Metamorphic Rocks

Metamorphic rocks are pre-existing rocks that have been changed by exposure to unusual temperatures and/or pressures. The pre-existing rocks, called protoliths, may be igneous, sedimentary or other metamorphic rocks. After metamorphosis, these new rocks typically contain distinctive new minerals, reoriented into characteristic patterns. Metamorphic rocks are very common in the Jacksonville area and throughout the entire Appalachian mountain chain, so a knowledge of these rocks is essential.

 A. Formation of Metamorphic rocks.

Three general factors influence the changes that take place in rock during metamorphosis. Each of these has a unique effect on the resulting rock. These factors will be considered individually in the next sections.

1. Temperature.

Changes in temperature alter the mineral composition of rocks. Every mineral has a distinctive range of temperature within which it can exist. Beyond this range, the mineral structure breaks down and the atoms create a new mineral stable within this new range. For an analogy, consider a raw egg. A raw egg is stable at room temperature as a runny, slimy, transparent, unusually disgusting substance. What happens if you fry that egg? The egg is not stable at this new temperature. The molecules in that egg rearrange themselves and the slimy raw egg is metamorphosed into a hard, rubbery, opaque fried egg.

Every mineral has a precisely defined range of temperature within which it is stable. For some minerals, this range is very broad. For example, quartz and the feldspars are stable at almost any temperature. Other minerals have very narrow ranges. These minerals are called "index minerals. If we can identify an index mineral in a rock, this tells us the temperature to which that rock was heated.

A rock similar to shale is claystone. The clay minerals in this sedimentary rock are stable under surface conditions, but as temperature increases they break down and rearrange themselves into new, higher-temperature minerals. At about 200 degrees C., the clay transforms into the mineral chlorite. The name schist refers to metamorphic rocks with a certain arrangement of crystals. In this case, the crystals are composed of new mineral chlorite. As temperature continues to increase, chlorite breaks down, the atoms rearrange, and a new mineral is produced called muscovite. Examine the specimen of schist in the metamorphic set. The shiny flat sheets of this silicate should look familiar. As temperature continues to increase, muscovite is destroyed and garnets begin to form. Examine garnet schist. Note the small garnet crystals, shaped like little grains (the word garnet comes from the Greek word for "grain") growing within the rock matrix. If conditions are right, these garnet crystals may grow to the size of softballs. With further increases in temperature, the garnets break down and a very high-temperature mineral called kyanite is formed. You have seen this mineral before ground up to make the white ceramic tops of spark plugs. When automotive engineers needed a ceramic that could survive in extremely hot engine compartments, they turned to a mineral that is stable at very high temperatures. If temperatures continue to rise, the kyanite eventually melts. The resulting rocks are no longer metamorphic, they are igneous.

2. Pressure.

Exposure to high pressure does not change the kinds of minerals found in a rock. Pressure only reorients the mineral crystals. This reorientation is perpendicular (at right angles) to the direction of pressure. For an analogy, consider a jar full of pennies. If you pour out that jar on the table, the force of gravity exerts a downwards pressure on those pennies. The pennies react to that pressure by orienting themselves perpendicularly to that pressure, in other words they lay out more or less flat on the table top. You'd be surprised if they stood on end. Examine the schist specimen. Let the rock lay flat in your hand and look carefully at the individual flakes of mica. Note how they are all arranged in generally the same direction.

Reorientation in response to pressure is most pronounced when crystals are tabular in shape (flat and thin, like a tablet of writing paper or like pennies). If instead, the crystals are equidimensional in shape (the same length in all directions, like a sphere or a cube), the crystals will not re-align. For example, if equidimensional marbles are poured on a table top, they will not re-orient themselves in any particular direction.

The reorientation of minerals in response to pressure produces "foliation". The word foliation is related to the word "foliage", the leaves on trees, and "folio", a collection of sheets of paper. In general, foliations are layers. Pick up the schist specimen again, and look at it from the side. Observe the foliations. Pick up the specimen of slate. Look at the side. Slate also has very distinctive foliations. Examine the specimen of gneiss. Gneiss is a coarsely foliated rock, the layers appearing as alternating bands of dark and light-colored minerals.

3. Presence of water.

Water is an excellent solvent, capable of dissolving and transporting large quantities of material. The presence of water in protoliths greatly accelerates rates of chemical change. If this water is flowing through the zone of metamorphism, it may carry in new ions that will produce new metamorphic minerals not possible from just the protolith. This flow may also remove ions produced as the crystalline structures of the protoliths break down.

 

B. Metamorphic Environments.

The high temperatures needed for metamorphism are generated in the interior of the earth. Rocks near the earth's surface are relatively cold, typically with a more or less constant temperature of about 15 degrees Centigrade. Think about going down into a cave, or even a basement. The temperature is about the same, nice and cool, no matter what time of year. Rock temperature increases with depth. The rate of increase varies depending on your location, but an easy to remember average value is 2 degrees C for every 100 meters. For examples, at 100 meters, the average temperature of rock would be about 17 degrees, at 1000 meters, about 35 degrees, at 10,000 meters, 215 degrees, and so on.

Relatively cool surface rock is exposed to high temperatures in one of two ways. Either the heat of the interior is brought up to the surface rock, or the surface rock is transported down into the hot interior. Heat is conducted to the surface by large masses of rising magma associated with igneous activity. On the other hand, surface rock is transported down into the interior of the earth either by burial or by subduction. This burial also exposes these rocks to high pressures.

 

C. The Classification of Metamorphic Rocks.

All metamorphic rocks are divided into two general categories: foliated or non-foliated.

Foliated rocks have crystals that have been reoriented by pressure. If you look at these rocks carefully you will be able to see these foliations. A distinctive kind of foliation is called "schistosity". In a schist the individual mineral crystals have grown large enough to be visible to the unaided eye. The result is a rough, scaly appearance. Observe the schist specimen. There are many different kinds of schist (a term relating only to appearance) depending on the kind of mineral composing the layered crystals. In the case of the schist specimen, the crystals are of muscovite, and so this rock is called a muscovite schist.

Non-foliated rocks result from low-pressure metamorphism that does not reorient crystals, or from the metamorphism of minerals with equidimensional crystals. No matter how high the pressure, these crystals will not foliate. Non-foliated rocks are classified primarily on the basis of the minerals that they contain.