GMAT 考满分题库

Scientists generally credit violent collisions between tectonic plates, the mobilefragments of 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, a vast plateau over 1,000 miles across and nearly a mile high. Geologic evidence shows that southern African has been slowly rising for the past 100 million years, yet it has not experienced a tectonic collision for nearly 400 million years. The explanation may be in Earth's mantle, the layer of rock underlying the tectonic plates and extending down over 1,800 miles to the outer edge of Earth's iron core.

Since the early twentieth century, geophysicists have understood that the mantle churns and roils like a thick soup. The relative low density of the hottest rock makes that material buoyant, so it slowly ascends, while cooler, denser rock sinks until heat escaping the molten core warms it enough to make it rise again. While this process of convection was known to enable the horizontal movement of tectonic plates, until recently geophysicists were skeptical of its ability to lift or lower the planet's surface vertically. However, recent technological advances have allowed geophysicists to make three-dimensional "snapshots" of the mantle by measuring vibrations, or seismic waves, set in motion by earthquakes originating in the planet's outer shell and recording the time it takes for them to travel from an earthquake's epicenter to a particular recording station at the surface. Because geophysicists know that seismic waves become sluggish in hot, low-density rock, and speed up in colder, denser regions, they can now infer the temperatures and den- sities in a given segment of the interior. By compiling a map of seismic velocities from thousands of earthquakes across the globe, they can also begin to map temperatures and densities throughout the mantle. These methods have revealed some unexpectedly immense for- mations in the deepest parts of the mantle; the largest of these is a buoyant mass of hot rock directly below Africa's southern tip. Dispelling researchers' initial doubts, computer models have confirmed that this formation is buoyant enough to rise slowly within the mantle and strong enough to push Africa upward as it rises.