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Faults, isostasy, mountains
& landform development - Part 2
3.0 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.
3.1 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. Structures formed about 500 Ma ago are responding to a new forces and relieving strain in the mid-continent.
3.2 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.
3.3 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, 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.
3.4 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.
4.0 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).
4.1 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.
5.0 FORMATION OF ARCHIPELAGOS IN SOUTH ASIA
Like mountains and other land formations, archipelagos are formed in part by tectonic activity. When underwater volcanoes, or hot spots, allow magma (liquid rock) to seep out in the sea, rock formations are created under the water. As more and more magma is released, the rock formations eventually peak out over the surface of the water, creating an island.
The Malay archipelago situated between mainland south east Asia and Australia is a group of almost 25,000 islands. It is the largest archipelago areawise. In terms of plate tectonic setting the Indonesian archipelago is situated in the triple junction of the three major plates, which are-the Indo-Australian, the Eurasian and the Pacific Plates. The interaction of the three major plates creates a complex tectonic especially in the plate boundary that is situated on Eastern Indonesia. The subduction of the Indian oceanic plate beneath the Eurasian continental plate formed the volcanic arc in western Indonesia, one of the most seismically active areas on the planet with a long history of powerful eruptions and earthquakes. This chain of active volcanoes formed Sumatra, Java, Bali, and Nusa Tenggara islands, most of which, particularly Java and Bali, emerged within the last
2-3 million years. The Pacific and Australian plate movements controlled the tectonics of the eastern portion of Indonesia.
The islands of Lombok and Sumbawa lie in the central portion of the Sunda Arc. The oldest exposed rocks are Miocene, suggesting that subduction and volcanism began considerably later than in Java and Sumatra to the west, where there are abundant volcanic and intrusive rocks of Late Mesozoic age.
The territory of the Philippines is composed of many island arcs formed by several incidents of subduction. The island arcs are collectively called Philippines island arc system. Each major Philippine island had a complex natural history. With the exception of Palawan, Mindoro and Romblon, most of the Philippine islands are considered to have been parts of island arcs formed at the southern edge of the Philippine Sea plate millions of years ago. As part of the Philippine Sea plate, the islands moved northward as the plate rotated clockwise. These roving islands, known as the Philippine Mobile Belt, eventually collided with the Sundaland. The collision resulted, among others, in a series of subductions around Philippine archipelago.
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