Well, hello, again and welcome. I'm glad you could join us today for our program on "Metamorphic Rocks."

The metamorphic rock we call "marble," for example, is simply recrystallized calcite from the metamorphism of limestone. It often contains streaks of graphite and hematite, which represent the organic material or iron oxides that we find mixed in with the limestone, so before we moves on to the program, let me remind you of the lesson assignment. We're working in Chapter 15, which is metamorphism and metamorphic rocks and hydrothermal rocks, so you should read the introduction and the summary and be sure to note the position of metamorphism and metamorphic rocks in relation to igneous and sedimentary rocks on the rock cycle diagram on page 303. You might also want to note that both sedimentary and igneous rocks undergo metamorphism and note the line leading from metamorphic rock to weathering and also from metamorphic rock to magma in the rock cycle diagram and examine the photograph on page 329. Note the size of the microfold and the name of the red mineral that occurs and also look at Figure 15.1 on page 330 for a good photomicrograph of a metamorphic outcrop so read the entire chapter and pay close attention to the photographs and diagrams. At the end of the chapter the diagrams of subduction zones are especially helpful in understanding the origin of metamorphic rocks, and, of course, follow the study plan and the study guide, and when you've done that go back to the learning objectives and make sure you've learned each one, so again I won't go through the learning objectives with you, but I will remind you that they're there in the study guide, and it's important to make sure that you've got a sense of each of these various learning objectives.

Well, understanding how rocks are changed by heat, pressure, and chemical solutions can help us in understanding the variety of metamorphic rocks which we actually observe on the Earth's surface. "Metamorphism" is a process that changes the forms of rocks which are exposed to new conditions upon deep burial, so we have rocks that are in equilibrium usually weathering products, sediments and equilibrium at your surface, which are buried where they encounter new conditions of temperature and pressure. It's important to understand that in metamorphic rocks these changes take place in the solid state; that is, little, if any, melting occurs in the rocks at all. The high temperatures make atoms mobile. Remember that when atoms heat up or when a rock heats up, the atoms move around faster, so when the atoms become more mobile they're able to move around within the solid rock to find other atoms and form new combinations which amount to the formation of new minerals.

Metamorphic rock types are the most complex of all rock types, but like all rocks they're classified on the basis of composition and texture, that is, what minerals they contain and how those minerals are oriented, the size of the grains, and so forth. Luckily for us, even though there's a wide variety of compositions and textures, these can be classified into only a few basic types. There are many varieties of each type, so many different varieties, in fact, that each particular region where metamorphism has happened, may have rocks which are unique to that region and can be identified simply because they look so unique. It's also noteworthy that metamorphism may cover large regions, large regions meaning areas several hundred miles in diameter. Certain patterns in the types and sequences of rocks are generally observed over a given region, and we'll come back and look at this a little bit later on.

Okay, let's look at the agents of metamorphism; in other words, what is it that actually causes metamorphism? Basically there are three agents. I've already mentioned two of these: heat, and pressure, and chemically active fluids, most notably, hot water so let's look at heat first. At high temperatures atoms have more thermal energy and greater motion. This thermal energy may give them enough energy to break chemical bonds which hold them in crystal structures of the minerals in which they're already in as they're buried, so the atoms and ions then become free to roam about looking for new bonds to form new minerals which are stable at those higher temperatures. The source of this heat may be from friction as subducting plates slide past one another, from compression as sediments are folded and squeezed at a converging plate boundary, by burial, simply taking advantage of the geothermal gradient as the Earth temperature increases, or by being close to igneous intrusions. How about pressure? Pressure naturally increases with depth in the Earth simply from the weight of overlying rocks, but rocks exert both confining and directional pressure. Pressure in general is force per unit area. Confining pressure acts like a lid.

Okay, when you're under water, the water exerts a confining pressure on you. It's due to the weight of the water, but rocks also exert "directional pressure." What that means is that the pressure may be greater in the vertical direction squeezing this way than it is in the horizontal direction,and this differential pressure or directional pressure allows the movement of atoms horizontally more easily than vertically; that is, the atoms which are broken away from the existing mineral structures may find themselves free to move horizontally but not free to move vertically where they would encounter a pressure gradient which would tend to keep them from moving. How about chemically active fluids? Well, in general the fluid we're talking about here is water. Water is, in fact, the most chemically active fluid known, and it's even more active when it's warm. It's capable of dissolving and carrying ions and atoms in solution, and it's capable of dissolving and carrying more of these when it's warm. The water may move through hot rocks even when they're solid. There may be cracks, crevices, or the individual water molecules may simply migrate along with the other ions. The effect that the water has is that it aids in breaking the chemical bonds. It helps make the atoms more mobile. We encountered in an earlier lesson that water in the presence of rock at high temperature also lowers the melting temperature for the same reason. It interferes with the bonding of the silica tetrahedra. There are several different textures of metamorphic rocks that we need to become familiar with before we see the video. Basically, metamorphic rocks are either "foliated" or "nonfoliated." The word "foliate" or "folia" means sheets or layers,so "foliated" rocks are those which contain layers or sheets of any type. Usually, the foliations are a type of rock structure that's formed in metamorphic rocks under directional pressure. It's caused by separation as well as aggregation of minerals by physical characteristics such as "shape." What I mean by that is simple. Some silicate mineral crystals tend to be symmetrical or elongated while others are not. Keep in mind, recall back from the lesson on minerals and silica tetrahedra that certain ferromagnesian minerals, for example, amphibole and biotite are elongated because they consist of chains or sheets of tetrahedra, so that they have a preferred growth direction along the direction of the chains or along the directions of the sheets.

On the other hand, quartz and feldspar two other common silicate minerals, are not elongated because they consist of frameworks of silica tetrahedra in a three dimensional structure, so that there's no particular preferred growth direction, so those minerals which tend to become elongated may grow preferentially along the horizontal direction forming layers and sheets, and we'll see some examples of this after the video when we look at some of these metamorphic rocks. Nonfoliated rocks generally consist of those minerals which do not have preferred growth directions. They also occur in types of metamorphism that form at high temperatures but relatively low pressures, for example, in contact with igneous intrusions at the edges where the igneous intrusion contacts the country rock.

Okay, so we can make a general note here that foliated rocks do form at high temperatures but low pressures; whereas, foliated rocks tend to form at high pressures over a range of temperatures, and this usually happens in regional metamorphism in the hot deep cores of orogenic belts. Well, what are the common metamorphic minerals then? Are they the same minerals that we find in igneous rocks? Are they the same minerals that we find in sedimentary rocks? The answers are really quite simple. The common metamorphic minerals are those which are stable under various conditions of high temperature and pressure. The actual minerals that form depend upon the pressure, temperature, and of course, the kinds of atoms which are available. Well, what kind of atoms are available in metamorphic rocks? They're exactly the same atoms that are found in sedimentary and igneous rocks. In other words, those atoms of the eight major elements: oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, and potassium.

Okay, most of the minerals in metamorphic rocks are silicates because silica still occupies a significant portion of all the available atoms, so we find quartz, mica, amphibole and pyroxene in metamorphic rocks just like those found in igneous rocks because these form at high temperatures over a wide range of pressures, and these are found both in igneous and metamorphic rocks although they're not particularly common in sedimentary rocks. At least the ferromagnesians aren't. There are also certain minerals that are found almost exclusively in metamorphic rocks. These are minerals that are only stable at both high temperature and high pressure: minerals like staurolite and kyanite, and garnet, and siliminite, and graphite. These minerals are almost unheard of in igneous rocks and in sedimentary rocks because they only form under conditions of extreme temperature and pressure. Diamond, of course, is another mineral that we sometimes find in extreme metamorphic rocks that form way down deep in the crust, so I think this gives a fairly good background to understand the video, so when we come back from the video, I'll talk a little bit more about the various occurrences of metamorphic rocks and give you some examples of their various types, but with this background let's watch the video.

Major funding for "Earth Revealed" was provided by the Annenberg C.P.B. Project. Throughout history mountains have been deeply imbedded in the human experience. We've worshipped them, created nations using them as boundaries, stripped them of valuable resources, and returned to them for inspiration and recreation. If you were at all curious about the Earth, you've probably wondered why mountains exist. This question has intrigued Earth scientists ever since the emergence of geology as a science in the late Eighteenth Century, and the more we learn about mountains and what they're made of, the more fascinating these question becomes. Most mountains are forming today in tectonically active regionswhere the movements of plates deform the rocks of the Earth's lithosphere. The tremendous energy that's expended in the mountain building process often has a profound effect on the rocks. The geologic events that accompany mountain building, such as the collisions between plates, deep subsidence of portions of the Earth's crust, moving masses of magma, and the displacement of rock bodies along fault zones focus heat and pressure on the rocks. As the result, these rocks are changed dramatically. This process of change by the effects of heat and pressure is called "metamorphism", a term derived from the Greek words "meta," which means "change" and "morph" meaning "form."

Metamorphism changes the appearance of rocks, their mineral composition, and even their age as measured by radiometric data. During metamorphism atoms within the rock can dislodge themselves from mineral lattices and move about freely. This atomic reshuffling causes the existing minerals to recrystallize and new minerals to form. This process also resets the radioactive clock within the rock to the time of metamorphism. Metamorphism can result in complex structures and rare minerals that make these some of the most bazaar looking and strikingly beautiful of all crustal rocks, but to geologists the real beauty of metamorphic rocks is the information they contain about tectonic processes and Earth history. Metamorphic rocks can appear in many forms from platy, black, fine grained stone to granite- like layered rock, to the marble used by sculptors. One explanation why a wide variety of metamorphic rocks exists is simply that there are many different sedimentary and igneous rocks, each responding to metamorphic conditions in its own unique way. Geologists use the term "protolith" to refer to the original rock existing before metamorphism. For example, limestone is the protolith of marble, one of the most common metamorphic rocks, and basalt, a volcanic igneous rock, is the protolith of amphibolite, but geologists have found many more types of metamorphic rocks than protoliths, so factors other than original composition must also play a role in creating these rocks. Study of geologic structures such as folds and faults suggests that there's a wide range of pressures and temperatures inside growing mountain belts. Quite likely, this plays a critical role in explaining variations in metamorphic rocks. Laboratory experiments have helped geologists understand metamorphic conditions. The conditions under which metamorphism occurs is beneath the level of weathering and sedimentation to form the sedimentary rocks generally at temperatures about of a greater than 200 degrees and at conditions that do not produce a melt that goes into igneous rocks, so the range in temperatures are roughly about 200 degrees C to about 800 degrees C. They occur, the process and the formation of the rocks occur at depths generally from two to several tens of kilometers in depth beneath the Earth's surface. At the surface we are accustomed to the pressure of the air surrounding us. We don't notice the air because the pressure is equal all over our bodies. Deep underground, however, pressure is not equally applied. Rock can be squeezed strongly with pressure greatest in the direction of the squeezing. Sometimes opposing pressure an be applied on different parts of a rock causing it to bend or sheer apart like a sliding deck of cards. Whether from sheering or simple squeezing, the rock is experiencing what geologists refer to as "directed pressure" or "directed stress." The structure of many metamorphic rocks is a result of directed pressure. Directed sheer stress, for example, helps explain the origin of a spectacular form of crystal growth. These swirling images suggest several things: a cluster of spiral galaxies, the Chinese symbol for ying and yang. They are, in fact, snowball garnets, to the geologist, frozen slices of metamorphism in action. The swirling pattern in a snowball garnet is formed by planes of tiny mineral inclusions that are swallowed up by the garnet as it grows. Sheer stress causes the garnet to rotate during growth distorting the planes into swirls. The three dimensional form of the swirling pattern can be shown by means of a multi-ringed model. Each ring represents the edge of a plane of minerals incorporated by the growing garnet. These are the rings and let's do that process as it goes on so we can visualize it. First, we grow a little bit of garnet; then we rotate the ring. We rotate that and grow another ring and rotate it with the ring inside, and we grow another ring and so forth until we develop basically a shape like this in which we have a little pit over here and a little mound over here in the diameter of the rotation here. We can compare that with a real specimen, which is over here, and this is a real specimen in which we see the little pit here, the little mound here, the common axis through here, and it shows a snowball pattern. In the same specimen we can see a cross section of a garnet that's grown considerably more than that garnet has showing that rotation. Directed stress involving compression helps explain the origin of a very common metamorphic structure. As temperature and pressure increase, minerals recombine to make new, more stable minerals. The minerals grow in the directions of lowest pressure perpendicular to the directed stress. This results in a layering which geologists call "foliation." Sheer stress, too, can cause foliation. Out of a piece of a metamorphic rock we call a shist, a mica shist, and we can see it's very layered. That layering is a preservation of a stress field generated within a subduction zone environment. As the rock is recrystallized under great pressure in great temperature, it is also recording in it the intensity of the stress field. We see stresses that had to be in some sort of direction to platerize the micas forming the mica shist. Foliated rocks are easy enough to spot but are often taken for granted at some cost. Constructing roads, dams, or foundations on such rocks can create severe problems. The production of foliation within metamorphic rocks gives rise to the same type of structural heterogeneity at weak directions as you find within landslide prone,for instance, sedimentary rock, so although we consider these basement rocks to be quite stable, in reality shists can be fairly unstable. Any engineering firms that were wanting to construct either houses, or dams, or other types of constructions on metamorphic rock have to take into consideration the foliation and the direction of the foliation to make sure that it isn't in an unstable orientation with regards to any engineered works that could be constructed on it. In addition to directed stress, rising temperature will cause minerals in a metamorphic rock to react forming new crystal lattices and mineral types. This process called "recrystallization" generally causes minerals to grow larger developing an interlocking texture resembling that of igneous rocks. For example, when the sedimentary rock limestone is metamorphosed by heat into marble, the fine grains of calcite in the original limestone recrystallize into large calcite crystals which interlock to give the emerging marble a course texture. In some circumstances, the temperature of a deeply buried rock become so great the rock starts melting. When this happens, a rock having both igneous and metamorphic features results. Geologists call these intermediate rock types "megmatites " or mixed rocks. In some megmatites geologists find evidence for the origin of one of Earth's most common igneous rocks, granite. Yet another factor may be critical in creating metamorphic rocks. For many years, geologists believed that the overall composition of a rock rarely changes during metamorphism; however, this is no longer assumed to be the case. The other aspect of metamorphism the result of these metamorphic reactions is that the rocks undergo changes in composition, and mainly this shows up in the formation of the liberation of H20 and its departure from the rocks, so the rocks dry out in very much the same way that a fired pot dehydrates in a kiln. The main feature of higher temperatures, for example, is to cause the rock to undergo a loss of certain volatile components, and in most metamorphic rock that means mainly the loss of H20 and C02. Fluids are released not only by metamorphic rocks at high temperatures but from magmas intruding the metamorphic rocks as well. During mountain building, the crust in many places is saturated with migrating fluids. These accelerate some chemical reactions and may stop others, so the spectacular diversity of metamorphic rock is created by the numerous protoliths, the presence of fluid, and the wide range of temperatures and pressures possible within the Earth. When rocks are exposed to the heat and pressure of metamorphism, they undergo changes both in texture and mineral content. The specific changes that take place depend on a variety of factors including the composition of the original rock called the "protolith," how much heat or pressure is applied, the length of time of metamorphism, and whether or not water is present.

Most metamorphic rocks form in one of two geologic settings. The first is where cold rock is intruded by a hot magma. This is called "contact" metamorphism, and its effects are confined to a small area. The other setting is at a convergent plate margin where the motions of the plates generates metamorphic conditions over a wide area in places covering thousands of square kilometers. This is known as "regional" metamorphism. One of the main differences between "contact" and "regional" metamorphism is that temperature is the predominant cause of "contact" metamorphism; whereas "regional" metamorphism involves both temperature and pressure. The chemical changes that accompany contact metamorphism, especially of marbles, is seen on this hand sized sample of a contact. On the left side is an intrusive igneous rock, a tonalite. It came into contact with a limestone. The limestone was heated up, and at the same time it was heated up there were chemical elements that left the intrusive rock into the marble, and then elements that left the marble and went into the intrusive rock. The material leaving the intrusive rock consisted of some iron, aluminum, and silicon, which went to form garnets of brown material here. Silica continued in further into the marble than the other two elements and resulted in a formation of a layer of the mineral, wolastinite, which is a calcium silicate. This is a good example of a well layered rock produced by regional metamorphism and a so-called amphibolite phases. It contains of alternating layers, the light of quartzite and of dark, a mica shist bearing the aluminum silicate mineral sillimanite. At first thing you would think these are relic sedimentary beds, but they are not; they've been structurally transposed from sedimentary beds into tectonically bounded layers. Here you can see two of the biotite rich layers coming together to constitute a single layer, so there has been a great deal of directed sheerer pressure through this rock that is produced as well-layered aspect to it. Metamorphism has been compared to cooking. The dish that you wind up with depends both on your starting ingredients and on the way you cook them. Likewise laboratory experiments have shown that the composition of rocks changes very little during metamorphism, but as temperature and pressure increase the atoms within the rock become mobile and recombine to form new minerals. These experiments have also shown that the various metamorphic minerals or assemblages of minerals found together form only within specific ranges of temperature and pressure, so geologists can use minerals and metamorphic rocks as pressure gauges and thermometers to understand the conditions under which metamorphism took place. This chart is an example how minerals are used to interpret the metamorphism of basalt, the rock that makes up the crust of the world's ocean basin. Pressure on the chart increases downward, and temperature increases to the right. This is a piece of unmetamorphosed basalt, which formed at the Earth's surface. This rock would plot here on our chart at relatively low temperature and pressure. As basalt is metamorphosed to different combinations of temperature and pressure, its mineral composition changes as the rock re- equilibrates to its new condition. These zones on the chart are called "metamorphic phases," and each phase is defined by the formation during metamorphism of a particular mineral or mineral assemblage. For example, this is a metamorphosed basalt containing the mineral "amphibole." Amphibole forms at a temperature between 450 and 700 degrees Centigrade and at a pressure corresponding to a depth of at least six kilometers, so this metamorphosed basalt would plot here on a chart in the amphibolite phases. This is a metamorphosed basalt that contains zeolite, another mineral group, and it plots here in the zeolite phases. The zeolite phases corresponds to less intense metamorphic conditions than does the amphibolite phases, so rocks containing zeolites are said to be of lower metamorphic grade. Metamorphism is a process of progressive change. As rocks are exposed to higher and higher temperatures and pressures, they're altered in a predictable manner. As the intensity of metamorphism increases, the rocks become harder and more coarsely crystalline and develop special metamorphic texture.

Geologists refer to progressive metamorphism as an increase in metamorphic grade from low to high, and the best way to see this pattern of change is to begin with an unmetamorphosed protolith and watch it change as the intensity of metamorphism increases. Geologists can see how particular rock types undergo progressive metamorphism by tracing widespread rock formations from areas where no metamorphism has occurred into areas where metamorphism is extreme. If our starting material is like a clay stone like this it's sedimentary rock relatively alumina rich. On heating this under the low part of the regional metamorphism that we develop a very layered finely layered rock called a "slate," and it doesn't have any recognizable minerals in it because they have not grown large enough, but due to the growth of new minerals and of the directed pressure that we end up with a very well foliated rock, it's possible to cleave into very regular end layers. With increased temperature and pressure at the slate then is transformed into a slightly higher grade rock, which is called a phylitte, and this rock has a linear structure to it as well as to the foliated structure, and it is a little slightly different in luster due to the larger mica sized crystals. As the temperature and pressure increase further that we develop a shist; in this case, this is a garnet shist with large garnet crystals, lots of white mica, very coarse crystals. This would be formed at quite high metamorphic grade. At yet higher metamorphic grade, it should constitute a gneiss where you start having minerals segregate into definite layers. If metamorphic rocks form inside the Earth as temperatures and pressures rise, why aren't they unmetamorphosed as temperatures and pressures fall back down? In part, this is because loss of fluids during metamorphism makes it impossible for certain chemical reactions to reverse themselves. Also, as temperatures drop, ions cannot migrate easily through the rock, so minerals will not recrystallize, so in most metamorphic rocks geologists find a preserved record of the greatest temperatures and pressures occuring during crustal deformation. With the development of plate tectonics theory, the temperature and pressure changes geologists have long seen in metamorphic rocks finally began to make sense.

For many years geologists have been able to relate individual phases to the pressure and temperature conditions of metamorphism, but they had no satisfactory explanation for the geologic processes that form metamorphic rocks, that is, until the theory of plate tectonics emerged. One good example is this relatively rare metamorphic rock called "blue shist." Experimental work had shown that the minerals in blue shist form only under very unusual metamorphic conditions. These conditions are a pressure range equivalent to a depth of 15 to 30 kilometers in the crust and a very cool temperature, only 200 to 400 degrees centigrade, that's the approximate cooking temperature of a kitchen oven or toaster. At a depth of 15 to 30 kilometers, however, the temperature is normally about twice as high, 500 to 750 degrees centigrade, so the only way that rocks can be metamorphosed to blue shist phases , is to be quickly shoved down to those extreme depths and then rapidly brought back up before the rocks have time to heat up completely, and that's exactly what happens where two tectonic plates are colliding in a subduction zone; in fact, blue shist bearing rocks normally occur in long linear zones that mark ancient plate subduction boundaries. Metamorphic rocks provide geologists with the most complete picture of temperatures and pressures developed when plates collide. In addition, these rocks contain other fundamental information.

In the metamorphic regions of the northeastern United States, for example, snowball garnets preserve an important record of the building of the Appalachian Mountains. By comparing the amount of rubidium isotope decay at the center of these garnets relative to their margins, geologists can determine how fast the crystals grow. The garnets in Vermont took about ten, ten and a half million years to grow,and it's correspondent to a gross rate of roughly a few atomic diameters per year, and to give you some comparison of what that might be in terms that might be more in a human reference frame, that corresponds to about a millionth as fast as the diameter of a ordinary tree might grow, so it's a very slow process. The Vermont garnets began growing about 380 million years ago as the continents of Eurasia and Africa drifted toward the Americas to form the supercontinent of Pangea.The convergence of the plates gradually heaved up the rocks of Northeastern North America to create the Appalachian Mountain range pushing the rocks into huge flat lying folds called "nappes." Buried deep inside these giant folds of rock tiny garnet crystals echo the twisting and contorting going on around them, rotating and spiraling between 20 and 30 degrees every million years. The beauty about the garnets in Vermont is that these are, the ones we measured were snowball garnets, and so we get some other useful information from the garnets studied in Vermont. That information tells us how fast the rocks were deforming, and in other words, how fast the garnets were rotating. That tells us how fast the rocks in the surroundings around the garnets were causing that rotation were deforming, and that is something that has never been measured before. That's a considerable interest for people studying tectonism because we're actually measuring the rates at which the rocks get folded, and that's another factor we're interested in. Like a tiny black box flight recorder in an airplane or a trip odometer in a car, a snowball garnet provides geologists with a crystalline log of plate collision and mountain building spanning millions of years.

Metamorphism is a fundamental rock forming process on Earth. About 15 percent of all continental crust exposed at the surface is composed of metamorphic rocks,and much of the oceanic crust is metamorphosed to a low grade as it formed. Just as fossils are a record of life through time, metamorphic rocks are used to study the history of the Earth. They allow us to reconstruct the movement of plates that no longer exist and to study mountain ranges that have long since worn away. Like the opening of new oceans, the movement of continents, and the creation of mountain ranges, metamorphism is a consequence of plate tectonics. The rise in temperature and pressure that makes metamorphism possible is almost always linked to plate movement and mountain building. The collisions, the intrusions in fault zones that metamorphose the rocks are concentrated at plate margin. As rocks are depressed to great depth, say tens of kilometers in a subduction zone or placed under the great compression of a continental collision, metamorphic conditions can become so intense that the rocks begin to melt. The magma rises buoyantly toward the surface setting the stage for the formation of new rocks and new metamorphic transformations. When we study metamorphic rocks, we're seeing a brief glimpse of this cycle of rock formation and change, a cycle that's as old as the Earth itself. I like that video, especially the part of those garnets. That's really amazing that you can actually time the rate of metamorphism by looking at these garnets, something fairly new in geology. Well, let's review briefly the occurrence of metamorphic rocks. We find metamorphic rocks in certain kinds of places and not in other kinds of places. We find them, for example, at the boundaries of igneous intrusions. We call this "contact" metamorphism. We find them in and around the cores of old mountain ranges, for example, all along the Appalachian Mountain system. We find them underlying sedimentary rocks at the Cambrian unconformity. We find them, for example, underlying all the rocks at the bottom sequence in the Grand Canyon. We also find them in the continental shields because the continental shields themselves represent cores of ancient mountain ranges that have been eroded down, so let's look now at the classification and identification of metamorphic rocks. The video, I don't think, covered this as well as we can do it here in the studio, so we're going to look at the classification of metamorphic rocks by composition and by texture, but the texture of metamorphic rocks turns out to be the most important. We have here a classification based upon the foliation versus nonfoliation; in fact, the foliated rocks are named both by composition and texture, and the nonfoliated rocks are named almost entirely by composition, so let's take a look at some of these various types of metamorphic rocks on the closeup camera. The first one is a rock called "marble." Marble is, notice that it's a fairly massive piece. This particular piece has been polished on the outside and been cut, but if I turn it around you can see the individual grains of calcite glittering in the light. Marble's formed from the metamorphosis of limestone. In this particular case, if I can turn this around here, the marble's got some dark pieces in it over here on the edges. These dark pieces represent, in this case, organic material, which has been partly metamorphosed to graphite, so that we've got a separation of the original calcite from the materials that were contaminating it in the original state. Before it was metamorphosed, this piece was probably a dark gray piece of fine grained limestone. Okay, the next thing to look at is another nonfoliated rock called quartzite. It's a metamorphic rock composed almost entirely of recrystallized quartz grains; in this case, the quartz also is stained by a little bit of iron oxides, so it has this pinking or reddish tint to it. It was probably formed from a piece of sedimentary rock, a red quartz sandstone, much like the one that's in my fingers here now, in which case the grains have simply recrystallized, formed larger grains, and grown together to form this more dense massive structure of the quartzite.

Okay, another example of a type of metamorphic rock is a type of coal known as "anthracite." "Anthracite" is actually the end result of the burial of bituminous coal, which is itself a sedimentary rock. It's squeezed enough that basically what remains is pure carbon. The other volatile materials, the sulfur, and so forth,have been pretty much squeezed out of it, and this particular piece is quite shiny, almost glassy in appearance. Okay, turning attention to the foliated rocks, we have a type of rock known as "slate." I have an example here of a piece of slate and next to it is a piece of shale. As we'll learn here in a few minutes, the process of changing shale into slate is a continuum, and that shale is actually a sedimentary rock, and slate is a low grade metamorphic rock. They look very much alike, and in fact, about the only way you can tell the difference is by the sound that they make when they hit something hard. The reason for this is the same reason why pottery sounds different than a lump of clay. Pottery is baked at a relatively high temperature at a relatively low pressure, so I'm going to put the piece of marble back in here just for something to drop this against and see if you can tell the difference in the sound of these two things. Here's one of them. Okay, it's the sound, it has sort of a ringing sound, and I hope this comes across on the TV speakers as sort of a ringing sound. By comparison, this one makes a dull thud. It's kind of the same difference that you find between a piece of silver and a piece of lead. Hear the ring? The dull thud, of course, is the softer shale; whereas, the ringing sound is the more highly baked and more consolidated slate. In many cases, this is the only way you can tell them apart.

Okay, I also have another bit of comparisons here. Let me take the marble out of the way. I have a piece of black slate just to show you that the slate sometimes retains its organic material and stays dark, but in slate, you see, the grains are extremely fine. Their silt size grains, so it feels kind of gritty to the finger, but you can't see the individual grains. At a slightly higher grade of metamorphism, a type of rock called "phyllite " forms. The difference is that the phyllite although still fine grained has grains large enough that you start to see the individual sheen or the shine off of the individual grains. Usually, in this case, these are grains of mica, which are oriented along the directions of the differential pressure; in other words, the mica grains all tend to become alive. I think you can see the nice sheen on this piece of phyllite, as opposed to the rather dull piece of slate over here on this side, so the essential difference between the slate here and the phyllite here is the size of the grains which gives the phyllite this shiny appearance.

Okay, I have another piece of phyllite, which is a little bit lighter in color and also a larger piece, but here you can see, you can almost imagine that you can see the individual grains, but as I rotate it around in the light, you can see that it still catches that sheen from the light surface. You'll also notice on the edge here the foliation, okay, the layers like the pages of a book along the edge of this.

Okay, at a slightly higher grade of metamorphism we come across something that we might call a shist. A shist has larger grains yet, and this one although you can't see the individual textures and individual grains, as I move it around in the light you see the sparkles. The sparkles here come from the tiny plates of mica, in this case muscovite mica, which are all aligned, not completely flat, but sort of like what you would get if you threw playing cards into a pile, almost flat, overlapping, but you can see the individual sheens. This particular piece also has crystals of garnet. Those are the dark specks that you see that kind of look like raisins in the cookie. The garnet is a equilateral mineral, meaning it doesn't tend to have a preferred direction of growth, so it doesn't participate in the foliation, and, I think you can even see the foliation on the edge of this along here. Again, it looks like the pages of a book. At a slightly higher grade of metamorphism or a slightly higher degree of metamorphism the minerals may actually begin to separate themselves into layers. This is a piece of an amphibolite shist, where the main mineral is amphibole, and again you can see the individual sparkles on the surface; you can even see the elongation of the grains of amphibole. If I turn this on its side, you can begin to see the separation of the dark minerals and the light minerals into layers. I think you can probably see it better on this side. It's beginning to develop dark and light colored bands, so here we see the dark minerals beginning to separate and aggregate the mafic minerals; whereas, the lighter minerals, in this case mostly quartz and muscovite mica tend to concentrate together so that we begin to develop this banded appearance. This is also a shist, but it's tending toward gneiss, not nice like in very nice, but "gneiss," the German word G-N-E-I-S-S. In a true piece of gneiss, the banding is extremely well developed as seen in this rock. Notice here that again the black minerals, which are amphibole again have almost completely separated from the lighter minerals, quartz, and the grain size is fairly large. You can see the individual sparkles of the grains as I move it around, so all of these different types represent various degrees of metamorphism, and in some cases as in this piece of gneiss, you see the foliation is again very well developed, but you can also see the result of extreme compressional processes that cause this rock to form. Notice in this case the bands sort of take on the appearance of geologic folds. What's happened here, of course, is that this rock has been squeezed, but it's been squeezed while the minerals are forming, so instead of having nice straight pressure regions, the pressure regions are, in fact, folded, and you can see those folds preserved very nicely in this piece of gneiss. At different angles, you can see the folds from slightly different angles. This piece has been cut and polished. This, by the way, came from New Zealand in the mountain ranges of the Southern Alps in New Zealand, so let's move on and let's see if we can try to understand some of these processes. What we're viewing here with these samples is what's often called "progressive" metamorphism that a single parent rock can give rise to a variety of metamorphic types. A sequence is commonly observed when we approach the core of a mountain range over distances of hundreds of miles. For example, starting in Western Pennsylvania and working your way into the Appalachians, you would find that you note that the sequence of rock going from shale, to slate, to phyllite, to shist, to gneiss at the center of the mountain range. The exact degree of metamorphism or the extent of metamorphism depends upon the depth of burial and the amount of time. The temperature of formation of the gneiss, the banded variety, is probably close to the melting or crystallization temperature of granite. All of these other things, remember, take place at relatively lower temperatures, not in the solid state, not high enough to be melted, so embodied in this concept of the progressive metamorphism is the idea of metamorphic phases. In geology, the word "phases" simply means a change in the rock within a one stratigraphic layer. Each mineral has associated with it a stability field of temperature and pressure, an envelope of stability if you like. Certain index minerals are common in metamorphic rocks of various grades, and here by grades we mean the amount of metamorphism. These index minerals can be used to determine the temperature and pressure that were present during the formation of a given metamorphic rock, and even the metamorphic phases typically show a sequence around a large regionally metamorphosed body.

You might want to refer to Box 15.2 on page 342 in the text with this concept of metamorphic phases and to see the diagram. Well, it might seem natural to ask what happens if rocks do become completely hot enough to melt. Well, there we've crossed the boundary between igneous rocks and metamorphic rocks, and, in fact, one of the great controversies surrounding the origin of igneous rocks in the first place was do they form by melting, or do they form by extreme metamorphism. Well, recently geologists have discovered that there's a kind of a special process called "metasomatism," which is a metamorphic process that produces a rock that looks very much like granite. You might say that metasomatism represents the ultimate grade of metamorphism, which is melting, but there's a significant difference here that normally during metamorphic processes very little water or very little new atoms are added, but in metasomatism water brings new minerals or new ions in from the outside in solution and may actually remove other ions. The things that are brought in are usually silica rich and aluminum rich things like quartz and feldspar, and the things that are taken away are the mafic minerals or the ferromagnesians. These rocks may show both foliations and granitic structures. I have one example here of a particular piece of material called "migmatite," which, on one hand, looks like granite, and on this part looks like granite, but here you see these very distinct foliations which represent the same sort of foliations we find in a gneiss, so the question is: Is this rock metamorphic or is the rock igneous? Because it appears that these changes took place in the solid state, but yet it has the texture of granite. I might just close this discussion of metasomatism out by noting that geologists often say, "Well, when it comes to granite, there's "granite" and there's "granite." It sounds like a good Yogiism if you're into baseball. I might also, I want to call attention to the idea of hydrothermal rocks as well. "Hydrothermal" means hot water, and hydrothermal rocks are formed by the action of hot water which contains dissolved atoms or ions. Hydrothermal processes are responsible for the deposition of metallic ores. These ore minerals are carried as atoms in solution and may crystallize in a particular sequence as the hot water moves away from a magma source or from the region of metamorphism. The water itself cools as it moves away through cracks. It's significant to note here that certain ore minerals are almost always associated with one another, minerals like pyrite, and galena, and sphalerite, and cinnabar, which are all sulfides, generally occur in an aureole or a ring around metamorphic regions in a particular sequence. It's also noteworthy here that the water may pick up additional atoms while it's passing through the country rock. We will come back and look at ore minerals in somewhat more detail in the last couple of programs.

Well, I hope this has helped to unravel the complexities of metamorphism and metamorphic rocks. The processes themselves are relatively simple, but the products are many, and the results are amazingly complex. We have seen, I think, that we can learn much about the processes deep in the Earth by studying rocks which were formed there and brought to the surface by the interplay or erosional and tectonic forces as long as we understand the conditions under which these minerals are formed, which we can do in the laboratory, so in the next program we'll begin to direct our attention to the idea of stream processes and erosion, so the text assignment for next time is Chapter 16, "Streams and Landscapes," and will actually take two programs to cover this chapter, so at this point read the introduction and the summary and examine the photograph on page 349 and notice in this photograph the little waterfalls, the turbulence, the waterworn boulders, and also the mass wasting on the far side of the stream. Read the chapter at least as far as the section on "Valley Development," which would be pages 349 to 368 and pay close attention to photographs and diagrams. The remainder of this chapter then will be covered in the final lesson, which would be Program 24, Lesson 20, so with all this in mind, study hard, try to find some metamorphic rocks and enjoy them, and

I'll see you next time.