25 the explanation may be in earths mantle the layer

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(25) 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 (30) edge of Earth’s iron core. Since the early twentieth century, geophysicists have understood that the mantle churns and roils like a thick (35) soup. The relative low density of the hottest rock makes that material buoyant, 3
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so it slowly ascends, while cooler, denser rock sinks (40) until heat escaping the mol- ten core warms it enough to make it rise again. While this process of convection was known to enable the (45) horizontal movement of tec- tonic plates, until recently geophysicists were skeptical of its ability to lift or lower the planet’s surface vertically. (50) However, recent technolog- ical advances have allowed geophysicists to make three- dimensional “snapshots” of the mantle by measuring (55) 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 (60) travel from an earthquake’s epicenter to a particular recording station at the surface. Because geo- physicists know that seismic (65) waves become sluggish in hot, low-density rock, and speed up in colder, denser regions, they can now infer the temperatures and den- (70) sities in a given segment of the interior. By compiling a map of seismic velocities from thousands of earth- quakes across the globe, (75) they can also begin to map temperatures and densities throughout the mantle. These methods have revealed some unexpectedly immense for- (80) mations in the deepest parts of the mantle; the largest of these is a buoyant mass of hot rock directly below 4
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Africa’s southern tip. Dis- (85) pelling researchers’ initial doubts, computer models have confirmed that this formation is buoyant enough to rise slowly within the mantle (90) and strong enough to push Africa upward as it rises. -------------------------------------------------------------------------------- Q6: The passage is primarily concerned with A. describing a traditional explanation for the vertical movement of Earth’s surface B. discussing recent geophysical research that may explain certain features of Earth’s surface that were previously difficult to account for C. combining elements of several theories in order to develop a plausible explanation for a poorly understood feature of Earth’s surface D. providing new evidence in support of a long-standing belief about how certain of Earth’s massive surface features were created E. questioning the implications of geophysicists’ recent findings regarding the composition and density of Earth’s mantle Answer: -------------------------------------------------------------------------------- Q7: The passage suggests that geophysicists can now infer the characteristics in a given segment of the mantle by doing which of the following? A. Observing how long it takes for seismic waves to travel from an earthquake’s epicenter to a particular surface location and applying knowledge of how rock temperature and density affect the velocity of those waves B. Analyzing the results of studies linking the formation of some of Earth’s surface
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