Earth’s surface consists of rigid plates that are constantly shifting and jostling one another. Plate movements are the surface expressions of motions in the mantle—the thick shell of rock that lies between Earth’s crust and its metallic core. Although the hot rock of the mantle is a solid, under the tremendous pressure of the crust and overlying rock of the mantle, it flows like a viscous liquid. The mantle’s motions, analogous to those in a pot of boiling water, cool the mantle by carrying hot material to the surface and returning cooler material to the depths. When the edge of one plate bends under another and its cooler material is consumed in the mantle, volcanic activity occurs as molten lava rises from the downgoing plate and erupts through the overlying one.

Most volcanoes occur at plate boundaries. However, certain “misplaced” volcanoes far from plate edges result from a second, independent mechanism that cools the deep interior of Earth. Because of its proximity to Earth’s core, the rock at the base of the mantle is much hotter than rock in the upper mantle. The hotter the mantle rock is, the less it resists flowing. Reservoirs of this hot rock collect in the base of the mantle. When a reservoir is sufficiently large, a sphere of this hot rock forces its way up through the upper mantle to Earth’s surface, creating a broad bulge in the topography. The “mantle plume” thus formed, once established, continues to channel hot material from the mantle base until the reservoir is emptied. The surface mark of an established plume is a hot spot—an isolated region of volcanoes and uplifted terrain located far from the edge of a surface plate. Because the source of a hot spot remains fixed while a surface plate moves over it, over a long period of time an active plume creates a chain of volcanoes or volcanic islands, a track marking the position of the plume relative to the moving plate. The natural history of the Hawaiian island chain clearly shows the movement of the Pacific plate over a fixed plume.

Q22:

The passage is primarily concerned with discussing

A. the composition of Earth’s mantle

B. how the Hawaiian Islands were created

C. what causes Earth’s surface plates to move

D. two different mechanisms by which volcanoes are formed

E. why most volcanoes occur at plate boundaries

Q23:

It can be inferred from the passage that a chain of volcanoes created by a mantle plume would most likely be characterized by

A. a curved outline

B. constituent volcanoes that differ from each other in age

C. occurrence near a plate boundary where one plate bends under another

D. appearance near many other volcanic chains

E. rocks with a wide range of chemical composition

Q24:

The author’s reference to the Hawaiian Islands serves primarily to

A. provide an example of a type of volcanic activity that does not occur elsewhere

B. identify the evidence initially used to establish that the Pacific plate moves

C. call into question a theory about the source of the volcanoes that created the Hawaiian Islands

D. illustrate the distance from plate edges at which volcanoes typically appear

E. provide an example of how mantle plumes manifest themselves on Earth’s surface

Q25:

According to the passage, a hot spot on Earth’s surface is an indication of which of the following?

A. An untapped reservoir of hot rock in the base of the mantle

B. Volcanic activity at the edge of a plate

C. Solid mantle rock under tremendous pressure

D. The occurrence of a phenomenon unique to the Pacific plate

E. A plume of hot mantle rock originating near Earth’s core