The serpentinite is formed by alteration and hydration of the original ultramafic rock in the presence of warm, watery fluids. Hydration is the process by which water is incorporated into the crystal structures of new minerals in the rock, replacing the old minerals. This makes serpentinite a form of metamorphic rock. Serpentinite is a distinctive type of rock with a greasy-smooth feel and mottled green to black colors.
If you have ever experienced serpentinite, you are likely to remember it. For a long time the origin of ophiolites was something of a mystery to geologists, because although they clearly had oceanic affinities, they were commonly found in mountain ranges such as the Alps in Europe or the Coast Ranges in northern California. Most ophiolites form at divergent plate boundaries. Beneath a mid-ocean spreading ridge, the upwelling asthenosphere, made mostly of the ultramafic rock known as peridotite, melts to produce bodies of mafic magma.
Crystals settle out of the magma at the base of the magma chambers, forming layered gabbro. The top of the magma body forces molten rock up through cracks in the rifting crust, forming sheeted dikes as the crust keeps cracking and spreading apart and new dikes keep intruding.
On the floor of the ocean the lava erupts from fissures fed by the dikes. The lava cools quickly, quenching against the cold ocean water. This produces piles of basalt blobs on the ocean floor known as pillow basalt. Divergent plate boundaries are like factories which keep producing these rocks that spread away from the mid-ocean ridge as if on conveyor belts.
As the newly-formed oceanic lithosphere moves away from the spreading center and ages, oceanic sediments accumulate on top of it, completing the ophiolite sequence. Ophilites are found in places where terranes have accreted onto new plates. It is common for an ophiolite to mark a "suture zone," the boundary zone along the margin of an accreted terrane, along which the terrane was accreted onto a new plate.
Such ophiolites were apparently caught up between the terrane and the plate that the the terrane was being accreted to. Terranes are separated from their surroundings by faults, most commonly thrust faults or reverse faults.
Each terrane records its own geologic history in its stratigraphy , geologic structures, rock types, and fossils. The geologic history of an accreted terrane is different from the geologic history of nearby rocks that are native to the continent, indicating that it is exotic. Fossils in some terranes provide strong evidence of an exotic origin. Most species of plants, animals, and other organisms in the world do not exist on all continents or in all parts of the oceans - they have a limited geographic range.
Many terranes contain fossils that do not match the continent's fossil sequence but do match the sequence of other places. This is taken to indicate that the terranes came from other places. The direction of the earth's magnetic field changes over time. In addition, if you use a three-dimensional compass, you find that the earth's magnetic field at a given time is oriented nearly horizontal near the equator and vertically at the magnetic poles.
When rocks that contain magnetized minerals first form, the magnetized minerals "lock in" the direction of the earth's magnetic field at that time by aligning toward the earth's magnetic poles. Once the rock is solid, the alignment of the magnetized minerals remains.
This characteristic, called paleomagnetism , allows determination of the approximate latitude - its position relative to the equator and poles -at which a magnetized rock first formed. For a paleomagnetic measurement to be useful, the original horizontal position of the rocks must be known. That is why paleomagnetic measurements work best on lava flows or sedimentary beds that show in their bedding which way was originally horizontal.
Paleomagnetic measurement indicates only original latitude, not longitude. Therefore the distance that a plate may have moved in the east-west direction cannot be determined by paleomagnetism. There are several other ways the past motions of tectonic plates and terranes can be detected from paleomagnetism. They can also check for other physical properties such as if there is a taste, if it is radioactive or if it gives off an odor.
Jacques Terrane's birth name is Jacques Tartire. Jacques Terrane was born on August 23, , in Paris, France. Geologists identify minerals that are too small to be seen in a hand specimen using powerful microscopes.
Minerals are classified according to their physical properties such as color, streak, cleavage, hardness and crystal form. Some characteristics geologists use to identify minerals are fracture, luster, streak, density, cleavage and hardness.
From the choices given, D is the one that is not a characteristic. Jacques Terrane died on June 20, , in Damascus, Syria of shot. Geologists use index fossils to define and identify geologic periods. The investigative process by which geologists identify and match sedimentary strata and other rocks of the same ages in different areas is referred to as correlation.
Log in. Earth Sciences. See Answer. Best Answer. By looking at it. Study guides. Q: How can geologists identify terrane? Write your answer Related questions. How do you identify suspect terrane?
What do geologists call small accreted crustal fragments that have a geologic history distinct from adjacent blocks? What characteristics do geologists use to identify rocks?
When we look at modern day mountain belts, we recognize there are really two portions, an interior portion of metamorphic rocks and igenious rocks that are intrinsic to that continent and then an outboard portion on amalgamation of exotic or suspect terrane that have come from various distances and representing various ages of ancient rock. Regardless of how they formed, mountain belts along convergent boundaries stop growing when subduction ends. They gradually deteriorate to become part of the low-lying craton itself.
Ultimately, of course, mountain building ends, and that signals the end of convergent plate motion, a settling back or perhaps low angle distributive faulting occurs, which extend the mountain belt rather than compress it, and the forces of erosion once this constructional stage is over take over. Gone, too, also is the volcanism that characterizes early and middle stages of many mountain belts, but the actual geologic mountains are then terminated by erosive processes.
There may be later uplift, which provide strong relief and gives you topographic mountains, but this later process is not strictly speaking a mountain- building process; it is simply an uplift and erosional process. In Eastern North America, the Appalachian Mountains continue to exist more than million years after the plate collisions that formed them. Given rates of erosion, these mountains should have worn flat tens of millions of years ago; yet they still stand, indicating that some uplift must be continuing.
The cause of this puzzling late stage uplift was discovered in by British surveyor, G. While working in India, Airy discovered that plumb bobs, iron weights used to level sighting instruments were less attracted by the gravity from the nearby Himalayan Mountains than they should be if the Himalaya were directly underlain by the same dense rock presumed to form most of the Earth's interior.
This suggested there was less mass present beneath the Himalaya than previously thought. To explain this discrepancy Airy concluded that a low density root must lie beneath the range. Geophysical studies have since confirmed that the crust beneath the Himalaya extends to a depth of 75 kilometers, twice as thick as ordinary continental crust.
It's now known that most mountain ranges are underlain by crustal roots floating atop the hot plastically deforming mantle. The roots grow as a result of compression during plate convergence. As mountain ranges are worn down, their roots are buoyed upward by the mantle. Because the mantle is far stiffer than the most fluid lava, the crust flows upward quite slowly sustaining a hilly topography in the landscape for hundreds of millions of years. As the crust rises, rocks from ever deeper levels inside the Earth are brought to the surface and worn away.
The floating of Earth's crust atop the mantle is termed "isostasy". This is similar to what happens at sea, where large icebergs float with more ice extending beneath the surface than small ones do. In the same way, tall mountains usually have roots extending deeper into the Earth than low mountains made up of the same rock type.
In both cases, far more mass lies hidden from view than can be seen at the surface. Isostasy is the process by which different thickness and different density irregularities in the outer Earth float in gravitational equilibrium with one another. When you build up a large mountain range, you're liable to have a root underneath and a lot of material piled up high on the Earth's surface, and, ultimately, if you don't have forces to keep it piled up, that is going to tend to want to equilibrate and float in gravitational equilibrium with the other areas around it.
As mountain belts uplift and late in their stages, they may begin to actually undergo extensional collapse or breaking apart at the high levels due to the force of gravity.
At their deeper levels, there may be plastic flow underneath them or compensation by flow in the mantle in order to let whatever root that exists to equilibrate and to come to gravitational equilibrium with the mantle and a lower crust around it. During this stage of ultimate isostatic equilibration, if there are no longer major forces uplifting the mountain range, then erosion will ultimately win out over the uplift process, and the mountain belt will be beveled to a much flatter lower relief surface.
At this stage the mountain belt is well on its way to becoming part of the craton. Through geologic time, the amount of continental material on Earth has slowly grown in size at the expense of the ocean basins. But tracing the history of growth on individual continents is a great challenge for each continent today has been joined to other continents in the past. The general pattern in continents is to find the oldest material in the interiors of the cratons, and this is because the cores of the continents formed and then successive mountain belts and continent-edge accretions occurred around their margins.
But geologists find that pattern to be imperfect because continental masses tend to break and rift apart during their growth.
And so if they break apart, they may break apart across older interiors of continents across younger mountain belts, and then subsequently they may form a new mountain belt across a broken edge, so that leaves us with a competing series of processes of marginal growth, and breaking apart and drifting, and then colliding back together and growing again.
Mountain ranges, newly forming and ancient, mark the growth of continents in response to plate movements. Floating on Earth's plastic mantle, these gigantic topographic features disappear slowly as their low-density roots are buoyed up. So mountains owe their existence to two factors: the heat that drives plate tectonics and the effects of gravity. In time, mountains wear flat, adding new crust to the cratons, the oldest, most stable lands on the planet Earth.
One of the benchmark discoveries in geology over the last half-century is the origin of mountain ranges. Continents and oceanic crust have collided or subducted at tectonic plate margins. Mountain ranges have been formed, and processes of erosion have torn them down. Eventually, the continents are split apart by renewed plate divergence and are on their way to new collisions, often forming a supercontinent.
This tectonic cycle, sometimes referred to as the "dance of the continents" has been repeated many times in the geologic past with each complete cycle lasting several hundred million years.
Some aspects of this tectonic dance have surprisingly complicated steps. Alaska, for example, is largely composed of plate fragments that have been packed together by successive collisions.
Some of these terrane have been tectonically transported thousands of kilometers by sea floor spreading and strike slip faulting before colliding with North America to form Alaska. The Mediterranean Sea is a shrinking ocean basin caught in a collision between the colliding continents of Africa and Europe, the famous volcanoes and earthquakes and intensely deformed mountains of this region are evidence of the profound mountain building that accompanies the death of an ocean.
Tectonic cycles and mountain building are nearly as old as the Earth itself, and the forecast for the geologic future is continued change, change in the ocean basins, and continents, and mountain ranges, that together are the face of the Earth. A map of the world a billion years from now will be a scant resemblance of the world we know today. This is such a fascinating process, this mountain building, and when you think about it, it's amazing that we, as geologists, could figure this out at all considering how little of the Earth we can see at any one time and how many different ways we have to put all this information together, but can't you see how nicely this fits into the model of plate tectonics?
That much of the facts about mountain building: the forces, the folding of the rocks, the igneous intrusions, the metamorphisms. All of these things were known long before the plate tectonics theory came around.
The model seems to provide the mechanism which ties everything together. We also have new evidence as well that goes into examining the mountain building process, and we'll try to come back and look at some of that later on. From the video we see that mountain belts start out as thick piles of sediment, as much as 10, meters thick; that's six miles thick. This sediment is accumulated mostly on passive continental margins, which have since become active. I'll remind you that passive continental margins are those found on the trailing edges of continents, where usually it's tectonically stable; not much change happens over long periods of time; whereas, active continental margins are found on the leading edges of continents, where the continents are distorted.
It's evident that the sediment must accumulate on the continental margins for long periods of time; in fact, we can do a little bit of calculation. The average sedimentation rate on continental margins is around 25 centimeters or about 10 inches per year; that's about the length of my hand. Did I say per year?
Twenty-five centimeters per thousand years. It makes a little bit of difference. At this rate it takes about 40 million years to accumulate 10, meters of sediment. We might also note here that nowhere is the ocean 10, meters deep, so in order for these sediments to be deposited underwater, there must also be an isostatic adjustment taking place; in other words, the weight of the sediment depresses the continental margin due to the extra load of the sediment.
We might note here that this 40 million year period is really quite short in geologic terms although it's very long in human terms, and we might also note that the 40 million years to accumulate this 10, meter thick layer of sediments is about the same amount of time that it takes a mountain range to erode. So we have an older mountain range on the continent giving up sediments to the sea floor, and those sediments then later become compressed and a new mountain range added or accreted to the edge of the continent.
It's probably caused by changing plate motions. Now, keep in mind that the rising convection currents are rather chaotic, and as spreading centers change location, as continents bump into each other, as ocean plates stall and move at different speeds, a passive continental margin may eventually change into an active continental margin. The resulting compressive forces at the new active margin deform sediments, change them into sedimentary rock, and if they're subducted deep enough, may even melt them.
Various different things happen as the rocks are subducted depending upon the rock type and the rate at which the process happens, the temperature, and how deep the rocks are subducted.
We'll examine the nature of these forces and the types of deformation that rocks undergo in the next lesson. The evidence shows us that some mountain ranges conform and reform several times. The Appalachians, for example, show at least three separate episodes of mountain formation, which represents successive openings and closing of the Atlantic Ocean; in other words, before the supercontinent of Pangea broke up million years ago, the Appalachians had already been formed, and the new rift opened up along a different seam from the old rift; in fact, the seam never opens in exactly the same place, and parts of the Northeastern United States, which are now attached to the North American Continent actually belong to the Continent of Africa, or in some cases, the Continent of Europe.
We also note that the structures of the Appalachian mountains: the folds, and the faults, and the metamorphic belts, and so forth, continue on the other side of the Atlantic. There are mountain ranges with the same structures that are found in Ireland, and Scotland, and Greenland, and in Scandinavia, and this continuation of the structures was noted by Wegener in his Continental Drift Theory; in fact, he used this as one of the lines of evidence to suggest that the continents might have at one time been connected in the supercontinent.
We see that the Appalachians and other mountain ranges may have rivaled the Himalayas in size more than once. This is difficult to imagine because on our paltry scale of human existence, mountains seem to be an everlasting feature, and, in fact, the changes in a given mountain range that have happened during the time humans have been on the Earth are really insignificant. It's safe to say that even the Appalachians looks pretty much the way they do now when the first people arrived on the North American Continent as much as 50, 60, 70 thousand years ago.
All of this is part of our difficulty in visualizing geologic time. When we try to compare geologic time scales with human time scales, our time really is insignificant in relation to the age of the Earth.
This just points to the fact that all Earth processes, including mountain building and erosion, are slow but relentless, and like all geologic processes, as slow as they are, over a long enough period of time, they can produce tremendously large results. You see, the way that this mountain building process works is a little bit like scraping peanut butter and jelly off of a knife onto a piece of bread.
The peanut butter and jelly get deformed and mixed around and stick to the edge of the bread. Now, of course, the actual process is somewhat more complicated than this, not only because of the large time scale, but because rocks are really a little more complicated than peanut butter and jelly.
But sediments, old sea floor pieces of continents, or whatever else happens to be drifting along with the lithospheric plates are deformed as they're incorporated into active margins at subduction zones. One good example of this is ophiolites. You may remember that ophiolites are fragments of the upper mantle and crust, which involve the gabbros, the sheeted dikes, and the pillow of basalts.
The Island of Cypress, as noted in the last lesson, is one of these pieces of ophiolites, which was rafted for some distance over from a spreading center along the lithospheric plate, which is now embedded, in this case, in oceanic crust, which protrudes above sea level to become an island. There are other examples of ophiolites, which are incorporated into the structures of folded mountain ranges on several continents.
We may also find simply fragments of oceanic crust. In the Cascade Mountains and on the Olympic Peninsula in the western part of Washington State, we find mixed in with sedimentary sequences of various types, pieces of basaltic oceanic crust that seem somehow really out of place with the continental rocks.
We also find evidence for pieces of broken continents being incorporated into the mountain ranges. These have just recently been discovered, recently being in the last 10 or 15 years and had been called "terranes.
Studies of the West Coast of the United States revealed that along the newly emergent coastal ranges we find rock types that don't seem to correspond to one another. You find a piece of one type of rock next to a piece of different kind of rock, and it's not clear exactly the relationship under which these different types of rocks formed until you incorporate the plate tectonics model.
If we look at the world map today, we see next to some of the major continents, small pieces of continental fragments. One example, the islands of New Zealand. Now, these happen to correspond to a subduction zone, and there are active andesite volcanoes on the island of New Zealand, but it's a small crustal fragment, and another example is the island of Madagascar off the east coast of Africa.
If you imagine these continental fragments embedded in the lithospheric plates, they may be rafted great distances across the Earth's surface, where they eventually will come in contact with a continent and be scrunched up against the edge of the continent and partially subducted.
Keep in mind that the continental fragments themselves can't really be subducted because they're lighter. They're lighter in density, and they basically float on top of the oceanic crust and the mantle, so they can't be carried down completely and subducted, so the West Coast of the United States is made of many of these terranes. As many as 25 or 30 different terranes have been identified, some of them having moved as much as 10, miles across the Pacific Ocean from the Southern Hemisphere.
The rocks that make up these terranes, the continental fragments, are distorted and blended with the other folded rocks that form from the orogenic process of distorting of sediments. Also note here that descending plate motions create intense heat and pressure.
Now, this pressure and the heat increases with depth. On one hand, the Earth's temperature naturally increases as you go deeper; on the other hand, some of the energy of the movement of the plates, the kinetic energy of the plates is translated into heat, which causes the temperature to rise even more than it would otherwise. It is a problem with geologists, or maybe we shouldn't say it's a problem; should say it's been discussed how these cold descending basaltic plates can generate heat in this way, but it's one of those minor problems with the Theory of Plate Tectonics that needs to be worked out.
Because of the difference in heat and pressure as you go deeper in the Earth, different effects happen, and as we'll see in the next lesson, the way in which rocks respond to forces depends upon several factors, which include how strong the forces are, what the temperature of the rock is, and the time period over which the forces operate.
For now we can just note that rocks near the surface tend to be brittle; that means that when they're subjected to forces, they tend to crack and break to cause earthquakes, so we find most of the earthquakes associated with the active plate margins either near the surface or down to a fairly limited depth.
As the sediments and assorted pieces are buried a little deeper, we find that the heat lithifies the rocks; that is, changes them into different forms and causes them to fold. Rocks which are subjected to high temperatures tend to bend rather than break, so the folding of the rocks then takes place with some fracturing but mostly in what we call a plastic deformation.
At yet greater depths, the atoms that make up the individual minerals in the rocks can actually rearrange themselves without melting. This is what happens when regional metamorphism occurs. Here the rocks are drastically changed as new minerals form from the minerals in the original rock and leave a signature for us to examine later to give us clues about the depth at which these transformations occurred. At yet greater depth, we find that the rocks actually begin to melt.
0コメント