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Plate tectonics and formation of mountains - Part 2
4.0 PLATE BOUNDARIES
The three types of plate boundaries are divergent, convergent, and transform.
4.1 Divergent Boundaries
Divergent boundaries are formed when the directions of motion of two plates are opposite to each other. They split apart segments of continental crust along rift valleys. Narrow oceans represent youthful divergent boundaries and wide oceans are indications of a long-lived ocean basin.
Ocean ridges and subduction zones are boundaries between plates of lithosphere. A gap is created when oceanic lithosphere separates along the oceanic ridge. The gap is filled by magma that rises from the asthenosphere. The magma cools and solidifies to create new oceanic lithosphere.
The evolution of a divergent plate boundary passes through three stages. The birth of a divergent boundary requires that an existing plate begin to divide. This is happening today in East Africa, in an area known as the East African Rift zone. The African continent is slowly splitting in two. As the continental crust divides, magma from the asthenosphere fills in the gap. Several volcanoes are present in the rift zone.
Lithospheric plates are much thicker than oceanic or continental crust. Their boundaries do not usually coincide with those between oceans and continents, and their behaviour is only partly influenced by whether they carry oceans, continents, or both. The Pacific Plate, for example, is entirely oceanic, whereas the North American Plate is capped by continental crust in the west (the North American continent) and by oceanic crust in the east; it extends under the Atlantic Ocean as far as the Mid-Atlantic Ridge.
As plates move apart at a divergent plate boundary, the release of pressure produces partial melting of the underlying mantle. This molten material, known as magma, is basaltic in composition and is buoyant. As a result, it wells up from below and cools close to the surface to generate new crust. Because new crust is formed, divergent margins are also called constructive margins.
Upwelling of magma causes the overlying lithosphere to uplift and stretch. If the diverging plates are capped by continental crust, fractures develop that are invaded by the ascending magma, prying the continents farther apart. Settling of the continental blocks creates a rift valley, such as the present-day East African Rift Valley. As the rift continues to widen, the continental crust becomes progressively thinner until separation of the plates is achieved and a new ocean is created. The ascending partial melt cools and crystallizes to form new crust. Because the partial melt is basaltic in composition, the new crust is oceanic, and an ocean ridge develops along the site of the former continental rift. Consequently, diverging plate boundaries, even if they originate within continents, eventually come to lie in ocean basins of their own making.
Eventually the gap will form a narrow ocean (youth) much like the Red Sea to the north of the East African Rift zone. The Red Sea separates Saudi Arabia from Africa. A similar narrow sea, the Gulf of California, lies between much of Mexico and Baja California.
It takes millions of years to form a mature ocean, as rates of plate motions are slow (10-100 mm/yr). The oldest oceanic crust in the Atlantic and Pacific Oceans is the same age (~180 million years) but the Pacific is much wider than the Atlantic because it is spreading 2 to 3 times as fast.
4.2 Convergent Plate Boundaries
There are three types of convergent plate boundaries depending upon the types of lithosphere that are juxtaposed.
Oceanic Plate vs. Oceanic Plate Convergence: When plates of oceanic lithosphere collide the older of the two plates descends into the subduction zone along a trench. The descending plate carries water-filled sediments from the ocean floor downward into the mantle. The presence of water alters the physical and chemical conditions necessary for melting and causes magma to form. The magma rises up through the overriding oceanic plate, reaching the surface as a volcano. As the volcano grows, it may rise above sea level to form an island.
Trenches often lie adjacent to chains of islands (island arcs) formed by magma from the subducted plate.
Oceanic Plate vs. Continental Plate Convergence: When oceanic lithosphere collides with continental lithosphere, the oceanic plate will descend into the subduction zone. Oceanic lithosphere is denser than continental lithosphere and is therefore consumed preferentially. Continental lithosphere is almost never destroyed in subduction zones. The Nazca Plate dives below South America in a subduction zone that lies along the western margin of the continent. Convergence between these plates has resulted in the formation of the Andes Mountains (the second highest mountain range on Earth), extensive volcanism, and widespread earthquake activity. The largest earthquakes are concentrated along subduction zones.
Continental Plate vs. Continental Plate Convergence: The Himalayan mountains were formed due to the collision between the Indian and Eurasian plates which began over 40 million years ago.
4.3 Transform Boundary
Transform Plate Boundaries are locations where two plates slide past one another. The fracture zone that forms a transform plate boundary is known as a transform fault. Most transform faults are found in the ocean basin and connect offsets in the mid-ocean ridges. A smaller number connect mid-ocean ridges and subduction zones.
Transform boundaries join sections of convergent and/or divergent boundaries. Most transform boundaries occur in ocean basins where they offset oceanic ridges. Plates on either side of a transform boundary slide past each other without either plate being consumed and without a gap opening between the plates.
Some transform boundaries such as the San Andreas Fault in California or the North Anatolian Fault in Turkey occur on land. The San Andreas Fault joins two oceanic ridges. The southern end of the fault begins in the Gulf of California at the north end of a young ocean. The northern end of the fault becomes the Mendocino fracture zone offsetting a section of the oceanic ridge that defines one side of the small Juan de Fuca plate offshore from Washington and Oregon.
Land on the west side of the San Andreas fault, including Los Angeles and San Diego, is part of the Pacific Plate. San Francisco lies east of the fault and is on the North American Plate. Western California is being slowly displaced to the northwest relative to the rest of the state. It is not going to drop off into the ocean but it will eventually migrate along the western boundary of the North American Plate, eventually colliding with Alaska millions of years from now.
5.0 FAULTS
Faults are surfaces across which Earth material has lost cohesion and across which there is perceptible displacement. It is a break in the earth's crust along which movement can take place causing an earthquake. The center of the fault is the most deformed and is where most of the offset or slip between the surrounding rock occurs. The region can be quite small, about as wide as a pencil is long, and it is identified by the finely ground rocks called cataclasite (we call the ground up material found closer to the surface, gouge). From all the slipping and grinding, the gouge is composed of very fine-grained material that resembles clay. Surrounding the central zone is a region several meters across that contains abundant fractures. Outside that region is another that contains distinguishable fractures, but much less dense than the preceding region. Last is the competent "host" rock that marks the end of the fault zone.
Strike and dip refer to the orientation of a geological feature. Fault strike is the direction of a line created by the intersection of a fault plane and a horizontal surface, 0° to 360°, relative to North. Strike is always defined such that a fault dips to the right side of the trace when moving along the trace in the strike direction. Fault dip is the angle between the fault and a horizontal plane, 0° to 90°. Rake is the direction a hanging wall block moves during rupture, as measured on the plane of the fault. It is measured relative to fault strike, ±180°. For an observer standing on a fault and looking in the strike direction, a rake of 0° means the hanging wall, or the right side of a vertical fault, moved away from the observer in the strike direction (left lateral motion). A rake of ±180° means the hanging wall moved toward the observer (right lateral motion). For any rake>0, the hanging wall moved up, indicating thrust or reverse motion on the fault; for any rake<0° the hanging wall moved down, indicating normal motion on the fault.
5.1 Active, Inactive, and Reactivated Faults
Active faults are structure along which we expect displacement to occur. By definition, since a shallow earthquake is a process that produces displacement across a fault, all shallow earthquakes occur on active faults.
Inactive faults are structures that we can identify, but which do no have earthquakes. As you can imagine, because of the complexity of earthquake activity, judging a fault to be inactive can be tricky, but often we can measure the last time substantial offset occurred across a fault. If a fault has been inactive for millions of years, it's certainly safe to call it inactive. However, some faults only have large earthquakes once in thousands of years, and we need to evaluate carefully their hazard potential.
Reactivated faults form when movement along formerly inactive faults can help to alleviate strain within the crust or upper mantle. Deformation in the New Madrid seismic zone in the central United States is a good example of fault reactivation. Structure formed about 500 Ma ago are responding to a new forces and relieving strain in the mid-continent.
5.2 Dip-Slip Faults
In a normal fault, the block above the fault moves down relative to the block below the fault. This fault motion is caused by tensional forces and results in extension.
In a reverse fault, the block above the fault moves up relative to the block below the fault. This fault motion is caused by compressional forces and results in shortening. A reverse fault is called a thrust fault if the dip of the fault plane is small.
5.3 Strike-slip Faults
In a strike-slip fault, the movement of blocks along a fault is horizontal. If the block on the far side of the fault moves to the left, as shown in this animation, the fault is called left-lateral. If the block on the far side moves to the right, the fault is called right-lateral. The fault motion of a strike-slip fault is caused by shearing forces.
A transform fault is a type of strike-slip fault wherein the relative horizontal slip is accommodating the movement between two ocean ridges or other tectonic boundaries.
5.4 Oblique Fault
Oblique-slip faulting suggests both dip-slip faulting and strike-slip faulting. It is caused by a combination of shearing and tension or compressional forces. Nearly all faults will have some component of both dip-slip (normal or reverse) and strike-slip, so defining a fault as oblique requires both dip and strike components to be measurable and significant.
6.0 FORMATION OF MOUNTAINS
The process of mountain formation is one of the most remarkable geological processes. These processes are caused due large-scale movements of the earth's crust (plate tectonics). The orogenic process of mountain building includes processes like folding, faulting, volcanic activity, igneous intrusion and metamorphism. The understanding of specific landscape features in terms of the underlying tectonic processes is called tectonic geomorphology, and the study of geologically young or ongoing processes is called neotectonics.
Mountains form along the boundaries where the tectonic plates move towards each other (convergent boundaries). The tectonic plates collide triggering deformation and thickening of the crust. This in turn leads to crustal uplift and mountain formation. This process is a horizontal compression that leads to deformation folding and faulting of layers into folds or wrinkles along the convergent plate boundaries. This crustal uplift can be either a hill or a mountain depending upon the height and slope of the formation. But also to balance the weight of the earth surface, much of the compressed rock is forced downward, producing deep mountain roots making mountains for both upward and downward.
The Himalaya in Asia formed from one such massive collision that started about 55 million years ago. Thirty of the world's highest mountains are in the Himalaya. The summit of Mount Everest, at 29,035 feet (8,850 meters), is the highest point on Earth.
The tallest mountain measured from top to bottom is Mauna Kea, an inactive volcano on the island of Hawaii in the Pacific Ocean. Measured from the base, Mauna Kea stands 33,474 feet (10,203 meters) tall, though it only rises 13,796 feet (4,205 meters) above the sea.
Mountains can also be formed along fault lines. Blocks of Earth are uplifted and tilted over as two plates grind together. The uplifted part forms a mountain, and the lowered parts are filled in with eroded material. An example of this is the Sierra Nevada mountain range in California.
Another way that mountains are formed is when magma from beneath the Earth's surface is pushed up, but doesn't actually crack through. This bulge of magma eventually cools and hardens into hard rock, like granite. The layers of softer rock above the magma erode away and you're left with a large dome-shaped mountain. If the magma actually cracks through the surface a volcano is formed. Regular eruptions of lava, ash and rock build up a volcano to large heights. In fact, some of the largest, tallest mountains in the world are volcanoes. For example, Mauna Loa and Mauna Kea are examples of volcanoes. Measured from the bottom of the sea floor, they're actually taller than Mount Everest.
The final way to form a mountain is through erosion. If you have a high plateau, rivers will carve deep channels into the area. Eventually, you have mountains in between the river valleys.
6.1 Types of mountains
Fold Mountains: The most common type of mountain in the world are called fold mountains. Fold mountains occur near convergent or compressional plate boundaries. Where an area of sea separates two plates, sediments settle on the sea floor in depressions called geosynclines. These sediments gradually become compressed into sedimentary rock. When the two plates move towards each other again, the layers of sedimentary rock on the sea floor become crumpled and folded.
Eventually the sedimentary rock appears above sea level as a range of fold mountains. Where the rocks are folded upwards, they are called anticlines. Where the rocks are folded downwards, they are called synclines. Severely folded and faulted rocks are called nappes. Human activities surrounding fold mountains are:
Some examples of fold mountain ranges include the Rocky Mountains in North America, and the Himalayan Mountains in Asia.
Fault-Block Mountains: Fault-Block mountains are formed by folded rock strata broken along young fault lines into blocks that are uplifted to different heights. They usually arise in folded zones that once had a mountain relief but have lost their plasticity and been smoothed by denudation. Due to repeated tectonic action sections of the earth's core do not form folds but break into separate blocks, some of which rise as horsts and begin to form ranges ("reborn mountains"), and others sink as grabens, forming depressions. Sometimes with repeated orogeny, the smoothed surface of the earth is subjected to plicate deformation, which leads to the formation of broad and gently sloping folds accompanied by faults. Instead of folding, like the plate collision we get with fold mountains, block mountains break up into chunks and move up or down. Fault-block mountains usually have a steep front side and then a sloping back side.
Examples of fault-block mountains include the Sierra Nevada mountains.
Dome Mountains: Dome mountains are formed due to volcanism. Melted rock in the interior of the Earth squeezes together into vast pools of magma beneath the ground. As it is less dense than the surrounding rock, it makes its way upward to the surface. If the magma crashes through the surface it results in a volcano. But if the magma pushes up but doesn't actually crack through the surface dome mountains are formed.
Dome mountains don't usually get as high as folded mountains because the force of the magma underneath doesn't push hard enough. Over a long period, the magma cools to become cold, hard rock. The result is a dome-shaped mountain.
Volcanic Mountains: These mountains are formed due expulsion of materials from deep within the earth in the form of enormous amounts of lava or cinders. This material builds up around the volcanic vent and piles up building a mountain. Some of the largest mountains in the world were created this way, including Mauna Loa and Mauna Kea on the Big Island of Hawaii. Other familiar volcanoes are Mt. Fuji in Japan and Mt. Rainier in the US. A recent example of creation of a volcaninc mountain occurred on February 20, 1943, when a farmer's cornfield in Mexico suddenly began to erupt. By the second day, the cone had risen to 100 feet (30.5 m.). By two weeks it was 450 feet high (137 m.), and when the eruptions finally ceased in 1952 the cone had risen to 1,350 feet (411 m.). The nearby villages of Paricutin and Parangaricutiro had been completely buried under debris from the new volcano. Lava flows extended six miles from the crater and all vegetation for miles around had been choked out from the accumulations of dust and rock. The volcano itself was named Paricutin from one of the nearby villages it destroyed.
Plateau Mountains: Plateau mountains are not formed by internal activity. Instead, these mountains are formed by erosion. Plateaus are large flat areas that have been pushed above sea level by forces within the Earth, or have been formed by layers of lava. Plateau mountains are created when running water carves deep channels into a region, creating mountains. Over billions of years, the rivers can cut deep into a plateau and make tall mountains. Plateau mountains are usually found near folded mountains.
7.0 ISOSTASY
In 1705, Pierre Bouguer observed that massive mountains like the Andes and the Himalayas do not exert the kind of gravitational pull as expected of their size and height. These observations were later on confirmed by Sir George Everest, indicating a lack of compensating mass beneath the visible mountain ranges. These were called gravitational anomalies which set the thinking towards the principle of isostasy.
Isostasy is a natural adjustment or balance maintained by blocks of crust of different thicknesses to also maintain gravity. Isostasy uses energy to balance mass. The energy comes from the hydrologic cycle, which is the path of a drop of water that originates in the ocean, evaporates to form a cloud, falls on the mountain as a raindrop, and flows back to the sea carrying particles of rock and soil. The hydrologic cycle derives its energy from gravity and solar radiation. As water flows or a glacier slowly grinds over land, energy is lost in that now-isolated system.
In the theory of isostasy, a mass above sea level is supported below sea level, and there is thus a certain depth at which the total weight per unit area is equal all around the Earth; this is known as the depth of compensation. The depth of compensation was taken to be 113 km (70 miles) according to the Hayford-Bowie concept, named for American geodesists John Fillmore Hayford and William Bowie. Owing to changing tectonic environments, however, perfect isostasy is approached but rarely attained, and some regions, such as oceanic trenches and high plateaus, are not isostatically compensated.
The Airy hypothesis says that Earth's crust is a more rigid shell floating on a more liquid substratum of greater density. Sir George Biddell Airy, an English mathematician and astronomer, assumed that the crust has a uniform density throughout. The thickness of the crustal layer is not uniform, however, and so this theory supposes that the thicker parts of the crust sink deeper into the substratum, while the thinner parts are buoyed up by it. According to this hypothesis, mountains have roots below the surface that are much larger than their surface expression. This is analogous to an iceberg floating on water, in which the greater part of the iceberg is underwater.
The Pratt hypothesis, developed by John Henry Pratt, English mathematician and Anglican missionary, supposes that Earth's crust has a uniform thickness below sea level with its base everywhere supporting an equal weight per unit area at a depth of compensation. In essence, this says that areas of the Earth of lesser density, such as mountain ranges, project higher above sea level than do those of greater density. The explanation for this was that the mountains resulted from the upward expansion of locally heated crustal material, which had a larger volume but a lower density after it had cooled.
The Heiskanen hypothesis, developed by Finnish geodesist Weikko Aleksanteri Heiskanen, is an intermediate, or compromise, hypothesis between Airy's and Pratt's. This hypothesis says that approximately two-thirds of the topography is compensated by the root formation (the Airy model) and one-third by Earth's crust above the boundary between the crust and the substratum (the Pratt model).
7.1 Isostatic equilibrium
Isostatic equilibrium requires balancing of forces (associated with different weights on different areas) acting against each other through a fluid column. A motor vehicle lifting jack is a common example. In the case of Earth movement, isostatic equilibrium is associated with the balancing of forces due to different weights of landmasses in relatively close proximity. According to this principle the 'column' supporting the lighter Pacific plate will need to be longer than the opposing 'column’ supporting the heavier African plate with its large mass of continental crust. In simper terms this situation is analogous to placing the point of balance of a tapered shaft away for its mid or central geometrical centre to a position nearer the heavier end.
As a mountain range block erodes, the block rises because it is not heavy due to the erosion of the material, and it does not need to "ride" as low in the mantle. The eroded material is deposited as sediment on the adjacent thinner continental blocks, which increases their weight, and they then sink farther into the plastic asthenosphere. Areas that are tectonically stable tend to be isostatically balanced. The viscosity of the mantle can be calculated based on the rates of the isostatic adjustment of the crustal blocks.
Some geologists believe that plate subduction generates large bodies of magma that adhere to the bottom of the continental mass and cool, locally thickening the crust. In order to maintain isostasy, the crust would then have to rise through the formation of a mountain range. This idea has not been widely accepted, however.
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