教甄◆英文科題庫下載題庫

 (第 1 篇) One of the most basic concepts in plate tectonics, the study of the movements of the earth’s crust, is the idea of continental drift. As even small children often realize when studying an atlas or a globe, the continents as we know them now seem to be pieces of an enormous jigsaw puzzle. ( A ) This is because the continents were once, in fact, parts of a single supercontinent called Gondwanaland. For eons, however, they have been moving apart, and the oceans separating them have been growing wider. In order for the seas to expand, there must be some mechanism by which the ocean floor can itself spread out to fill the space vacated by the continents, a process which was finally discovered by scientists in the 1920’s. ( B ) Using sonar, a device that reads the reflected echoes of sound waves, and a specially developed magnetometer to measure magnetic fields, scientists discovered that the rocks closest to the mid-ocean ridges were younger than rocks found at a distance. ( C ) Further, closer inspection revealed that, while the sea floor close to these ridges was relatively free from debris, marine sediment became thicker as the distance from the ridges increased. Still more advanced testing uncovered the fact that there were unusually warm currents emanating from the area of the ridges. All of these pieces of evidence pointed to one inevitable conclusion: new rock was somehow being born there as the seafloor spread out from these ridges. So just how is this rock created? Magma (molten lava from the earth’s mantle) flowing up through the relatively thin crust of the ocean floor meets the cooler ocean water and quickly coalesces, forming the rock of the mid-ocean ridges. ( D ) Because, however, this new rock has no place to go, it exerts pressure against the sides of the fissure through which it emerged, widening the crack and “spreading” the floor of the ocean. It is precisely this mechanism that forces the continental plates apart, and is responsible, at least in part, for continental drift.