Metamorphic Rocks

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INTRODUCTION As with igneous and sedimentary rocks, metamorphic rocks are classified according to their texture and mineral composition, with texture being the predominant characteristic. Most metamorphic rock classification schemes first separate the rocks into two textural categories; those that are layered (or foliated), and those that are not. This approach is used below.

FOLIATED METAMORPHIC ROCKS The generally-crude layering or foliation of many metamorphic rocks is due to the intense directional pressure they experienced while buried deep underground, usually along a convergent plate tectonic boundary. In this natural setting and over a long period of time, flat mineral crystals and fragments (such as volcanic pyroclasts and stream pebbles) gradually become oriented perpendicular to the directions of pressure. In addition to the effects of pressure, increasing temperature can cause minerals such as mica and chlorite to recrystallize into larger, more visible crystals. The result of rising pressure and temperature can be the foliation shown in the images below of slate, phyllite and schist. Note that the sizes of mineral crystals increases from slate to phyllite and finally to schist, that is from fine to coarse-grained in texture.

The metamorphic rock gneiss becomes layered as increasing pressure and heat mobilize ions which migrate together to form the distinctive banding of light and dark-colored minerals. Field studies of metamorphic rocks and their distributions relative to each other indicate that, as temperature and pressure increase, slate metamorphoses into phyllite, which can then metamorphose into schist. In some situations, schist may metamorphose into gneiss. The changes that occur to metamorphic rocks can be related to the degree or grade of metamorphism that they experienced, that is from low-grade, to medium-grade, and eventually to high grade metamorphism given the right conditions and enough time.

SLATE The most common parent rock for slate is shale, a relatively soft sedimentary rock. But, the volcanic rock basalt or some other fine-grained rock can also metmorphose into slate. As shale becomes deeply buried its clay minerals neomorphose into chlorite and muscovite mica crystals. These crystals fuse together forming a denser and harder rock that has planes of weakness, called cleavage, formed due to the flattening of the crystals perpendicular to the directions of pressure. Simultaneoulsy, the pressure causes the rock to fracture along shear planes, forming slaty cleavage along which slate will readily split. Note that slate is a low-grade metamorphic rock, formed due to relatively low pressure and temperature conditions. Big slabs of slate have been used as chalkboards in the past, and can still be seen in the Alps as shingles on the roofs of houses. Smaller pieces of slate work well as skipping stones on a lake. Below are some images of slate that you can click on for a larger view.

A slate quarry in Pennsylvania. Note the person in the red jacket at the bottom of the image for scale. Photo is courtesy of Dr. Stanley Finney, CSU Long Beach.

The same sample as above, but with a view of the cleavage plane. Enlarge the image to see patches of the green mineral chlorite.

Side-view comparison of handsamples of slate (above) and its parent rock, shale (below). Enlarge the image to see the differences between slate and shale. Note that the layering (bedding) in shale results from the deposition of clay-sized sediment grains in thin layers, which then become compacted together. The layering you can see in the slate sample is either foliation that formed as the tiny mica and chlorite crystals reacted to increasing pressure, or relic bedding from shale. The flat surfaces along which slate cleaves or breaks are shear surfaces along which the rock broke and shifted in response to increasing pressure. The technical term for this is slaty cleavage.

Oblique-view comparison of handsamples of slate (above) and shale (below). Since slate has experienced more pressure than shale, it is usually harder than shale. So, most samples of shale can be scratched with your fingernail, but you can't do that with slate.

This is a microscopic, thin-section view of slate. Enlarge the image to see the faint foliation and tiny mineral crystals characteristic of slate. Used by permission of Dr. Allen Glazner, University of North Carolina.

PHYLLITE The parent rock for phyllite is slate. As slate becomes more deeply buried underground and pressure and temperature continue to rise, chlorite and mica crystals recrystallize into larger crystals. The crystals do not respond evenly to pressure, and often have a distinctly wavy appearance when viewed from the side or especially from the top. Note the characteristic waviness of the surface of the first image of phyllite below. Phyllite is considered to be a product of low-grade metamorphism. It is transitional between slate and schist.

This is a microscopic, thin-section view of phyllite. Enlarge the image to see the more-distinct and wavier foliation of phyllite as compared to slate. Used by permission of Dr. Allen Glazner, University of North Carolina.

SCHIST The parent rock for schist is phyllite. Schist is a medium-grade metamorphic rock, so it has experienced more heat and pressure than both slate and phyllite. The main change from phyllite to schist is that foliation is much more distinct due to the recrystallization of mica and chlorite mineral crystals. These larger crystals tend to reflect light very well, so schist usually has a high luster than phyllite and slate. Samples of schist often contain some larger, unusual mineral crystals such as garnet and tourmaline, which are generally referred to as porphyroblasts. Such minerals can form as heat and pressure transform several different minerals into a new and distinct mineral, a process referred to as neomorphism. There are a variety of schists, each named for the dominant mineral(s) that comprise(s) that particular metamorphic rock specimen.

Muscovite-mica schist at an outcrop in the San Gabriel Mountains of southern California.

A microscopic, thin-section view of a muscovite mica-quartz schist. Enlarge the image to see the individual quartz crystals (gray) and the blue (in polarized light) muscovite mica crystals which define the foliation of this sample. Used by permission of Dr. Allen Glazner, University of North Carolina.

This is a microscopic, thin-section view of muscovite-biotite mica schist with wavier, more distinct foliation than the sample shown above. Used by permission of Dr. Allen Glazner, University of North Carolina.

This sample of schist is mostly composed of muscovite mica, but it also contains garnet porphyroblasts (the small red crystals), so this is a garnet-mica schist.

Side view of the garnet-mica schist sample above. Note the wavy layering, or foliation.

This microscopic, thin-section view shows a garnet porphyoblast within a garnet-mica schist similar to the samples above. Enlarge the image to see the tiny quartz inclusions within the garnet crystal. Used by permission of Dr. Allen Glazner, University of North Carolina.

This is a photograph of muscovite mica schist (large specimen in the middle) and two pieces of phyllite (laid atop the schist) taken in the San Gabriel Mountains. Note the larger crystal sizes of the schist, and the higher reflectivity of the phyllite samples. The camera lens is two inches in diameter.

Diane standing next to a contorted outcrop of mica schist in the San Gabriel Mountains.

GNEISS Gneiss is a high-grade metamorphic rock that can have many different parent rocks, with the most common being granite, diorite and schist. The buildup of intense heat and pressure over a very long period of time enables the migration of atoms within the rock leading to the formation of mineral bands that are oriented perpendicular to the directions of pressure. This banding develops a crude sort of foliation, typically as alternating layers of light minerals (quartz and feldspar) and dark minerals (hornblende and biotite mica). As gneiss approaches its melting point, the dark and light-colored mineral bands can bend and become quite contorted. Of course, if temperature rises too high, the rock will melt and return to the igneous phase of the rock cycle.

Diane with a sample of gneiss (and Skittles) in the San Gabriel Mountains of southern California.

A closer look at the rock above, with potassium feldspar comprising the light bands, and biotite mica and amphibole the dark bands. Technically, this is a biotite mica-amphibole-potassium feldspar gneiss.

A beautiful handsample of gneiss from the San Gabriel Mountains. The light bands are primarily composed of quartz, and the dark bands are composed of amphibole, so this is a quartz-amphibole gneiss. Enlarge the image to see the porphyroblast in the top right corner of the handsample.

This sample of gneiss is also from the San Gabriel Mountains. Based on its extremely deformed banding, it was approaching its melting point while it was deeply buried underground.

NONFOLIATED METAMORPHIC ROCKS As opposed to the foliated metamorphic rocks, the nonfoliated rocks are not distinctly layered. This is probably because nonfoliated rocks were exposed to high temperature conditions, but not to high directional pressure conditions. This being the case, nonfoliated metamorphic rocks tend to be highly recrystallized, and sometimes neomorphosed as well. Listed below are a few of the more common nonfoliated metamorphic rocks.

MARBLE The parent rock for marble is limestone. As temperature and pressure increase on a body of limestone underground, calcite crystals begin to fuse together and recrystallize. Pure limestone is white, but marble can be any color based on the impurities that may have existed in the original limestone rock. Marble is an excellent building stone due to its uniform texture, softness (calcite is a 3.5 on Mohs hardness scale), and inherent beauty.

A beautiful sample of pure marble from Anza Borrego State Park, California. Enlarge the image to see the uniform texture of the recrystallized calcite crystals.

A sample of less-pure marble. The pink color is probably due to the presence of iron within this sample.

This microscopic, thin-section view shows the intergrown calcite crystals which develop as limestone recrystallizes into marble. Used by permission of Dr. Allen Glazner, University of North Carolina.

Comparison of the marble from the second image above to a sample of fine-grained crystalline limestone, a parent rock for marble (below).

QUARTZITE The parent rock for quartzite is quartz-rich sandstone. As sandstone becomes deeply buried, rising temperature will fuse the quartz grains together forming the extremely hard and weather-resistant rock quartzite. Like marble, quartzite comes in many colors, but when pure it is light-colored. Quartzite tends to have a sugary appearance, and when broken the fractures cut through the sand grains, not around them as with a sandstone.

A sample of quartzite. Enlarge the image to see the sugary appearance of this specimen.

This microscopic, thin-section view of quartzite shows the tight packing and recrystallization of quartz grains characteristic of quartzite. Used by permission of Dr. Allen Glazner, University of North Carolina.

A sample of quartzite collected from the Anti Atlas Mountains of Morocco. Note the complete mineral replacement that occurred to the trilobite fossil on this rock specimen.

Another quartzite sample from southern Morocco. This specimen was shaped into a tool by ancient humans living in northern Africa thousands of years ago.

A comparison of quartzite (above) to quartz sandstone (below). Note that individual sand grains are readily visible on the surface of the sandstone, but not on the quartzite.

SERPENTINITE The most common parent rock for serpentinite is the ultramafic igneous rock peridotite. Less likely precursors to serpentinite are the the mafic igneous rocks basalt and gabbro. Serpentinite is composed of the mineral serpentine which forms as the igneous minerals olivine and pyroxene are subjected to rising temperature and pressure conditions along a subduction plate boundary. Serpentinite can be cut and polished for use as ornamental stone inside buildings.

The glossy, glazed luster and green-black color are characteristic of serpentinite. This specimen is from northern California.

A comparison of serpentinite (above) with one of its parent rocks, basalt (below).

METACONGLOMERATE The parent rock for metaconglomerate is the sedimentary rock conglomerate. The latter tends to form from large, rounded sediment grains such as pebbles and cobbles depoisited by a stream. As such a sedimentary deposit becomes buried, compaction and cementation occur forming conglomerate rock. If burial continues to great depth, the pebbles and cobbles become more flattened and stretched out. Recrystallization of the pebbles and cobbles, and surrounding sediment grains also occurs. Metaconglomerates are high-grade metamorphic rocks.

This sample of metaconglomerate shows the flattened, stretched pebbles characteristic of this rock.

A closer view of a sample similar to the one above. Courtesy of Dr. Stanley Finney, CSU Long Beach.

A comparison of metaconglomerate (above) to conglomerate (below). Enlarge the image to see how widely separated the pebbles are from each other in the conglomerate as compared to the metaconglomerate.

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