Historical Geology/Metamorphic rocks

Metamorphic rocks are those in which a pre-existing rock (the parent rock) has been altered chemically or texturally by heat and/or pressure. In this article we shall look at metamorphic processes and their effects on the resulting rocks.

Types of metamorphism

There are two main types of metamorphism; contact metamorphism and regional metamorphism.

In contact metamorphism, the heat of magma intruding through rocks causes metamorphism to the surrounding rocks, leaving an aureole of metamorphic rocks around the igneous rocks formed from the magma.

Regional metamorphism is caused by tectonic events involving both heat and pressure, and will affect large, elongated regions of rock.

Besides these broad classifications, we can also class metamorphic rocks according to their grade of metamorphism; the temperature to which they have been exposed. As we shall see, this determines the chemical changes that metamorphic rocks undergo. Grades of metamorphism range from very low (below 300°C) to low (300°C - 500°C) to medium (500°C - 600°C) to high (600°C and upwards).

Chemical changes

Garnets in metamorphic rock.

Metamorphism often causes chemical changes to the minerals of which the affected rocks are composed. For example, at low temperatures clay minerals will be converted into chlorite; at higher temperatures the chlorite will itself be transformed into other minerals. Hence, if we find a rock with chlorite in it, we know that it has undergone low-grade metamorphosis. Minerals such as this, which reveal the grade of metamorphosis, are known as index minerals.

Geologists can correlate index minerals with grades of metamorphism because it is simple enough to repeat the processes of metamorphism in the laboratory; that is, they can take a piece of non-metamorphic rock, subject it to various regimes of temperature and pressure, and see which characteristic minerals form at which temperatures. So in shale, for example, we see a sequence (as temperature increases) from unaltered shale to rocks containing chlorite; then biotite; then garnet; then staurolite; then kyanite; and finally silmanite. The image to the right shows some particularly large garnets embedded in metamorphic rock.

We see this same sequence arranged spatially as we approach the center of an area of metamorphism: from unaltered shale through the "chlorite zone", the "biotite zone", the "garnet zone" the "staurolite zone", the "kyanite zone", and the "silmanite zone". We may not get all the way up to silmanite, that depends how intense the metamorphism was at the center of metamorphism.

The sequence, of course, depends on the parent rock; the sequence given above is specific to shale, and we would see a different sequence if we were looking at (for example) mafic igneous rocks.

Metasomatism

Skarn.

In the section above on chemical changes we dealt with the case where the parent rock reacts with itself as a result of heat or pressure acting on the rock. But in the case of contact metamorphism, we can also see metasomatism taking place: the parent rock mixes and/or reacts with the intrusive igneous rock and the hot fluids associated with its eruption.

The picture to the right shows a particularly attractive example of skarn, a very distinctive product of metasomatism.

Textural changes

Besides chemical changes, rocks that undergo metamorphosis suffer textural changes, such a recrystallization, foliation, and lineation.

In recrystallization, the original texture of the rock is lost as the minerals, under the effect of high temperatures, reform as a collection of interlocking crystals of similar size. The effect of this is seen most dramatically in sedimentary rocks.

So, for example, quartz sandstone loses its sedimentary structure of cemented grains to become quartzite, with a smooth texture consisting of interlocking crystals. As a result, except at very low grades of metamorphism, any bedding of the rock will be destroyed, as will any fossils that the rock contains. Similar textural changes produce marble from limestone, and hornfels from mudrock.

Foliation in schist, as seen under a microscope and illuminated by plane polarized light.

When rocks are metamorphosed by pressure as well as heat, they undergo foliation, in which sheet silicates, if they are present in the rock, rearrange themselves so that the sheets are at right angles to the direction of pressure. The picture to the right shows a view of foliation under a microscope.

Lineation is a similar phenomenon affecting silicates with the structure of a chain or double chain; the direction of the chain ends up, again, at right angles to the direction of pressure.

Not every metamorphic rock will display foliation or lineation: some rocks simply don't contain any sheet or chain silicates: an example would be limestone, which metamorphoses to marble. Also some metamorphic rocks are formed by heat without any significant pressure, as is usually the case with contact metamorphism; so, for example, mudrocks, which will form foliated slate or schist under pressure, will produce non-foliated hornfels without pressure.

Foliation comes in several varieties:

Slatey foliation is caused by the alignment of sheet silicates such as clay minerals and chlorite (which is produced by chemical changes to clay minerals). It results in rocks which cleave easily into thin layers.

Schistosity is caused by sheet silicates such as biotite and muscovite. Not only do they align, but they tend to separate out from the non-sheet silicates such as quartz, producing a rock that breaks easily into thicker leaves than those found in slatey rocks.

Gneiss.

Finally, we come to gneiss. At high grades of metamorphism, sheet silicates tend to break down, and dark-colored chain silicates such as hornblende and pyroxene begin to appear. These are separated out into dark bands, again at right-angles to the direction of pressure, giving gneiss a distinctive streaky appearance, as shown in the photograph.

How do we know?

How can we recognize rocks as metamorphic?

First of all, as we have observed, we can reproduce metamorphic processes in the laboratory. Marble, for example, is what we get if we heat limestone; quartzite is what we get if we heat quartz sandstone; schist is what we get if we heat mudrock and apply pressure. It would seem downright perverse to maintain that metamorphic rocks should have been produced by other processes not as yet discovered. (Note that the textures of metamorphic rocks exclude the possibility that they are sedimentary rocks, and their chemical composition usually excludes the possibility that they are igneous rocks.)

We can also look at the patterns we find in the rocks. I shall give some examples of the kind of predictions we can make from the theory of metamorphism; the reader will doubtless be able to think of other examples.

To take a simple example: when we look at an aureole of marble, we should expect to find it embedded in an outer ring of limestone, and not of (for example) sandstone, which would go with an aureole of quartzite.

Then again, according to our notion that metamorphic rocks are indeed produced by metamorphosis, we should not (and we do not) find, looking horizontally at sequences of rocks, alternating bands of unaltered rocks and high grade metamorphic rocks. Instead, as we have noted above, we find concentric zones of rocks with high-grade metamorphic rocks at the center of metamorphism, progressing to lower and lower grades of metamorphism until we reach unaltered rocks.

If we find a foliated rock like schist, then according to our interpretation of schist as produced by temperature and pressure, we should find other evidence of the pressure; we should expect to find the beds of rock buckled and deformed. And this is indeed what we see.

If, on the other hand, we find hornfels, which, laboratory experiments show, requires temperature without significant pressure (otherwise it would be foliated) then we expect to find (and do) that it forms an aureole around igneous rock, with progressively lower grade metamorphism in concentric zones around the igneous rock.

Other patterns are discernible: for example, we would not expect to find schist overlying limestone, because the events that created the schist would also have turned the limestone into marble.

In summary, the chemical composition and texture of the rocks that we have classed as metamorphic, together with their arrangement and relation to other rocks in the geological record, is just what we should expect to see if they are indeed produced by metamorphosis.