Since oceanic ridges are areas where new oceanic crust is created by intrusion and eruption of basaltic magmas, these water-rich fluids are heated by the hot crust or magma and become hydrothermal fluids. The hydrothermal fluids alter the basaltic oceanic crust by producing hydrous minerals like chlorite and talc. Because chlorite is a green colored mineral the rocks hydrothermal metamorphic rocks are also green and often called greenstones.
Subduction Related Metamorphism - At a subduction zone, the oceanic crust is pushed downward resulting in the basaltic crust and ocean floor sediment being subjected to relatively high pressure. But, because the oceanic crust by the time it subducts is relatively cool, the temperatures in the crust are relatively low.
Under the conditions of low temperature and high pressure, metamorphism produces an unusual blue mineral, glaucophane. 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. The heat may be enough to raise the temperature to the melting temperature of the earth rock. The shock wave produces high enough pressure to cause quartz to change its crystal structure to more a dense polymorph like coesite or stishovite.
Ancient meteorite impact sites have been discovered on the basis of finding this evidence of shock metamorphism. In general, metamorphic rocks do not undergo significant changes in chemical composition during metamorphism. The changes in mineral assemblages are due to changes in the temperature and pressure conditions of metamorphism.
Thus, the mineral assemblages that are observed must be an indication of the temperature and pressure environment that the rock was subjected to. This pressure and temperature environment is referred to as Metamorphic Facies.
The sequence of metamorphic facies observed in any metamorphic terrain, depends on the geothermal gradient that was present during metamorphism. A high geothermal gradient such as the one labeled "A" in the figure shown here, might be present around an igneous intrusion, and would result in metamorphic rocks belonging to the hornfels facies.
Under a normal geothermal gradient, such as "B" in the figure, rocks would progress from zeolite facies to greenschist, amphibolite, and eclogite facies as the grade of metamorphism or depth of burial increased. Before moving on to the rest of the course, you should read Interlude C in your textbook pages Now that we have discussed the three types of rocks, it is important to understand how the atoms that make up these rocks cycle through the earth.
This cycling involves process that will be discussed in detail throughout the remainder of this course. Since the rock cycle links the rock forming processes to tectonic process and to surface process most of which will be discussed throughout the rest of the course , it is important to understand the concept of the rock cycle and the various linkages involved.
We here start our discussion with Volcanoes and Volcanic eruptions and processes that are involved in the production of igneous rocks at the earth's surface.
Metamorphism and Metamorphic Rocks. Factors that Control Metamorphism Metamorphism occurs because rocks undergo changes in temperature and pressure and may be subjected to differential stress and hydrothermal fluids.
Rounded grains can become flattened in the direction of maximum stress. Minerals that crystallize or grow in the differential stress field can have a preferred orientation. This is especially true of the sheet silicate minerals the micas: biotite and muscovite, chlorite, talc, and serpentine.
These sheet silicates will grow with their sheets orientated perpendicular to the direction of maximum stress. 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. Metamorphism is the changing of rocks by heat and pressure. They change so much that they become an entirely new rock. Metamorphic rocks start off as igneous, sedimentary, or other metamorphic rocks. One ways rocks may change during metamorphism is by rearrangement of their mineral crystals.
When heat and pressure change the environment of a rock, the crystals may respond by rearranging their structure. They will form new minerals that are more stable in the new environment. Extreme pressure may also lead to the formation of foliation , or flat layers in rocks that form as the rocks are squeezed by pressure.
Foliation normally forms when pressure was exerted on a rock from one direction. 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. Whether from sheering or simple squeezing, the rock is experiencing what geologists refer to as "directed pressure" or "directed stress.
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. 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 shallow rocks, shear forces simply grind and crush the mineral grains cataclasis to yield cataclasite. Continued grinding yields the hard and streaky rock mylonite.
Different degrees of metamorphism create distinctive sets of metamorphic minerals. These are organized into metamorphic facies , a tool petrologists use to decipher the history of metamorphism.
Under greater heat and pressure, as metamorphic minerals such as mica and feldspar begin to form, strain orients them in layers. The presence of mineral layers, called foliation, is an important feature for classifying metamorphic rocks. As strain increases, the foliation becomes more intense, and the minerals may sort themselves into thicker layers.
The foliated rock types that form under these conditions are called schist or gneiss, depending on their texture. Schist is finely foliated whereas gneiss is organized in noticeable, wide bands of minerals. Non-foliated rocks occur when heat is high, but pressure is low or equal on all sides. This prevents dominant minerals from showing any visible alignment. The minerals still recrystallize, however, increasing the overall strength and density of the rock. The sedimentary rock shale metamorphoses first into slate, then into phyllite, then a mica-rich schist.
The mineral quartz does not change under high temperature and pressure, although it becomes more strongly cemented. Thus, the sedimentary rock sandstone turns to quartzite. Intermediate rocks that mix sand and clay—mudstones—metamorphose into schists or gneisses.
The sedimentary rock limestone recrystallizes and becomes marble. Igneous rocks give rise to a different set of minerals and metamorphic rock types. These include serpentinite , blueschist, soapstone, and other rarer species such as eclogite. Metamorphism can be so intense, with all four factors acting at their extreme range, that the foliation can be warped and stirred like taffy; the result of this is migmatite. With further metamorphism, rocks can begin to resemble plutonic granites.
0コメント