Types of Metamorphic Rocks

Andrew Alden is a geologist based in Oakland, California. He works as a research guide for the U.S. Geological Survey.

Metamorphic rocks are an important topic in geology. These are the rocks that form by the effects of heat, pressure, and shear upon igneous and sedimentary rocks. Some form during mountain-building by forces of others from the heat of igneous intrusions in regional metamorphism others from the heat of igneous intrusions in contact metamorphism. A third category forms by the mechanical forces of fault movements: cataclasis and mylonitization.

Amphibolite

Amphibolite is a rock composed mostly of amphibole minerals. Usually, it's a hornblende schist like this as hornblende is the commonest amphibole.

Amphibolite forms when basaltic rock is subjected to higher temperatures between 550 C and 750 C) and slightly greater pressure range than that which yields greenschist. Amphibolite is also the name of a metamorphic facies—a set of minerals that typically forms at a specific range of temperature and pressure.

Argillite

This is the rock name to remember when you find a hard, nondescript rock that looks like it could be slate but doesn't have slate's trademark cleavage. Argillite is a low-grade metamorphosed claystone that was subjected to mild heat and pressure without strong directionality. Argillite does have a glamorous side that slate can't match. It is also known as pipestone when it lends itself to carving. The American Indians favored it for tobacco pipes and other small ceremonial or decorative objects.

Blueschist

Blueschist signifies regional metamorphism at relatively high pressures and low temperatures, but it isn't always blue, or even a schist.

High-pressure, low-temperature conditions are most typical of subduction, where marine crust and sediments are carried beneath a continental plate and kneaded by changing tectonic motions while sodium-rich fluids marinate the rocks. Blueschist is a schist because all traces of original structure in the rock have been wiped out along with the original minerals, and a strongly layered fabric has been imposed. The bluest, most schistose blueschist—like this example—is made from sodium-rich mafic rocks like basalt and gabbro.

Petrologists often prefer to talk about the glaucophane-schist metamorphic facies rather than blueschist, because not all blueschist is all that blue. In this hand specimen from Ward Creek, California, glaucophane is the major blue mineral species. In other samples, lawsonite, jadeite, epidote, phengite, garnet, and quartz are also common. It depends on the original rock that is metamorphosed. For instance, a blueschist-facies ultramafic rock consists mainly of serpentine (antigorite), olivine and magnetite.

As a landscaping stone, blueschist is responsible for some striking, even garish effects.

Cataclasite

Cataclasite (kat-a-CLAY-site) is a fine-grained breccia produced by grinding rocks into fine particles, or cataclasis. This is a microscopic thin section.

Eclogite

Eclogite ("ECK-lo-jite") is an extreme metamorphic rock formed by regional metamorphism of basalt under very high pressures and temperatures. This type of metamorphic rock is the name of highest-grade metamorphic facies.

This eclogite specimen from Jenner, California, consists of high-magnesium pyrope garnet, green omphacite (a high-sodium/aluminum pyroxene) and deep-blue glaucophane (a sodium-rich amphibole). It was part of a subducting plate during Jurassic times, about 170 million years ago, when it formed. During the last few million years, it was raised and mixed into younger subducted rocks of the Franciscan complex. The body of eclogite is no more than 100 meters across today.

Gneiss

Gneiss ("nice") is a rock of great variety with large mineral grains arranged in wide bands. It means a type of rock texture, not a composition.

This type of metamorphic was created by regional metamorphism, in which a sedimentary or igneous rock has been deeply buried and subjected to high temperatures and pressures. Nearly all traces of the original structures (including fossils) and fabric (such as layering and ripple marks) are wiped out as the minerals migrate and recrystallize. The streaks contain minerals, like hornblende, that don't occur in sedimentary rocks.

In gneiss, less than 50 percent of the minerals are aligned in thin, foliated layers. You can see that unlike schist, which is more strongly aligned, gneiss doesn't fracture along the planes of the mineral streaks. Thicker veins of large-grained minerals form in it, unlike the more evenly layered appearance of schist. With still more metamorphism, gneisses can turn to migmatite and then totally recrystallize into granite.

Despite its highly altered nature, gneiss can preserve chemical evidence of its history, especially in minerals like zircon which resist metamorphism. The oldest Earth rocks known are gneisses from Acasta, in northern Canada, that are more than 4 billion years old.

Gneiss makes up the largest part of the Earth's lower crust. Pretty much everywhere on the continents, you will drill straight down and eventually strike gneiss. In German, the word means bright or sparkling.

Greenschist

Greenschist forms by regional metamorphism under conditions of high pressure and fairly low temperature. It isn't always green or even a schist.

Greenschist is the name of a metamorphic facies, a set of typical minerals that form under specific conditions—in this case relatively cool temperatures at high pressures. These conditions are less than those of blueschist. Chlorite, epidote, actinolite, and serpentine (the green minerals that give this facies its name), but whether they appear in any given greenschist-facies rock depends on what the rock originally was. This greenschist specimen is from northern California, where seafloor sediment has been subducted beneath the North American plate, then thrust to the surface soon afterward as tectonic conditions changed.

This specimen consists mostly of actinolite. The vaguely defined veins running vertically in this image may reflect the original bedding in the rocks from which it formed. These veins contain mainly biotite.

Greenstone

Greenstone is a tough, dark altered basaltic rock that once was solid deep-sea lava. It belongs to the greenschist regional metamorphic facies.

In greenstone, the olivine and peridotite that made up the fresh basalt have been metamorphosed by high pressure and warm fluids into green minerals—epidote, actinolite or chlorite depending on the exact conditions. The white mineral is aragonite, an alternative crystal form of calcium carbonate (its other form is calcite).

Rock of this kind is manufactured in subduction zones and is seldom brought to the surface unchanged. The dynamics of the Californian coastal region make it one such place. Greenstone belts are very common in Earth's oldest rocks, of Archean age. Exactly what they mean is still not settled, but they may not represent the kind of crustal rocks that we know today.

Hornfels

Hornfels is a tough, fine-grained rock that is made by contact metamorphism where magma bakes and recrystallizes the surrounding rocks. Note how it breaks across the original bedding.

Marble

Marble is made by regional metamorphism of limestone or dolomite rock, causing their microscopic grains to combine into larger crystals.

This type of metamorphic rock consists of recrystallized calcite (in limestone) or dolomite (in dolomite rock). In this hand specimen of Vermont marble, the crystals are small. For fine marble of the sort used in buildings and sculpture, the crystals are even smaller. The color of marble can range from the purest white to black, ranging through the warmer colors in between depending on the other mineral impurities.

Like other metamorphic rocks, marble has no fossils and any layering that appears in it probably does not correspond to the original bedding of the precursor limestone. Like limestone, marble tends to dissolve in acidic fluids. It is quite durable in dry climates, as in the Mediterranean countries where ancient marble structures survive.

Commercial stone dealers use different rules than geologists to distinguish limestone from marble.

Migmatite

Migmatite is the same material as gneiss but brought close to melting by regional metamorphism so that the veins and layers of minerals became warped and mixed.

This type of metamorphic rock has been buried very deep and squeezed very hard. In many cases, the darker part of the rock (consisting of biotite mica and hornblende) has been intruded by veins of lighter rock consisting of quartz and feldspar. With its curling light and dark veins, migmatite can be very picturesque. Yet even with this extreme degree of metamorphism, the minerals are arranged in layers and the rock is clearly classified as metamorphic.

If mixing is even stronger than this, a migmatite can be hard to distinguish from granite. Because it isn't clear that true melting is involved, even at this degree of metamorphism, geologists use the word anatexis (loss of texture) instead.

Mylonite

Mylonite forms along deeply buried fault surface by crushing and stretching of rocks under such heat and pressure that the minerals deform in a plastic way (monetization).

Phyllite

Phyllite is one step beyond slate in the chain of regional metamorphism. Unlike slate, phyllite has a definite sheen. The name phyllite is from scientific Latin and means "leaf-stone." It's typically a medium-gray or greenish stone, but here sunlight reflects off its finely wavy face.

Whereas slate has a dull surface because its metamorphic minerals are extremely fine-grained, phyllite has a sheen from tiny grains of sericitic mica, graphite, chlorite and similar minerals. With further heat and pressure, the reflective grains grow more abundant and join each other. And whereas slate usually breaks in very flat sheets, phyllite tends to have a corrugated cleavage.

This rock has nearly all of its original sedimentary structure erased, although some of its clay minerals persist. Further metamorphism converts all of the clays into large grains of mica, along with quartz and feldspar. At that point, phyllite becomes schist.

Quartzite

Quartzite is a tough stone composed mostly of quartz. It may be derived from sandstone or from chert by regional metamorphism.

This metamorphic rock forms in two different ways. In the first way, sandstone or chert recrystallizes resulting in a metamorphic rock under the pressures and temperatures of deep burial. A quartzite in which all traces of the original grains and sedimentary structures are erased may also be called metaquartzite. This Las Vegas boulder is a metaquartzite. A quartzite that preserves some sedimentary features is best described as a metasandstone or metachert.

The second method in which it forms involves sandstone at low pressures and temperatures, where circulating fluids fill the spaces between sand grains with silica cement. This kind of quartzite, also called orthoquartzite, is considered a sedimentary rock, not a metamorphic rock because the original mineral grains are still there and bedding planes and other sedimentary structures are still evident.

The traditional way to distinguish quartzite from sandstone is by viewing quartzite's fractures across or through the grains; sandstone splits between them.

Schist

Schist is formed by regional metamorphism and has schistose fabric—it has coarse mineral grains and is fissile, splitting into thin layers.

Schist is a metamorphic rock that comes in almost infinite variety, but its main characteristic is hinted at in its name: Schist comes from the ancient Greek for "split," through Latin and French. It is formed by dynamic metamorphism at high temperatures and high pressures that aligns the grains of mica, hornblende, and other flat or elongated minerals into thin layers, or foliation. At least 50 percent of the mineral grains in schist are aligned this way (less than 50 percent makes it gneiss). The rock may or may not be actually deformed in the direction of the foliation, although a strong foliation probably is a sign of high strain.

Schists are commonly described in terms of their predominant minerals. This specimen from Manhattan, for example, would be called a mica schist because the flat, shiny grains of mica are so abundant. Other possibilities include blueschist (glaucophane schist) or amphibole schist.

Serpentinite

Serpentinite is composed of minerals of the serpentine group. It forms by regional metamorphism of deep-sea rocks from the oceanic mantle.

It is common beneath the oceanic crust, where it forms by the alteration of the mantle rock peridotite. It is seldom seen on land except in rocks from subduction zones, where oceanic rocks may be preserved.

Most people call it serpentine (SER-penteen) or serpentine rock, but serpentine is the set of minerals that make up serpentinite (ser-PENT-inite). It gets its name from its resemblance to snakeskin with a mottled color, waxy or resinous luster and curving, polished surfaces.

This type of metamorphic rock is low in plant nutrients and high in toxic metals. Thus the vegetation on the so-called serpentine landscape is dramatically different from other plant communities, and serpentine barrens contain many specialized, endemic species.

Serpentinite can contain chrysotile, the serpentine mineral that crystallizes in long, thin fibers. This is the mineral commonly known as asbestos.

Slate

Slate is a low-grade metamorphic rock with a dull luster and strong cleavage. It is derived from shale by regional metamorphism.

Slate forms when shale, which consists of clay minerals, is put under pressure with temperatures of a few hundred degrees or so. Then the clays begin to revert to the mica minerals from which they formed. This does two things: First, the rock grows hard enough to ring or "tink" under the hammer; second, the rock gets a pronounced cleavage direction, so that it breaks along flat planes. Slaty cleavage is not always in the same direction as the original sedimentary bedding planes, thus any fossils originally in the rock are usually erased, but sometimes they survive in smeared or stretched form.

With further metamorphism, slate turns to phyllite, then to schist or gneiss.

Slate is usually dark, but it can be colorful too. High-quality slate is an excellent paving stone as well as the material of long-lasting slate roof tiles and, of course, the best billiard tables. Blackboards and handheld writing tablets were once made of slate, and the name of the rock has become the name of the tablets themselves.

Soapstone

Soapstone consists largely of the mineral talc with or without other metamorphic minerals, and it is derived from hydrothemal alteration of peridotite and related ultramafic rocks. Harder examples are suitable for making carved objects. Soapstone kitchen counters or tabletops are highly resistant to stains and cracking.