Gouged out of the face of the earth by the Colorado River. This is one of the most intricate systems of canyons, gorges, and ravines in the world. The spectacular beauty of this awesome abyss draws millions of visitors each year. But the grand canyon is more than just a tourist attraction. It is a stunning microcosm of this planets geologic history. The first geologic study of the grand canyon was led by geologist John Wesley Powell in 1869. Powell later wrote that rocks exposed in these canyon walls were an open book of geology. The Colorado River had opened the landscape, revealing one of the most complete rock records on earth, spanning almost two billion years of earth history. To a geologist, a layered sequence of sedimentary rocks is a historical record to be read like a book. Analyzing these rocks from the base to the top, were sifting through a wealth of information about conditions at the earths surface when the sediments were deposited. Rock characteristics such as mineral composition, grain size and shape, structures within the rock, and even rock color, tell us a great deal about the climate, vegetation patterns, position of the shoreline, and the topography of the earths surface in the geologic past. The challenge of sedimentary geology is interpreting these clues in the rocks. But we must first understand how sediment is formed and how its transformed into solid rock. Sediment is the product of mechanical and chemical weathering, and of erosion by wind. Water. And ice. Biological activity also plays a role. Sediment may be deposited in the form of sand building up as a dune. Or a beach. As pebbles piling up in a stream. Or shells and organic matter accumulating on the ocean floor. When a thick pile of sediment accumulates, the particles near the base of the pile are compacted under the overlying deposits. Eventually, they are cemented together to form a solid aggregate rock. Although 95 of the earths crust is made up of igneous or metamorphic rocks, the surface itself consists primarily of sedimentary material. The composition of this sediment is controlled by two factors weathering and erosion. Weathering and erosion influence the composition of sedimentary rocks because, first of all, the mechanical weathering breaks the rocks into smaller pieces, making them more easily eroded. Chemical weathering breaks down certain minerals in preference to others. One of the most common forms of sediment is referred to as clastic, from the greek word for broken. Fragments of rocks and minerals falling from an eroding outcrop are examples of clastic sediment. Clastic sediments are classified by size, ranging from large boulders through smaller cobbles and pebbles. Then grains of sand. Even smaller grains of silt. And finally down to the finest sediment of all claywhich has the consistency of flour. Sediment can be transported in various ways. It can slide down a hillside, be blown by the wind, or be carried along by a flowing stream. As sediment is transported, it tends to be smoothed and rounded as fragments hit and scrape against one another. Sorting is controlled by the size and weight of particles, with the heaviest sediment being deposited first, and finer sediment transported considerable distances. [dee trent] a deposition typically occurs wherever theres a slowing of the running water. A good example would be at the base of a Steep Mountain range, where the rivers are coming out of a steep canyon into a flat valley. The kinds of materials that would be formed there would be very coarse grained. The further away you get from the front of the mountain range, the finer the grain is of the sediment being carried. Consequently, you would get finer grained sandstones or siltstones further away. The same thing happens in an ocean, where a river enters the ocean. The first things that drop out are the coarse grain materials, generally sandsized materials. Further out to sea there will be fine grain materials. The sands are usually found on the coasts, where the muds and clays would be further offshore. The process which converts loose clastic sediment into solid sedimentary rock is known as lithification, from the greek lithos, meaning stone. The first mechanism is one of simple compaction. The weight of the overlying sediments squeezing down, grain by grain, causing the grains to ultimately rearrange so that they get into close packing and finally distortion and perhaps solution of the grains so that, ultimately, you have a very tightly packed assemblage of sedimentary grains. At the same time, there may very well be chemical precipitation within the pore space, and thats called theres two terms that are used here, but the best term for that is cementation. These two processes acting in concert go to make a loose sediment into a hard rock. In contrast to lithification, sedimentary rocks may also form from the precipitation of chemicals out of water. One common site of chemical sedimentation is desert lakes and lagoons. As evaporation occurs, the chemicals become increasingly concentrated in the water until they can no longer remain dissolved. They combine with one another, forming minerals such as calcite, gypsum, and salt. Deposits formed from evaporation are called evaporites. Chemical sedimentation also takes place in the ocean. Biological processes play a crucial role in triggering this phenomenon. Algae, coral, and invertebrate organisms all utilize Calcium Carbonate in constructing shells and reefs. When these organisms die, their carbonate hard parts accumulate on the sea floor to form limestone, one of the most common sedimentary rocks on earth. Limestones vary greatly in appearance, from formations packed with large fossils to beds of chalk formed from the microscopic shells of plants and animals. Life also contributes to the formation of sedimentary rocks other than limestone. In the cool, nutrientrich water near some continental shelves, radiolaria and diatoms thrive. These suspended microscopic organisms use silica to make their shells. When they die and settle to the sea floor, the silica accumulates to form layers of chert and diatomite. In swampy bayous and deltas on shore, the remains of moss, leaves, roots, and tree trunks may gradually compact over millions of years, giving rise to another sedimentary rock coal. Coal is formed in areas of swamps, quiet water, like okefenokee swamp and areas in Southeast Asia where you have lots of vegetation in Shallow Water over millions of years and material grows, dies, settles down. Due to the chemistry of the water, the material does not rot away. Layer after layer builds up and with time, sufficient pressure, its converted first to peat and eventually to coal. The places where sediment is deposited vary enormously, from glacial valleys. Lakes. Beaches. River deltas. To the sea floor. These environments of deposition are nearly always initially associated with water, but may eventually transform into dry land. Geologists interpret the characteristics of sedimentary rocks using the principle of uniformity. This principle is a model of the way sedimentary rocks form. According to this model, we accept that sedimentary rocks have formed throughout geologic time in exactly the same way that sediments are forming today. Throughout earth history the same geologic processes, such as weathering, running water, wind, tides, changing sea level, have created and deposited sediments that eventually were hardened into rock. The principle of uniformity says that the present is the key to the past, so as we study sediments being deposited right now and associate their characteristics with the conditions at the earths surface, we are creating models for interpreting environments in the geologic past. This principle can be very easy to use. This limestone, for example, is made of gray Calcium Carbonate mud, a little quartz sand, and the fossils of marine organisms. Sediment being deposited in shallow tropical seas, such as the bahamas and the florida keys, would look very much like this limestone if they were hardened into rock, so in this case uniformity is telling us that this area was covered by a shallow tropical sea about 250 million years ago. When geologists use the principle of uniformity to analyze an entire sequence of sedimentary rocks, the changing environments recorded in that sequence shows us how the earths surface itself changed and evolved through time. The deposition of sediment is recorded in the rock record as sedimentary structures. [walter reed] sedimentary structures are useful to us because they can allow us to reconstruct the environments of deposition of the sediments. Thats where the sedimentary structures are formed. Theyre not formed during erosion, theyre not formed during transportation, but during deposition, so the sedimentary structures and theres a very wide variety of them are indicative of a given site of deposition. One of the most obvious of these structures is bedding the layer cake pattern of rock strata. The contact between two layers of rock is called a bedding plane. [reed] one of the very distinctive, and probably the most compelling sedimentary structure that one sees when one looks at a stack of sediments, is beds. Bedding surfaces represent interruptions. They may be long interruptions or short interruptions. They may be just pulses of sediment where an interruption lasts just a momentary interruption. They may be interruptions of thousands or even millions of years. What is very obvious from everything we know now is that the sedimentary column that we have, the bedding surfaces themselves, almost certainly represent far more time than the stack of sediments that we have preserved. The law of original horizontality states that most bedding initially forms in a horizontal orientation, as material settles to a lake bed or the sea floor. But in some cases, sloping layers of sedimentary rock build up. For example, wind can pile up sand as dunes. As mineral grains of diverse color and composition are blown across the dune, discernible layering can develop inclined at an angle parallel to the slope of the dune. Such angled layers also develop in sand bars and stream deltas. Geologists refer to this as crossbedding because it cuts across the direction of ordinary horizontal bedding. Sets of crossbeds often develop. For example, at any gigiven point in a stream, periods of deposition may alternate with periods of erosion as the velocity of the wind or water changes. A set of crossbeds in a bar will be truncated by erosion. Then covered by another set of crossbeds when deposition resumes. Crossbedding is a sedimentary structure that is very revealing. If we look at a set of crossbeds that are very steep, that are truncated by the next set of crossbeds, then we know that we had a river system or a depositional system that was constantly interrupting itself and shifting around. If we have a smooth progradation of crossbeds, we know theres no interruption and its just a steady stream of deposition in more or less a constant fashion. Geologists find crossbeds useful in determining the direction of sediment transport in ancient river and dune systems. Crossbeds form perpendicular to the direction of the water current and tend to slope downstream. If youve ever looked at a river in any kind of detail, you know that a river doesnt run in straight line. It meanders around. So if we get many crossbed measurements, then we can come up with a statistical average for the direction the river was flowing. If we go downstream, down that river well find that sorting becomes better, sediment size tends to become less, and so forth. Another common sedimentary structure, ripple marks, often develops in soft beds of sand lying in Shallow Water. The toandfro motion of waves creates symmetrical ripples. When theyre sculpted by the oneway motion of currents, they are asymmetric, with their steeper sides facing in the downcurrent direction. Sometimes the surface of a fine sedimentary bed is also broken up into a pavement of fossil mud cracks, either with the fissure still open or with the gaps filled in by later deposits of sediment. [reed] mud cracks are a very good indicator of environments. Weve all been in river beds or in tidal flats in which weve seen desiccation and formation of mud cracks. These are preserved in the geologic record. If a sand, uh, is if sand is washed over that mud crack, it infiltrates down into the end of the crack and preserves the crack. We can identify them in river deposits. They show up on natural levees. They show up on flood plains and so forth. They show up in meandering, as well as braided streams. They show up in deltaic deposits where a given delta distributary is stranded or abandoned. And so they can be very useful us because they tell us that the situation at the time of sedimentation was such that it became dry. By understanding how mud cracks, ripple marks, crossbedding, and other sedimentary structures and textures form, geologists can, in a sense, read the sedimentary rock record. This allows them to reconstruct the physical appearance of ancient landscapes. One place such work is being done is known as the ridge basin, a thick sequence of sedimentary rocks sandwiched between the san andreas and the san gabriel faults in Southern California. Today, this area is not a basin at all. It is instead part of a young, growing mountain range. Yet 5 to 10 million years ago, it was a deep depression. To determine how the ridge basin formed sedimentologists like cathy busbyspera attempt to find out what the ridge basin once looked like. Here were looking at a very thick succession of finegrain deposits that we can interpret as lake deposits. There are several lines of evidence that we can use to determine this was deposited in a lake. First of all, the fine grain size and the thinness of the beds indicates slow deposition by finegrain sediment settling through a water column. Another thing is that the beds are very laterally continuous, as you can see in this outcrop, which would suggest deposition in a big feature like a lake rather than a small feature like a river channel. The third thing that we cant really see from here is that some of the thinner beds in this succession are limestones and they bear micro fossils that indicate deposition in fresh water. Another feature we can use to identify a lake is mud cracks and wave ripples. These features all taken together the wave ripples, the mud cracks, the freshwater fossils, and the finegrain nature of the deposit, give us real convincing arguments that this is a lake setting. In places, the edges of the ancient lake can be seen in the rocks. The rivers which once poured into this lake have also left evidence of their previous existence. This lake was the site of deposition of finegrain sediments or muds, but periodically, rivers built into this lake. What were looking at here then are the deposits of a river that built out into a lake. This is referred to as a delta. We see a thickening and coarsening upward sequence of beds. The initial deposits of the river are thin to mediumbedded sandstones and these represent sands that jetted out into the lake from the mouth of the river. The relatively coarsegrained river deposits stand in contrast to the finergrained, thinlybedded lake deposits. Another sedimentary structure, known as scourandfill, indicates that the velocity of the river fluctuated over time. Within this sandstone bed, we can see a sedimentary structure referred to as scourandfill. This is a surface running through the rock right here. What we see is that the underlying sandstone has bedding thats truncated by this surface. What this indicates is that. Flow was so strong during the initial deposition of this sandstone that the current eroded the previously deposited sands, and then as the current velocity slowed, it began to deposit material. First, it deposited pebbly sands and also deposited these ripup fragments of mud and then deposited sand above that. So what we have in this scourandfill surface is evidence again of fluctuating current activity within the river channel. At the margin of the basin, next to the san gabriel fault, evidence suggests that the fault was active at the time the basin formed. Were looking at coarsegrained sedimentary rocks of the basin margin. You can see that the clasts are large and very angular and that the rocks are very poorly sorted that is, its a mixture of all different grain sizes from sand on up to cobbles. The angularity and the poor sorting indicate that this material has not traveled very far from the source area and was probably shed from the active fault scarp as the basin was downdropped. Sedimentologists have concluded that this fault activity could itself have created the ridge basin. Owing to a bend in the san gabriel fault, movement would have stretched the crust in the area of the basin, causing it to sag downward. Through careful field work combining the study of sedimentary rocks with local tectonic history, geologists have reconstructed a detailed view of the ancient ridge basin. Geologists use their understanding of sedimentary rocks to do more than reconstruct the history of the earths surface. Most of the economically valuable resources that are extracted from the earths crust come from sedimentary rocks. Most people know that sedimentary rocks are the source of fossil fuels, such as oil, natural gas, and coal, but the economic value of sedimentary rocks influences almost every part of our lives. For example, virtually all buildings and public structures require sedimentary rocks in their construction. The cement and sand and gravel used to make concrete, iron ore for steel, bauxite used in making aluminum, brick and tile, cutstone used for facing large buildings, and even asphalt for the roads which make these buildings accessible. In fact, almost everywhere you look you can find examples of the commercial and industrial uses of sedimentary rocks. Though considerably less dramatic than such phenomena as volcanoes or earthquakes, sedimentary rocks are ultimately very important to our modern civilization. Not only can they be economically valuable resources, but in the composition and structure of sedimentary rocks lies the best record of earths long and complex history. To a geologist, this great stack of rocks in the walls of the grand canyon contains a fascinating story of earth history. Its a unique record of changing conditions at the earths surface through time and a storehouse of information about the mountains and the source rocks that provided the sediments themselves. Once we understand the significance of these rocks and of the clues they contain, we can read the record of earth history, like pages in a book, in the rocks of the canyon walls. The oldest rocks in the canyon were deposited as sediments about two billion years ago. A great pile of sand and mud, interbedded with volcanic ash and lava flows, filled a rapidly subsiding marine basin. These rocks were deformed in a mountain building episode into an ancient mountain range, the mazatzal mountains, about 1. 5 billion years ago. The mountains were deeply eroded and then covered with a mixture of limestone and shale, as sea level rose and flooded the region, creating an unconformity about one billion years ago. Mountain building and volcanic activity were then reactivated, followed by yet another rise in sea level and deposition of more marine sediment. In fact, the rocks of the grand canyon record no fewer than four more repetitive episodes of structural uplift and erosion, followed by deposition of more sediment as sea level encroached on the land and nearby mountains shed sediment into the region. And this process continues today. The Colorado River is now cutting an extraordinary unconformity surface, which will eventually be covered with sediments once again. I sometimes wonder what the grand canyon will look like when that happens. Will it be covered by an ocean . Will the sediments be shed from mountains yet to be formed . No one yet knows, but one things certain. The answers will be recorded in the sedimentary rocks. Captioning performed by the national captioning institute, inc. Captions copyright 1991 the corporation for Community CollegeTelevision Major funding forearth revealed was provided by the Annenberg Cpb project. Major funding for earth revealed was provided by. Captioning made possible by Southern California consortium throughout history, mountains have been deeply embedded in the human experience. Weve worshipped them, created nations using them as boundaries, stripped them of valuable resources, and returned to them for inspiration and recreation. If you are at all curious about the earth, youve probably wondered why mountains exist. This question has intrigued earth scientists ever since the emergence of geology as a science in the late 18th century. The more we learn about mountains and what theyre made of, the more fascinating this question becomes. Most mountains are forming today in tectonically active regions where the movements of plates deform the rocks of the earths lithosphere. Their Tremendous Energy thats expended in the mountainbuilding process often has a profound effect on these rocks. The geologic events that accompany mountainbuilding, such as the collisions between plates, deep subsidence of portions of the earths crust, moving masses of magma, and the displacement of rock bodies along fault zones focus heat and pressure on the rocks. As a result, these rocks are changed dramatically. This process of change by 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 dating. During metamorphism, atoms within the rock can dislodge themselves from mineral lattices and move about freely. This atomic reshuffling causes 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 bizarrelooking and strikingly beautiful of all crustal rocks. But to a geologist, 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 platey, black, finegrained stone, to granitelike 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 is 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 condition under which metamorphism occurs is beneath the level of weathering and sedimentation to form the sedimentary rocks, generally at temperatures greater than 200 degrees and at conditions that do not produce a melt such as 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 formation of the rocks occur at depths generally from 2 to several 10s of kilometers in depth beneath the earths surface. At the surface, we are accustomed to the pressure of the air surrounding us. We dont 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 can be applied on different parts of a rock, causing it to bend or shear apart like a sliding deck of cards. Whether from shearing 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 shear 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 yin 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. Shear stress causes the garnet to rotate during growth, distorting the planes into swirls. The threedimensional form of the swirling pattern can be shown by means of a multiringed model. Each ring represents the edge of a plane of Minerals Incorporated by the growing garnet. These are the rings. And lets 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. We have a little pit here, a mound over here, and the diameter of rotation here. We can compare that with a real specimen, which is over here. This is a real specimen in which we see the little pit here, the mound here, the common axis through here, and it shows the snowball pattern. We can see a crosssection of a garnet thats growing 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. Shear stress, too, can cause foliation. Ill hold a piece of a metamorphic rock we call a mica schist. We can see its 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 and great temperature, it is also recording in it the intensity of the stress field. We see stresses that had to be in some direction to have planerized the micas, forming the mica schist. 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 heterogeneneity and weak directions as you find within landslideprone, for instance, sedimentary rock. Although we consider these basement rocks to be quite stable, in reality, schists can be very unstable. Many engineering firms that were wanting to construct either houses or dams or other constructions on metamorphic rock has to take in consideration the foliation and the direction of the foliation to make sure that it isnt 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 coarse texture. In some circumstances, the temperature of a deeply buried rock can 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 migmatites, or mixed rocks. In some migmatites, geologists find evidence for the origin of one of earths 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 as a result of these metamorphic reactions is that rocks undergo changes in composition. And mainly, this shows up in the formation of the liberation of h2o and its departure from the rocks. 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. In most metamorphic rocks, that means mainly the loss of h2o and co2. Fluids are released not only by metamorphic rocks at high temperatures, but from magmas intruding the metamorphic rocks as well. During mountainbuilding, 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 rocks is created by the numerous protoliths, the presence of fluid, and the wide range of temperature 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 and pressure are 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 wide areas, 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 handsized sample of a contact. On the left side is an intrusive igneous rock, a tonalite that came into contact with a limestone. The limestone was heated up, and at the same time there were elements that left both 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 the brown material here. Silica continued in further into the marble than did the other two elements, resulting in a layer of the mineral wollastonite, which is a calcium silicate. This is a good example of a welllayered rock produced by regional metamorphism in the socalled amphibolite facies. It consists of alternating layers the light of quartzite, and of dark, a mica schist bearing the aluminum silicate mineral sillamanite. At first, youd think these are relic sedimentary beds, but theyre not. Theyve been structurally transposed from sedimentary beds into tectonically bounded layers. Here you can see two of the biotiterich layers coming together to constitute a single layer. So there has been a great deal of directed shear pressure through this rock that has produced this welllayered aspect to it. Metamorphism has been compared to cooking. The dish that you wind up with depends on your starting ingredients and 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 an example of how minerals are used to interpret the metamorphism of basalt, the rock that makes up the ocean basins crust. Pressure on the chart increases downward and temperature increases to the right. This is a piece of unmetamorphosed basalt, which formed at the earths 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 reequilibrates to its new condition. These zones on the chart are called metamorphic facies. Each facies 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 6 kilometers. This metamorphosed basalt would plot here on the chart in the amphibolite facies. This is a metamorphosed basalt that contains zeolites, another mineral group, and it plots here in the zeolite facies. The zeolite facies corresponds to less intense metamorphic conditions than does the amphibolite facies, so rocks containing zeolites are said to be of lower metamorphic grade. Metamorphism is a process of progressive change. As rock are exposed to higher temperatures and pressures, theyre altered in a predictable manner. As the intensity of metamorphism increases, the rocks become harder and more coarsely crystalline, and develop special metamorphic textures. Geologists refer to progressive metamorphism as an increase in metamorphic grade from low to high. 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 claystone like this, as sedimentary rock, relatively luminous rich. On heating this under the low part of the regional metamorphism, we develop a very layered finelineared rock called a slate. It doesnt have any recognizable minerals because they havent grown large enough, but due to the growth of new minerals and of the directed pressure, we end up with a wellfoliated rock, possible to cleave into regular thin layers. With increased temperature and pressure at the slate, its transformed into a slightly higher grade rock which is called a phyllite. This rock has a linear structure as well as to the foliated structure that it is slightly different in luster due to the larger micasized crystals. As the temperature and pressure increase further, we develop a schist. This is a garnet schist with large garnet crystals. Lots of white mica. Very coarse crystals. This would be formed at quite high metamorphic grade. At a higher metamorphic grade that you 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 arent 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 occurring 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 facies to the pressure and temperature conditions of metamorphism, but they had no satisfactory explanation for the geologic processes that from metamorphic rocks. That is, until the theory of plate tectonics emerged. One good example is this relatively rare metamorphic rock called blueschist. Experimental work had shown that the minerals in blueschist form under very unusual metamorphic conditions. These conditions are a pressure range equivalent to a depth of 1530 kilometers in the crust, and a very cool temperature, only 200400 degrees centigrade. Thats the approximate cooking temperature of a kitchen oven or toaster. At a depth of 1530 kilometers, the temperatures normally about twice as hot, 500750 degrees centigrade. The only way that rocks can be metamorphosed to blueschist facies is to be quickly shoved down to those extreme depths and rapidly brought back up before the rocks have time to heat up. Thats exactly what happens where two tectonic plates are colliding in a subduction zone. In fact, blueschistbearing 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 1o, 1o 1 2 million years to grow, and is correspondent to a growth rate of roughly a few atomic diameters per year. 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 an ordinary tree might grow, so its 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 pangaea. 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 flatlying 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 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. In other words, how fast the garnets were rotating. That tells us how fast the rocks around the garnets that were causing that rotation were deforming. That is something that has never been measured before. Its of considerable interest for people studying tectonism, because were actually measuring the rates at which the rocks get folded, and thats another factor were 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 mountainbuilding spanning millions of years. Metamorphism is a fundamental rockforming process on earth. About 15 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 forms. 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 movements 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 mountainbuilding. The collisions and intrusions and fault zones that metamorphose the rocks are concentrated at plate margins. As rks are depressed to great depth, say 10s 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 are seeing a brief glimpse of this cycle of rock formation and change, a cycle thats as old as the earth itself. Captioning performed by the national captioning institute, inc. Captions copyright 1991 the corporation for Community CollegeTelevision Major funding for earth reveal was provided by. Funding for this program [with captioning] was provided by additional funding is provided by and narrator each video episode has three parts. Watch the program, read your book, discuss the program and. Rebecca that would be enough, enough for me Everybody Needs a dream catcher