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Q23~Q26: TTGWD2-Q6 to Q9:
Scientistsgenerally credit violent collisions between tectonic plates, the mobile fragmentsof Earth’s rocky outer shell, with sculpting the planet’s surface, as, for example,when what is now the Indian subcontinent collided with Asia, producing the Himalayan Mountains
.However, plate tectonics cannot fully explain certain massive surface features,such as the “superswell” of southern Africa, avast plateau over 1,000 miles across and nearly a mile high. Geologic evidence shows that southern Africanhas been slowly rising for the past 100 million years, yet it has notexperienced a tectonic collision for nearly 400 million years. The explanationmay be in Earth’s mantle, the layer of rock underlying the tectonic plates andextending down over 1,800 miles to the outer edge of Earth’s iron core. Since theearly twentieth century, geophysicists have understood that the mantle churnsand roils like a thick soup. Therelative low density of the hottest rock makes that material buoyant, so itslowly ascends, while cooler, denser rock sinks until heat escaping the moltencore warms it enough to make it rise again. While this process of convection was known to enable the horizontal movement of tectonic plates, untilrecently geophysicists were skeptical of its ability to lift or lower the planet’ssurface vertically. However, recent technological advances have allowedgeophysicists to make three-dimensional “snapshots” of the mantle by measuring vibrations,or seismic waves, set in motion by earthquakes originating in the planet’s outershell and recording the time it takes for them to travel from an earthquake’s epicenterto a particular recording station at the surface. Because geophysicists know that seismic wavesbecome sluggish in hot, low-density rock, and speed up in colder, denser regions,they can now infer the temperatures and densities in a given segment of theinterior. By compiling a map of seismicvelocities from thousands of earthquakes across the globe, they can also beginto map temperatures and densities throughout the mantle. These methods have revealed some unexpectedlyimmense formations in the deepest parts of the mantle; the largest of these isa buoyant mass of hot rock directly below Africa’ssouthern tip. Dispelling researchers’initial doubts, computermodels have confirmed that this formation is buoyant enough to riseslowly within the mantle and strong enough to push Africa
upward as it rises.------------------------------------------------------------------
Q26:
Accordingto the passage, the computer models referred to in line 86 have had which ofthe following effects?
A. They have confirmedresearchers’ doubts regarding the theory that convection within the mantle canlift or lower Earth’s surface vertically.
B. They have cast doubt on the validity ofplate-tectonics theory as an explanation for the sculpting of Earth’s mountainranges.
C. They have proved geophysicists’ theory thatthe behavior of seismic waves in Earth’s mantle is related to the temperatureof the rock through which they are traveling.
D. They have convinced formerlyskeptical researchers that rock formations deep in Earth’s mantle belowsouthern Africa could have created the superswell.
E. They have confirmed researchers’ speculationthat masses of hot rock are buoyant enough to rise to the upper part of Earth’smantle.
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发表于 2011-10-1 22:59:32
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