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Theory of Plate Tectonics
Earth, either lithosphere or asthenosphere, is not absolute solid or rigid, in a relative sense. Assuming the water density is 1, lithosphere has a density like 3.7 (can't remember exactly by me, knowledge of 20+ years old), while asthenosphere, mainly magma, has that from 1.4 to 2.7.
Lighter Magma will flow upward while heavier earth crust will sink. Such a flow will shift the earth plate, causing plates to clash and building pressure along the edges or faultlines of the lithosphere..
Pressure once hitting the threshold will release, causing vibration of the plates, which is earthquake.
In China, most dangerous earthquake areas include:
1. Taiwan. Pacific plate moving into Asian plate.
2. Beijing-Tinjian-Tangshan due to the interaction of the smaller Northeast Asian plate and the Pacific plate.
3. SZ, Yunan, Szechuan and the extension all the way to Shanxi due to the mountain building of the Himalayas.
Source of information:
http://ca.encarta.msn.com/encycl ... late_Tectonics.html
Plate Tectonics, theory that the outer shell of the earth is made up of thin, rigid plates that move relative to each other. The theory of plate tectonics was formulated during the early 1960s, and it revolutionized the field of geology. Scientists have successfully used it to explain many geological events, such as earthquakes and volcanic eruptions as well as mountain building and the formation of the oceans and continents.
Plate tectonics arose from an earlier theory proposed by German scientist Alfred Wegener in 1912. Looking at the shapes of the continents, Wegener found that they fit together like a jigsaw puzzle. Using this observation, along with geological evidence he found on different continents, he developed the theory of continental drift, which states that today’s continents were once joined together into one large landmass.
Geologists of the 1950s and 1960s found evidence supporting the idea of tectonic plates and their movement. They applied Wegener’s theory to various aspects of the changing earth and used this evidence to confirm continental drift. By 1968 scientists integrated most geologic activities into a theory called the New Global Tectonics, or more commonly, Plate Tectonics.
II Tectonic Plates
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Tectonic plates are made of either oceanic or continental crust and the very top part of the mantle, a layer of rock inside the earth. This crust and upper mantle form what is called the lithosphere. Under the lithosphere lies a fluid rock layer called the asthenosphere. The rocks in the asthenosphere move in a fluid manner because of the high temperatures and pressures found there. Tectonic plates are able to float upon the fluid asthenosphere because they are made of rigid lithosphere. |
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楼主: 高田种小麦.
狗仔卡
发表于 2008-5-12 09:09:19
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