Metamorphic rocks are divided into two categories- Foliates and Non-foliates. Foliates are composed of large amounts of micas and chlorites. These minerals have very distinct cleavage.
Foliated metamorphic rocks will split along cleavage lines that are parallel to the minerals that make up the rock. Slate, as an example, will split into thin sheets.
Foliate comes from the Latin word that means sheets, as in the sheets of paper in a book. Silt and clay can become deposited and compressed into the sedimentary rock shale.
The layers of shale can become buried deeper and deeper by the process of deposition. Deposition is the laying down of rock forming material by any natural agent wind, water, glaciers over time. Because these layers are buried, temperatures and pressures become greater and greater until the shale is changed into slate.
Slate is a fine-grained metamorphic rock with perfect cleavage that allows it to split into thin sheets. Slate usually has a light to dark brown streak. Slate is produced by low grade metamorphism, which is caused by relatively low temperatures and pressures.
Slate has been used by man in a variety of ways over the years. One use for slate was in the making of headstones or grave markers. Slate is not very hard and can be carved easily.
The problem with slate though is its perfect cleavage. The slate headstones would crack and split along these cleavage planes as water would seep into the cracks and freeze which would lead to expansion. This freeze-thaw, freeze-thaw over time would split the headstone. Today headstones are made of a variety of rocks, with granite and marble being two of the most widely used rocks.
Slate was also used for chalk boards. The black color was good as a background and the rock cleaned easily with water. Today it is not very advantageous to use this rock because of its weight and the splitting and cracking over time. Schist is a medium grade metamorphic rock. This means that it has been subjected to more heat and pressure than slate, which is a low grade metamorphic rock. As you can see in the photo above schist is a more coarse grained rock.
The individual grains of minerals can be seen by the naked eye. Many of the original minerals have been altered into flakes. Because it has been squeezed harder than slate it is often found folded and crumpled.
Schists are usually named by the main minerals that they are formed from. Bitotite mica schist, hornblende schist, garnet mica schist, and talc schist are some examples of this. Gneiss is a high grade metamorphic rock. This means that gneiss has been subjected to more heat and pressure than schist. Gneiss is coarser than schist and has distinct banding. This banding has alternating layers that are composed of different minerals. The minerals that compose gneiss are the same as granite.
Feldspar is the most important mineral that makes up gneiss along with mica and quartz. Well, hello, again and welcome. I'm glad you could join us today for our program on "Metamorphic Rocks. We've seen how chemical weathering rearranges atoms; we've seen how atoms in collections, piles of sediments are deposited and turned into sedimentary rock, but rearrangements of atoms also take place when rocks are buried deep in the Earth's crust and exposed to heat,pressure, and hot water.
Metamorphic rocks are formed from pre- existing rocks as atoms arrange themselves in new crystal structures. Everything we know comes indirectly from occurrences of rocks and laboratory studies, but we do know that metamorphic rocks usually form at plate boundaries in connection with mountain building processes and with igneous intrusions.
The conditions deep in the earth are different enough that these different conditions cause minerals different from those which form at the surface. For example, surface weathering process, which is also a change of the types of minerals causes hard rocks to change into small crumbly pieces.
Metamorphism, on the other hand, takes place at high temperature and pressure and changes those same soft sedimentary rocks back into hard crystalline solids. Metamorphic rocks are complicated because you have only a few parent rock types which can provide different kinds of atoms for starting materials like the basic ingredients used in cooking, but the different conditions create many different products.
It's like being able to use the same ingredients in different recipes to come up with different final products. In the process of metamorphism assemblages of minerals are created, which tell us the conditions at the time of their formation. We rely upon comparisons of laboratory studies of these various minerals and their occurrences in the field to tell us the conditions under which they formed.
We also know that usually no new atoms are added during metamorphism, but new minerals are created by simply rearranging the existing atoms into new crystal structures. 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 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 Note the size of the microfold and the name of the red mineral that occurs and also look at Figure 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. 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.
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. 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.
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. 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 degrees and at conditions that do not produce a melt that goes into igneous rocks, so the range in temperatures are roughly about degrees C to about 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. Preferred orientation of sheet silicates causes rocks to be easily broken along approximately parallel sheets. Such a structure is called a foliation. Fluid Phase.
This fluid is mostly H 2 O, but contains dissolved ions. The fluid phase is important because chemical reactions that involve changing a solid mineral into a new solid mineral can be greatly speeded up by having dissolved ions transported by the fluid. If chemical alteration of the rock takes place as a result of these fluids, the process is called metasomatism. Time - Because metamorphism involves changing the rock while it is solid, metamorphic change is a slow process.
During metamorphism, several processes are at work. Recrystallization causes changes in minerals size and shape. Chemical reactions occur between the minerals to form new sets of minerals that are more stable at the pressure and temperature of the environment, and new minerals form as a result of polymorphic phase transformations recall that polymorphs are compounds with the same chemical formula, but different crystal structures.
Laboratory experiments suggest that the the sizes of the mineral grains produced during metamorphism increases with time. Thus coarse grained metamorphic rocks involve long times of metamorphism. Experiments suggest that the time involved is tens of millions of years. Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form.
Low-grade metamorphism takes place at temperatures between about to o C, and relatively low pressure. Low grade metamorphic rocks are characterized by an abundance of hydrous minerals minerals that contain water, H 2 O, in their crystal structure. Examples of hydrous minerals that occur in low grade metamorphic rocks: Clay Minerals Serpentine Chlorite High-grade metamorphism takes place at temperatures greater than o C and relatively high pressure.
As grade of metamorphism increases, hydrous minerals become less hydrous, by losing H 2 O and non-hydrous minerals become more common. Examples of less hydrous minerals and non-hydrous minerals that characterize high grade metamorphic rocks: Muscovite - hydrous mineral that eventually disappears at the highest grade of metamorphism Biotite - a hydrous mineral that is stable to very high grades of metamorphism.
Pyroxene - a non hydrous mineral. Garnet - a non hydrous mineral. Retrograde Metamorphism As temperature and pressure fall due to erosion of overlying rock or due to tectonic uplift, one might expect metamorphism to a follow a reverse path and eventually return the rocks to their original unmetamorphosed state.
Metamorphic Rock Types There are two major subdivisions of metamorphic rocks. Non-foliated Metamorphic Rocks Non-foliated rocks lack a planar fabric. Absence of foliation possible for several reasons: Rock not subjected to differential stress. Dominance of equant minerals like quartz, feldspar, and garnet. Absence of platy minerals sheet silicates. Protolith Composition Although textures and structures of the protolith are usually destroyed by metamorphism, we can still get an idea about the original rock from the minerals present in the metamorphic rock.
General terms used to describe the chemical composition of both the protolith and the resulting metamorphic rock are: Pelitic Alumina rich rocks, usually shales or mudstones. Types of Metamorphism Metamorphism can take place in several different environments where special conditions exist in terms of pressure, temperature, stress, conditions, or chemical environments. Contact Metamorphism also called thermal metamorphism - Occurs adjacent to igneous intrusions and results from high temperatures associated with the igneous intrusion.
Since only a small area surrounding the intrusion is heated by the magma, metamorphism is restricted to a zone surrounding the intrusion, called a metamorphic aureole. Outside of the contact aureole, the rocks are unmetamorphosed. The grade of metamorphism increases in all directions toward the intrusion. Because temperature differences between the surrounding rock and the intruded magma are larger at shallow levels in the crust, contact metamorphism is usually referred to as high temperature, low pressure metamorphism.
The rock produced is often a fine-grained rock that shows no foliation, called a hornfels. Burial Metamorphism - When sedimentary rocks are buried to depths of several hundred meters, temperatures greater than o C may develop in the absence of differential stress.
New minerals grow, but the rock does not appear to be metamorphosed. The main minerals produced are the Zeolites. Burial metamorphism overlaps, to some extent, with diagenesis, and grades into regional metamorphism as temperature and pressure increase. Dynamic Metamorphism - This type of metamorphism is due to mechanical deformation, like when two bodies of rock slide past one another along a fault zone.
Heat is generated by the friction of sliding along the zone, and the rocks tend to crushed and pulverized due to the sliding. Dynamic metamorphism is not very common and is restricted to a narrow zone along which the sliding occurred.
The rock that is produced is called a mylonite. Regional Metamorphism - This type of metamorphism occurs over large areas that were subjected to high degrees of deformation under differential stress. Thus, it usually results in forming metamorphic rocks that are strongly foliated, such as slates, schists, and gneisses.
The differential stress usually results from tectonic forces that produce a compression of the rocks, such as when two continental masses collide with one another. Thus, regionally metamorphosed rocks occur in the cores of mountain ranges or in eroded mountain ranges. Compressive stresses result in folding of the rock, as shown here, and results in thickening of the crust which tends to push rocks down to deeper levels where they are subjected to higher temperatures and pressures See Figure 8.
Compressional stresses acting in the subduction zone create the differential stress necessary to form schists and thus the resulting metamorphic rocks are called blueschist Shock Metamorphism - When a large meteorite collides with the Earth, the kinetic energy is converted to heat and a high pressure shock wave that propagates into the rock at the impact site.
Metamorphic Facies In general, metamorphic rocks do not undergo significant changes in chemical composition during metamorphism. If a low geothermal gradient was present, such the one labeled "C" in the diagram, then rocks would progress from zeolite facies to blueschist facies to eclogite facies.
Thus, if we know the facies of metamorphic rocks in the region, we can determine what the geothermal gradient must have been like at the time the metamorphism occurred. The Rock Cycle Before moving on to the rest of the course, you should read Interlude C in your textbook pages The rock cycle involves cycling of elements between various types of rocks, and thus mostly involves the lithosphere. The rock cycle involves the three types of rocks as reservoirs 1 igneous, 2 sedimentary, and 3 metamorphic.
Chemical elements can reside in each type of rock, and geologic processes move these elements into another type of rock. Energy for the parts of the crustal cycle near the Earth's surface is solar and gravitational energy which control erosion and weathering , whereas energy that drives processes beneath the surface is geothermal and gravitational energy which control uplift, subsidence, melting, and metamorphism.
Questions on this material that might be asked on an exam Define the following: a geothermal gradient, b metamorphism, c differential stress, d prograde metamorphism, e metasomatism f protolith, g foliation, i metamorphic aureole, j isograd, k greenstone, l blueschist.
Starting with a shale, describe the textural changes that would occur to the rock during prograde metamorphism with differential stress conditions present.
Why is retrograde metamorphism uncommon? Describe the following non-foliated metamorphic rocks a amphibolite, b quartzite, c marble, d hornfels. What are the terms used to describe the general chemical composition of metamorphic rocks?. Describe the type of rocks and minerals four nd in each.
What are the various types of metamorphism?
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