2014年4月托福考试模拟试卷及答案(第四套)_第15页

　　Volcanic fire and glacial ice are natural enemies. Eruptions at glaciated volcanoes

　　typically destroy ice fields, as they did in 1980 when 70 of Mount Saint Helens

　　ice cover was demolished. During long dormant intervals, glaciers gain the upper hand

　　Line cutting deeply into volcanic cones and eventually reducing them to rubble. Only rarely

　　(5) 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

　　(10) 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

　　(15) 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 larger of

　　Rainier's the crater's, forming a continuous passageway that extends two-thirds of the

　　(20) 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

　　(25) ice, replenished annually by winter snowstorms, will expand, pushing against the

　　enclosing crater walls and smothering the present caverns in solid firm ice.

　　The word "they" in line 2 refers to

　　(A) fields

　　(B) intervals

　　(C) eruptions

　　(D) enemies