Scientists generally credit violent collisions between tectonic plates

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.

1) In the highlighted text (Himalayan Mountains), 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 be 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



2) It can be Inferred from the passage that prior to the technological advances referred to in the highlighted text (recent technological), geophysicist were unable to

(A) understand exactly how the collisions of tectonic plates created the Earth's mountains
(B) establish that mantle material becomes more buoyant as it heats and more sluggish as it cools
(C) start mapping the densities and temperatures of rock throughout the mantle
(D) connect the phenomenon of convection within the mantle to the horizontal movement of tectonic plates
(E) prove that different types of rock within the mantle have different densities



3) According to the passage, the process of convection mentioned in the highlighted text (this process of convection) was regarded until recently by geophysicists as

(A) a process unlikely to occur deep within Earth's mantle
(B) a probable explanation for the rising of the southern African plateau
(C) an improbable explanation for the horizontal movement of tectonic plates
(D) unlikely to account for the vertical rising or lowering of Earth's surface
(E) unrelated to the creation of Earth's most massive surface features




4) According to the passage, which of the following pieces of geological evidence makes plate tectonics an inadequate explanation for the existence of the superswell of southern Africa?

(A) The depth of the mantle underlying the tectonic plates surrounding southern Africa
(B) The absence of any significant mountain ranges in the vicinity of the superswell
(C) The vast size of the plateau comprising the superswell's most visible feature
(D) The rate at whit the superswell has been rising above sea level over the past 100 million years
(E) The absence of any tectonic collisions in southern Africa for several hundred million years prior to the origin of the superswell