| Scientists generally credit violent collisions between tectonic plates, the mobile fragments 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 densities 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 formations 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.
Q6:
The passage is primarily concerned with
- describing a traditional explanation for the vertical movement of Earth’s surface
- discussing recent geophysical research that may explain certain features of Earth’s surface that were previously difficult to account for
- combining elements of several theories in order to develop a plausible explanation for a poorly understood feature of Earth’s surface
- providing new evidence in support of a long-standing belief about how certain of Earth’s massive surface features were created
- questioning the implications of geophysicists’ recent findings regarding the composition and density of Earth’s mantle
答案是b，我选的d，为什么d错，有谁能帮我解释哈？谢拉～
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 features with the violent collisions of tectonic plates and inferring the causes of the information of other features from those results
C. Recording data about earthquakes throughout the world and correlating the relative force and duration of those earthquakes with information about their geological environment
D. Comparing data about earthquakes that occur in southern Africa’s superswell with data about other types of seismic events that occur there
E. Using computer models to estimate the length of time it would take for a hypothetical landmass 1,000 miles across to rise one mile vertically
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Q8:
In line 10, the author mentions the Himalayan Mountains most likely in order to
A. highlight certain similarities between the southern African superswell and other massive features on Earth’s surface
B. identify a feature of Earth’s surface that predates the origins of the southern African superswell
C. provide an example of a feature of Earth’s surface that can explained by plate tectonics
D. suggest that geophysicists are correct in attributing the sculpting of Earth’s surface to violent collisions between tectonic plates
E. give an example of a feature of Earth’s surface that scientists are unable to explain fully
Q9:
According to the passage, the computer models referred to in line 86 have had which of the following effects?
- They have confirmed researchers’ doubts regarding the theory that convection within the mantle can lift or lower Earth’s surface vertically.
- They have cast doubt on the validity of plate-tectonics theory as an explanation for the sculpting of Earth’s mountain ranges.
- They have proved geophysicists’ theory that the behavior of seismic waves in Earth’s mantle is related to the temperature of the rock through which they are traveling.
- They have convinced formerly skeptical researchers that rock formations deep in Earth’s mantle below southern Africa could have created the superswell.
- They have confirmed researchers’ speculation that masses of hot rock are buoyant enough to rise to the upper part of Earth’s mantle.
答案是d，我选的a，为啥错啊～
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