TPO托福阅读练习题37及参考答案（六）

　　A green iceberg that stranded just west of the Amery Ice Shelf showed two distinct layers: bubbly blue-white ice and bubble-free green ice separated by a one-meter- long ice layer containing sediments. The green ice portion was textured by seawater erosion. Where cracks were present, the color was light green because of light scattering; where no cracks were present, the color was dark green. No air bubbles were present in the green ice, suggesting that the ice was not formed from the compression of snow but instead from the freezing of seawater. Large concentrations of single-celled organisms with green pigments (coloring substances) occur along the edges of the ice shelves in this region, and the seawater is rich in their decomposing organic material. The green iceberg did not contain large amounts of particles from these organisms, but the ice had accumulated dissolved organic matter from the seawater. It appears that unlike salt, dissolved organic substances are not excluded from the ice in the freezing process. Analysis shows that the dissolved organic material absorbs enough blue wavelengths from solar light to make the ice appear green.

　　Chemical evidence shows that platelets (minute flat portions) of ice form in the water and then accrete and stick to the bottom of the ice shelf to form a slush (partially melted snow). The slush is compacted by an unknown mechanism, and solid, bubblefree ice is formed from water high in soluble organic substances. When an iceberg separates from the ice shelf and capsizes, the green ice is exposed.

　　The Amery Ice Shelf appears to be uniquely suited to the production of green icebergs. Once detached from the ice shelf, these bergs drift in the currents and wind systems surrounding Antarctica and can be found scattered among Antarctica's less colorful icebergs.

　　Paragraph 1: Icebergs are massive blocks of ice, irregular in shape; they float with only about 12 percent of their mass above the sea surface. They are formed by glaciers-large rivers of ice that begin inland in the snows of Greenland, Antarctica, and Alaska-and move slowly toward the sea. The forward movement, the melting at the base of the glacier where it meets the ocean, and waves and tidal action cause blocks of ice to break off and float out to sea.

　　1. According to paragraph 1, all of the following are true of icebergs EXCEPT:

　　○They do not have a regular shape.

　　○They are formed where glaciers meet the ocean.

　　○Most of their mass is above the sea surface.

　　○Waves and tides cause them to break off glaciers.

　　Paragraph 2: Icebergs are ordinarily blue to white, although they sometimes appear dark or opaque because they carry gravel and bits of rock. They may change color with changing light conditions and cloud cover, glowing pink or gold in the morning or evening light, but this color change is generally related to the low angle of the Sun above the horizon. However, travelers to Antarctica have repeatedly reported seeing green icebergs in the Weddell Sea and, more commonly, close to the Amery Ice Shelf in East Antarctica.

　　2. According to paragraph 2, what causes icebergs to sometimes appear dark or opaque?

　　○A heavy cloud cover

　　○The presence of gravel or bits of rock

　　○The low angle of the Sun above the horizon

　　○The presence of large cracks in their surface

　　Paragraph 2: One explanation for green icebergs attributes their color to an optical illusion when blue ice is illuminated by a near-horizon red Sun, but green icebergs stand out among white and blue icebergs under a great variety of light conditions. Another suggestion is that the color might be related to ice with high levels of metallic compounds, including copper and iron. Recent expeditions have taken ice samples from green icebergs and ice cores-vertical, cylindrical ice samples reaching down to great depths-from the glacial ice shelves along the Antarctic continent. Analyses of these cores and samples provide a different solution to the problem.

　　3. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

　　○One explanation notes that green icebergs stand out among other icebergs under a great variety of light conditions, but this is attributed to an optical illusion.

　　○One explanation for the color of green icebergs attributes their color to an optical illusion that occurs when the light from a near-horizon red Sun shines on a blue iceberg.

　　○One explanation for green icebergs attributes their color to a great variety of light conditions, but green icebergs stand out best among other icebergs when illuminated by a near-horizon red Sun.

　　○One explanation attributes the color of green icebergs to an optical illusion under special light conditions, but green icebergs appear distinct from other icebergs under a great variety of light conditions.

　　Paragraph 4: The ice shelf cores, with a total length of 215 meters (705 feet), were long enough to penetrate through glacial ice-which is formed from the compaction of snow and contains air bubbles-and to continue into the clear, bubble-free ice formed from seawater that freezes onto the bottom of the glacial ice. The properties of this clear sea ice were very similar to the ice from the green iceberg. The scientists concluded that green icebergs form when a two-layer block of shelf ice breaks away and capsizes (turns upside down), exposing the bubble-free shelf ice that was formed from seawater.

　　4. The word penetrate in the passage is closest in meaning to

　　○collect

　　○pierce

　　○melt

　　○endure

　　5. According to paragraph 4, how is glacial ice formed?

　　○By the compaction of snow

　　○By the freezing of seawater on the bottom of ice shelves

　　○By breaking away from the ice shelf

　　○By the capsizing of a two-layer block of shelf ice

　　6. According to paragraph 4, ice shelf cores helped scientists explain the formation of green icebergs by showing that

　　○the ice at the bottom of green icebergs is bubble-free ice formed from frozen seawater

　　○bubble-free ice is found at the top of the ice shelf

　　○glacial ice is lighter and floats better than sea ice

　　○the clear sea ice at the bottom of the ice shelf is similar to ice from a green iceberg