1. Explain the marked difference in the depth of earthquakes at mid-ocean ridges
ID: 291815 • Letter: 1
Question
1. Explain the marked difference in the depth of earthquakes at mid-ocean ridges and subduction zones
2. what is back arc spreading and how does it account for the presence of seas between the iska d's of the wester. Pacific
3. transform faults are so called because they permit one type of plate motion. to be transformed into another, what is meant by this and how is it achieved.
4. lines of volcanic islands such as those of the Hawaiian chain at enig readily accku red for by plate tectonic theory , yet can be used to deduce the direction. of the plate motion . Explain this paradox
Explanation / Answer
1.Magnitude of earthquake near oceanic ridges are less compared to subduction zones a d their range may be in between 5 to 7 on magnitude scale. While at subduction zones it is greater than 8
Both are shallow focus . But subduction zone can produce deep earthquakes from benioff zone
At ridges magma is mafic while at subduction magma type is intermediate ( andesitic)
At ridges eruption style is non explosive, while at subduction it is violent as there is more water present in contact with the magma
2. submarine basin that forms behind an island arc. Such basins are typically found along the western margin of the Pacific Ocean near the convergence of two tectonic plates. Back-arc basins are sites of significant hydrothermal activity, and the deep-sea vents that occur in these regions often harbour diverse biological communities. Examples of back-arc basins include the Sea of Japan, the Kuril Basin in the Sea of Okhotsk, the Mariana Trough in the Philippine Sea, and the South Fiji Basin.
A back-arc basin is formed by the process of back-arc spreading, which begins when one tectonic plate subducts under (underthrusts) another. Subduction creates a trench between the two plates and melts the mantle in the overlying plate, which causes magma to rise toward the surface. Rising magma increases the pressure at the top of the overlying plate that creates rifts in the crust above and causes the volcanoes on the island arc to erupt. As additional magma breaks through the cracks in the crust, one or more spreading centres develop, which widen the seafloor and expand the section of the overlying plate behind the trench. (Spreading centres that form in back-arc basins are much shorter than those found along oceanic ridges, however.) As the basin expands, the leading edge of the overlying plate may be forced oceanward, causing the trench to “roll back” over the subducting plate, or it may serve as a “sea anchor” by remaining fixed in place relative to the top of the subducting plate. In the latter case, the enlargement of the basin forces the trailing part of the overlying plate to move in the opposite direction.
3.A transform fault or transform boundary, also known as conservative plate boundary since these faults neither create nor destroy lithosphere, is a type of fault whose relative motion is predominantly horizontal in either sinistral or dextral direction. Furthermore, transform faults end abruptly and are connected on both ends to other faults, ridges, or subduction zones. While most transform faults are hidden in the deep oceans where they form a series of short zigzags accommodating seafloor spreading, the best-known (and most destructive) are those on land at the margins of tectonic plates. Transform faults are the only type of strike-slip fault that can be classified as a plate boundary.
The effect of a fault is to relieve strain, which can be caused by compression, extension, or lateral stress in the rock layers at the surface or deep in the Earth
4.The Hawaiian Islands are the tops of gigantic volcanic mountains formed by countless eruptions of fluid lava over several million years; some tower more than 30,000 feet above the seafloor. These volcanic peaks rising above the ocean surface represent only the tiny, visible part of an immense submarine ridge, the Hawaiian Ridge—Emperor Seamount Chain, composed of more than 80 large volcanoes.
This range stretches across the Pacific Ocean floor from the Hawaiian Islands to the Aleutian Trench. The length of the Hawaiian Ridge segment alone, between the Island of Hawai'i and Midway Island to the northwest, is about 1,600 miles, roughly the distance from Washington, D.C., to Denver, Colorado. The amount of lava erupted to form this huge ridge, about 186,000 cubic miles, is more than enough to cover the State of California with a layer 1 mile thick.
According to the plate tectonic theory, the Earth's rigid outer layer, or "lithosphere," consists of about a dozen slabs or plates, each averaging 50 to 100 miles thick. These plates move relative to one another at average speeds of a few inches per year-about as fast as human fingernails grow.
Over a span of about 70 million years, the combined processes of magma formation, eruption, and continuous movement of the Pacific Plate over the stationary hot spot have left the trail of volcanoes across the ocean floor that we now call the Hawaiian-Emperor Chain. A sharp bend in the chain about 2,200 miles northwest of the Island of Hawai'i was previously interpreted as a major change in the direction of plate motion around 43-45 million years ago (Ma), as suggested by the ages of the volcanoes bracketing the bend.
However, recent studies suggest that the northern segment (Emperor Chain) formed as the hot spot moved southward until about 45 Ma, when it became fixed. Thereafter, northwesterly plate movement prevailed, resulting in the formation of the Hawaiian Ridge "downstream" from the hotspot.
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