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草莓小菇凉:说的非常好,十分有道理,棒棒棒!

06-08 15:44:55

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TPO-27- The Formation of Volcanic Islands

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at a map of the Pacific Ocean reveals that there are many islands far out at sea that are actually volcanoes----many no longer active, some overgrown with coral----that originated from activity at points in the interior of the Pacific Plate that forms the Pacific seafloor.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.

地球的表面不是由一块形成地壳的岩石片组成的,而是由一些紧密配合的“构造板块”构成的,就像一块巨大的七巧板一样。一些板块承载了海底的其他岛屿或大陆。所有这些都在慢慢地移动,因为板块漂浮在密度更高的半液体地幔上,即地壳和地心之间的层。这些板块的边缘是扩展的脊(两块板正在分开,新的海底正在形成),俯冲带(两块板碰撞,另一块在另一块下面),或者转换断层(两块板块既不会聚也不分叉,只是越过另一个)。地球上大部分的火山活动和地震活动都发生在板块的边界。

 

一般来说,板块的内部在地质上是平淡无奇的。但是,也有例外。从太平洋的地图上可以看出,远在海上的许多岛屿实际上是火山----许多不再活跃,有的是珊瑚丛生的----这些岛屿起源于内陆地区的活动点形成太平洋海底的太平洋板块。

 

火山活动如何能从板块边界发生到目前为止?夏威夷群岛提供了一个非常有益的答案。像许多其他岛屿组织,他们形成一个链条。夏威夷群岛链从夏威夷岛向西北延伸。在十九世纪四十年代,美国地质学家詹姆斯·达利(James Daly)观察到,不同的夏威夷岛屿似乎有着类似的地质演化,但是逐渐地更加侵蚀,因此可能更加老旧,向西北方向发展。然后在1963年,板块构造理论发展的早期。加拿大地球物理学家Tuzo Wilson认识到,如果这些岛屿形成在一个在内陆固定火山源上的平台上,那么这个时代就会发展。威尔逊认为,从夏威夷向西北延伸的火山长链,就是位于地幔板块下方的长寿命火山源的表面表现。今天最西北的岛屿将是第一个形成。随着板块向西北方向缓慢移动,板块在火山源上方移动,新的火山岛将会形成。最近的一个岛屿,夏威夷,将在链条的尽头,现在在火山源上。

 

虽然这个想法并没有立即被接受,但是夏威夷(和其他)链条上的熔岩年代表明,他们的年龄正在增加,远离目前活跃的火山,正如达利所说的那样。威尔逊对这些资料的分析现在是板块构造学的核心部分。在板块内部发生的大多数火山被认为是由地幔柱,从地幔深处升起的熔岩柱形成的。只要火山喷发,火山仍然是一个活跃的“热点”。这些羽毛显然起源于深处,或许与核心和地幔的边界一样深,而且很多都活跃了很长一段时间。夏威夷热点地区最古老的火山年龄接近八千万年。其他岛屿,包括印度洋太平洋,留尼汪岛和毛里求斯的大溪地岛和复活节岛,以及世界大洋上的大多数大岛,都归功于地幔羽。

 

因此,海洋火山岛及其热点径迹对于地质学家来说特别有用,因为他们记录了过去固定源的位置。因此,它们允许重建海底扩张过程,从而重建过去的大陆和海盆地理。例如,鉴于太平洋板块的当前位置,夏威夷位于太平洋热点之上。因此,五千万年前的太平洋板块的位置可以通过移动这样一个热点小径中的一个五千万年前的火山位于夏威夷的位置来确定。然而,由于海洋盆地确实是地质时代尺度上的短命的特征,沿着热点小道回溯的世界地理重建只能在地质时代最后的5%左右发挥作用。


中文翻译

The author mentions “spreading ridges”, “subduction zones”, and “transform faults” in order to

  • spreading ridges subduction zones transform faults 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 A illustrate that the boundaries of tectonic plates are neat, thin lines. B explain why some tectonic plates carry islands or continents while others form the seafloor. C explain the complex nature of the edges of tectonic plates. D provide examples of areas of tectonic plates where little geologic action occurs. spreading ridges subduction zones transform faults
正确答案: C

网友解析

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    十三个  发表于 2018-11-22 20:01:37

    C 原文这部分介绍了各种不同的edges的现象,并做了详细的解释,前面也说到plate的组成就像是jigsaw puzzle,从两处可以看出应该是要体现其复杂性。A与原文矛盾,原文并没有对B做出解释,D原文给出的是geologic action常发的地方,而不是little ~ occurs

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    十三个  发表于 2018-11-22 20:01:20

    C 原文这部分介绍了各种不同的edges的现象,并做了详细的解释,前面也说到plate的组成就像是jigsaw puzzle,从两处可以看出应该是要体现其复杂性。A与原文矛盾,原文并没有对B做出解释,D原文给出的是geologic action常发的地方,而不是little ~ occurs

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