Questions 45-50
Volcanic fire and glacial ice are natural enemies. Eruptions at glaciated volcanoes typically destroy ice fields as they did in 1980 when 70 percent of Mount Saint Helens ice cover was demolished. During long dormant intervals, glaciers gain the upper hand cutting deeply into volcanic cones and eventually reducing them to rubble. Only rarely do these competing forces of heat and cold operate in perfect balance to create a phenomenon such as the steam caves at Mount Rainier National Park.
Located inside Rainier's two ice-filled summit craters, these caves form a labyrinth of tunnels and vaulted chambers about one and one -half miles in total length. Their creation depends on an unusual combination of factors that nature almost never brings together in one place. The cave-making recipe calls for a steady emission of volcanic gas and heat, a heavy annual snowfall at an elevation high enough to keep it from melting during the summer, and a bowl-shaped crater to hold the snow.
Snow accumulating yearly in Rainier's summit craters is compacted and compressed into a dense form of ice called firm, a substance midway between ordinary ice and the denser crystalline ice that makes up glaciers. Heat rising from numerous openings (called fumaroles) along the inner crater walls melts out chambers between the rocky walls and the overlying ice pack. Circulating currents of warm air then melt additional openings in the firm ice, eventually connecting the individual chambers and, in the large of Rainier's two craters, forming a continuous passageway that extends two-thirds of the way around the crater's interior.
To maintain the cave system, the elements of fire under ice must remain in equilibrium enough snow must fill the crater each year to replace that melted from below. If too much volcanic heat is discharged, the crater's ice pack will melt away entirely and the caves will vanish along with the snows of yesteryear. If too little heat is produced, the ice, replenished annually by winter snowstorms will expand, pushing against the enclosing crater walls and smothering the present caverns in solid firm ice.
【題組】46.The word " they" in line 2 refers to
(A) fields
(B) intervals
(C) eruptions
(D) enemies
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