红DeltaTest1听力Plate Tectonics原文文本答案解析+音频录音mp3

[00:00.00]We currently believe that the earth’s crust—the rocky part of the earth—is composed of several large, rigid plates. [00:08.94]These plates are being created at some edges and being destroyed at others. They’re also moving across the earth. [00:16.83]This theory is called plate tectonics. It was first put forth in 1963 by a Canadian geophysicist by the name of Tuzo Wilson.
[00:26.24]Tuzo Wilson was instrumental in advancing the theory of plate tectonics. [00:31.30]He suggested that the Hawaiian and other volcanic island chains might have formed as a result of the movement of a plate over a motionless “hotspot” in the earth’s mantle. [00:42.80]Hundreds of studies have proved that Wilson was right. [00:46.56]However, in the early 1960s, his idea was considered so radical that his “hotspot” manuscript was initially rejected by all the major international scientific journals.
[00:57.31]Basically, plates are areas of the earth’s crust that move as a unit. [01:03.35]At the present time, there are eight large plates, as well as a similar number of smaller plates.
[01:09.25]According to the theory of plate tectonics, a plate has three kinds of boundaries with other plates: oceanic ridges, oceanic trenches, and transform faults. [01:22.06]Most of the world’s earthquakes and volcanoes occur at plate boundaries. [01:27.00]This is what you’d expect because plate boundaries are where a great deal of friction and stress occur.
[01:32.56]At plate boundaries, a couple of things can happen. [01:35.13]One is that rock is forced up from the mantle in molten form as lava—at ridges. [01:42.00]Another is that rock is melted and forced back into the mantle—at trenches. [01:47.87]This process of rock being “swallowed” or forced back into the earth’s mantle is called subduction. [01:54.10]During subduction, as a plate dives into the depths, we think part of it finds its way back to the surface in the form of volcanoes.
[02:03.10]The theory of plate tectonics and the discovery of sea floor spreading have confirmed the theory of continental drift, the movement of continents. [02:12.12]Sea floor spreading was discovered in the North Atlantic, and soon afterward in all other oceans.[02:18.25]What we found is that—in the areas around oceanic ridges— the deep sea floor is formed by rising lava, which then spreads out sideways in both directions.
[02:30.00]So, does the spreading of the ocean floor mean that the surface of the earth is increasing? No, not in the least. [02:36.70]Sea floor spreading doesn’t cause an increase in the earth’s surface. [02:40.86]And why not? Because the lava that rises and spreads from the oceanic ridges sinks again elsewhere in subduction zones, which are nearly identical with the ocean trenches.
[02:53.00]Subduction zones are areas of frequent earthquakes and are usually associated with the rows of volcanic islands that accompany the oceanic trenches. [03:03.00]Subduction is currently happening beneath island arcs, like Japan. [03:07.59]Subduction is also taking place on the Pacific Northwest Coast of North America, beyond the end of the San Andreas Fault. [03:15.45]This is where a subducted plate is thought to have disappeared beneath the North American plate in recent geological time, leaving the volcanoes of the Cascade Range as evidence of its past existence.