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tpo-27 - Reading - Passage2 < 上一篇 下一篇 >
Question 1 of 14
1. The author mentions “spreading ridges”, “subduction zones”, and “transform faults” in order to
illustrate that the boundaries of tectonic plates are neat, thin lines
explain why some tectonic plates carry islands or continents while others form the seafloor
explain the complex nature of the edges of tectonic plates
provide examples of areas of tectonic plates where little geologic action occurs

  Earth’s surface is not made up of a single sheet of rock that forms a crust but rather a number of “tectonic plates” that fit closely, like the pieces of a giant jigsaw puzzle. Some plates carry islands or continents others form the seafloor. All are slowly moving because the plates float on a denser semi-liquid mantle, the layer between the crust and Earth’s core. The plates have edges that are spreading ridges (where two plates are moving apart and new seafloor is being created), subduction zones (where two plates collide and one plunges beneath the other), or transform faults (where two plates neither converge nor diverge but merely move past one another). It is at the boundaries between plates that most of Earth’s volcanism and earthquake activity occur.

  Generally speaking, the interiors of plates are geologically uneventful. However, there are exceptions. A glance at a map of the Pacific Ocean reveals that there are many islands far out at sea that are actually volcanoes----many no longer active, some overgrown with coral----that originated from activity at points in the interior of the Pacific Plate that forms the Pacific seafloor.

  How can volcanic activity occur so far from a plate boundary? The Hawaiian Islands provide a very instructive answer.Like many other island groups, they form a chain. The Hawaiian Islands Chain extends northwest from the island of Hawaii. In the 1840s American geologist James Daly observed that the different Hawaii islands seem to share a similar geologic evolution but are progressively more eroded, and therefore probable older, toward the northwest. Then in 1963, in the early days of the development of the theory of plate tectonics. Canadian geophysicist Tuzo Wilson realized that this age progression could result if the islands were formed on a surface plate moving over a fixed volcanic source in the interior. Wilson suggested that the long chain of volcanoes stretching northwest from Hawaii is simply the surface expression of a long-lived volcanic source located beneath the tectonic plate in the mantle. Today’s most northwest island would have been the first to form. They as the plate moved slowly northwest, new volcanic islands would have forms as the plate moved over the volcanic source. The most recent island, Hawaii, would be at the end of the chain and is now over the volcanic source.

  Although this idea was not immediately accepted, the dating of lavas in the Hawaii (and other) chains showed that their ages increase away from the presently active volcano, just as Daly had suggested. Wilson’s analysis of these data is now a central part of plate tectonics. Most volcanoes that occur in the interiors of plates are believed to be produced by mantle plumes, columns of molten rock that rise from deep within the mantle. A volcano remains an active “hot spot” as long as it is over the plume. The plumes apparently originate at great depths, perhaps as deep as the boundary between the core and the mantle, and many have been active for a very long time. The oldest volcanoes in the Hawaii hot-spot trail have ages close to 80 million years. Other islands, including Tahiti and Easter Islands in the pacific, Reunion and Mauritius in the India Ocean, and indeed most of the large islands in the world’s oceans, owe their existence to mantle plumes. 

  The oceanic volcanic islands and their hot-spot trails are thus especially useful for geologist because they record the past locations of the plate over a fixed source. They therefore permit the reconstruction of the process of seafloor spreading, and consequently of the geography of continents and of ocean basins in the past. For example, given the current position of the Pacific Plate, Hawaii is above the Pacific Ocean hot spot. So the position of The Pacific Plate 50 million years ago can be determined by moving it such that a 50-million-year-old volcano in the hot-spot trail sits at the location of Hawaii today. However because the ocean basins really are short-lived features on geologic times scale, reconstruction the world’s geography by backtracking along the hot-spot trail works only for the last 5 percent or so of geologic time.

地球的表面并不是由形成外壳的单层岩石组成的,而是许多的地壳版面严密的组合在一起的,就像是一个巨大拼图的拼图块。一些板块承载着岛屿或是大陆,其余的组成了海底平面。 这些都在缓慢的移动,因为这些板块是漂浮在密度更大的半液态地幔上的,地幔位于地壳和地核之间。板块的边缘是扩张脊(两个版块分离,新的海底形成的地方),俯冲带(两板块碰撞,一个投入到了另一个下面),或者是转换断层(两板块既不集合于一点也不偏离,但只是互相错过)。板块边界是地球上的火山爆发和地震的高发地。 通常,板块内部从地质学角度上来说是比较平静的。但是,也有例外。扫一眼太平洋的地图就能知道那里有许多在大海深处的岛屿,他们其实都是火山,其中有许多已经不活动了,一些长满了珊瑚,这些火山都是起源于当时太平洋板块内部的地质活动,因而形成了太平洋的海底。

为什么火山活动可以发生在离板块边缘这么远的地方呢?夏威夷群岛提供了一个非常有启发性的回答。就像其他的群岛一样,他们形成了一个链条。夏威夷群岛链从夏威夷岛向西北扩张。在十八世纪40年代,地理学家James Daly观察到不同的夏威夷岛屿看起来经历了相似的演变过程,但有一些被慢慢地腐蚀的更多,所以往西北方向可能更老一些。在1963年,在大陆板块理论的早期发展时期,加拿大的地质学家Tuzo Wilson意识到年轮的增加会引起形成在板块表面的岛屿移动到一个板块内部的固定火山的源头。Wilson解释说,火山的长链从夏威夷向西北延伸只是一个长时间存于板块下,地幔中的火山源头。最新的岛屿,夏威夷岛,应该是在链条的最后,现在应该在火山源头上。





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