks.
List and describe the characteristics of foliated metamorphic rocks, including textures and composition.
List and describe the characteristics of non-foliated metamorphic rocks, including textures and composition.

This section addresses, in whole or in part, the following Georgia GPS standard(s):

S6E5b. Classify rocks by their process of formation.
S6E5c. Describe processes that change rocks and the surface of the Earth.

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:

Rock is composed of different combinations of minerals. Smaller rocks come from the breakage and weathering of bedrock and larger rocks. Soil is made partly from weathered rock, partly from plant remains-and also contains many living organisms.
Sedimentary rock buried deep enough may be reformed by pressure and heat, perhaps melting and recrystallizing into different kinds of rock. These re-formed rock layers may be forced up again to become land surface and even mountains. Subsequently, this new rock too will erode. Rock bears evidence of the minerals, temperatures, and forces that created it.

This section addresses, in whole or in part, the following National Science Education Standards:

Some changes in the solid earth can be described as the "rock cycle." Old rocks at the earth's surface weather, forming sediments that are buried, then compacted, heated, and often recrystallized into new rock. Eventually, those new rocks may be brought to the surface by the forces that drive plate motions, and the rock cycle continues.

Introduction to Metamorphism

Metamorphism means "changed form". Metamorphic rocks are important because they remind us that rocks are changeable. The rock cycle describes how one type of rock can be converted into another. Metamorphic rocks are formed primarily by the action of heat and/or pressure on other types of rocks.

In many parts of the U.S., metamorphic rocks are deeply buried beneath layers of sedimentary rocks. In Georgia, however, metamorphic rocks are exposed throughout much of the central and northeastern part of the state in the Piedmont and Blue Ridge physiographic provinces.

Metamorphic processes are difficult to observe because they generally occur deep within the Earth, unlike the deposition of sediment or the eruption of volcanic igneous rocks. Much of what we know about metamorphic processes is the result of laboratory experiments and theoretical models that simulate conditions deep within the Earth.

Metamorphic rocks, and the minerals found within them, are economically value. They include marble and soapstone for carving, slate for billiard tables and roofing, various types of building stone, crushed stone for road construction, asbestos for fire-retardant materials, talc for talcum powder, graphite for lubricants and pencil "leads", garnet for abrasives, and a number of types of semi-precious stones including garnet and tourmaline. Economically valuable mineral deposits can be found in metamorphic rocks, including gold, copper, iron, lead, and zinc. In addition,anthracite, a type of coal (used for heating or electricity generation), is a metamorphic rock.

Agents of Metamorphism

Changes occur to rocks because of the action of:

Heat
Pressure
Chemical fluids

Rocks adjust to become more stable under new, higher temperatures and pressures.

Heat

Geothermal gradient
Temperature increases with depth at a rate of 20 - 30 degrees C per km in the crust.
Ultimate source of the heat? Radioactive decay.
Increase of temperature and pressure with depth causes Regional Metamorphism
Heat may come from large bodies of molten rock rising under a wide geographic area.
Intrusions of hot magma can bake rocks as it intrudes them. Lava flows can also bake rocks on the ground surface.
Lava or magma in contact with other rock causes Contact Metamorphism.
Hornfels is a common contact metamorphic rock.

Contact metamorphism along a narrow (approx. 1 meter wide) diabase dike in the Deep River Basin of North Carolina. Diabase weathers tan. Contact metamorphic aureole rocks (hornfels) are gray. Host rocks are red siltstones.

Pressure

Burial Pressure. Pressure increases with depth due to the weight of the overlying rocks. A cubic foot of granite weighs 167.9 pounds. Increase of pressure and temperature with depth causes Regional Metamorphism.
Regional metamorphism occurs at depths of 5 - 40 km.
Tectonic pressures associated with convergent plate boundaries and continental collision also cause Regional Metamorphism.
Pressure along fault zones causes Dynamic Metamorphism, the crushing and ductile flow of rock.
Rocks formed along fault zones are called mylonites.

Chemical Fluids

In some metamorphic settings, new materials are introduced by the action of hydrothermal solutions (hot water with dissolved ions). Many metallic ore deposits form in this way.

Hydrothermal (hot water) solutions associated with magma bodies can introduce new ions and remove existing ions (ion exchange)

How do rocks change?

Metamorphism causes changes in:

Texture
Mineralogy

The original rock (usually sedimentary or igneous) which is changed by metamorphism is referred to as the parent rock.

Texture

The processes of compaction and recrystallization change the texture of rocks during metamorphism.

Compaction
- The grains move closer together.
- The rock becomes more dense.
- Porosity is reduced.
- Example: clay to shale to slate
Recrystallization
Growth of new crystals. No changes in overall chemistry. New crystals grow from the minerals already present.
A preferred orientation of minerals commonly develops under applied pressure. Platy or sheet-like minerals such as muscovite and biotite become oriented perpendicular to the direction of force. This preferred orientation is called foliation.

Metamorphic Textures

Foliation is a broad term referring to the alignment of sheet-like minerals (such as the micas, muscovite and biotite).
Examples of foliated metamorphic rocks
(see pictures and more detailed information below):
1. Slate
2. Phyllite
3. Schist
4. Gneiss
Non-foliated or granular metamorphic rocks are those which are composed of equidimensional grains (such as quartz or calcite). The grains form a mosaic.
Examples of non-foliated metamorphic rocks
(see pictures and more detailed information below):
1. Quartzite derived from the parent rock, quartz sandstone
2. Marble derived from the parent rock, limestone.
  Note: Not all quartzites and marbles are pure. Some contain impurities that were originally mud interlayered with (or mixed with) the original quartz sand or lime mud. These clay impurities metamorphose to form layers of micas or other minerals, which may give marble a banded, gneissic appearance, or which may give a slight foliation to some quartzites.

The foliated metamorphic rocks

As shale is subjected to increasing grade of metamorphism (increasing temperatures and pressures), it undergoes successive changes in texture associated with an increase in the size of the mica grains.

Slate - very fine grained rock. Resembles shale. Has slaty cleavage which may be at an angle to the original bedding. Relict bedding may be seen on cleavage planes. Often dark gray in color. "Rings" when you strike it. (Unlike shale, which makes a dull sound. Temperature about 200 degrees C; Depth of burial about 10 km. Shale is the parent rock of slate.
Slate
Phyllite - fine-grained metamorphic rock. Has a frosted sheen, resembling frosted eye shadow. This is no coincidence. Cosmetics commonly contain ground up muscovite (ground to a size similar to that occurring naturally in phyllite.) Shale is the parent rock of phyllite.
Phyllite
Schist - metamorphic rock containing abundant obvious micas, several millimeters across. Shale is the parent rock of schist.
Several types of schist may be recognized, based on minerals which may be present:
- Mica schist (muscovite schist, biotite schist)
- Garnet schist
- Chlorite schist
- Kyanite schist
Muscovite Schist
Gneiss - (pronounced "nice") - a banded or striped rock with alternating layers of dark and light minerals. The dark layers commonly contain biotite, and the light layers commonly contain quartz and feldspar. Shale is the usual parent rock of gneiss. Granite is Shale is the parent rock of the parent rock of some gneisses.
Gneiss
Migmatite - a very high grade metamorphic rock that has been subjected to such high temperatures that it has partially melted. It is intermediate between the metamorphic and the igneous rocks. Look for swirled banding. The light colored minerals have undergone melting and flow. The dark colored minerals have been contorted by flow.

Mount Arabia Migmatite, part of the Lithonia Gneiss at Arabia Mountain, in southeastern DeKalb County near Stonecrest Mall.
Mylonite - A dynamic metamorphic rock which forms along fault zones.

Mylonite along the Linville Falls Fault, Linville Falls, NC. Relatively undeformed conglomeratic quartzite lies above the layered mylonite zone.

The non-foliated metamorphic rocks

Marble - fizzes in acid because its dominant minerals is calcite (or dolomite). The parent rock is limestone.

Marble
Quartzite - interlocking grains of quartz. Scratches glass. The rock fractures through the grains (rather than between the grains as it does in sandstone). The parent rock is quartz sandstone.
Quartzite
Hornfels - A fine-grained, tough, dense, hard, massive rock. Usually (but not always) dark in color. Finer grained than basalt, which it may superficially resemble. This rock forms through contact metamorphism. The parent rock is commonly siltstone or basalt, but may be other types of rock.
Serpentinite - A dark green, dense, tough, massive, hard rack. May contain veins of asbestos. The parent rock is peridotite, an ultramafic rock.
Serpentinite
Soapstone (sometimes called steatite) - a soft, easily carved rock with a slippery feel because it contains talc and chlorite. The parent rock is peridotite (ultramafic), probably with more water associated with it than in the formation of serpentinite. Example - Soapstone Ridge southeast of Atlanta.
Metabasalt (sometimes called greenstone if massive and green, or greenschist if foliated and green) - the green color comes from chlorite (soft and bluish green) and epidote (pea green). The parent rock is basalt. The grade of metamorphism is LOW.
Amphibolite - Abundant amphibole is present; may be lineated. Usually black. The parent rock is basalt. The grade of metamorphism is HIGH. Has been subjected to higher temperatures and pressures than metabasalt, greenstone, or greenschist.

Mineral changes in metamorphic rocks

Recrystallization - rearrangement of crystal structure of existing minerals. Commonly many small crystals merge to form larger crystals, such as the clay in shale becoming micas in slate, phyllite, and schist.
Also, fine-grained calcite in limestone recrystallizes to the coarse-grained calcite mosaic in marble.
Formation of new minerals - there are a number of metamorphic minerals which form during metamorphism and are found exclusively (or almost exclusively) in metamorphic rocks:
- Garnet - dark red dodecahedrons (12 sides)
- Staurolite - brown lozenge-shaped minerals, commonly twinned to form "fairy crosses". State mineral of Georgia.
- Kyanite - sky-blue bladed minerals with differential hardness. Scratch lengthwise with a knife or nail, but not sideways.
- Chlorite - dark bluish green, soft. Fe, Mg
- Talc - white or pale green and soft.
- Graphite - metamorphosed carbon
- Tourmaline - commonly black. Forms elongated crystals with a rounded triangular cross-section. Can see at Stone Mountain.
- Asbestos - fibrous mineral. Commonly light greenish. Occurs in veins (seems to fill a crack) with the fibers oriented perpendicular to the edged of the vein. Associated with lung diseases. Mesothelioma and asbestosis. Found in serpentinite. "Serpent rock" name due to snake-like veins of asbestos.
- Micas - muscovite (silvery), biotite (dark brown), phlogopite (light brown)