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.
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.
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.
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.
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, bubble-free 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.
冰山就是巨大的冰块，它们的形状各不规则；他们在海面上所呈现出来的部分大概只有总量的12%。冰山是由冰川----从格陵兰岛, 南极洲, 和阿拉斯加的内陆降雪开始积累成为大河中的冰----然后缓慢流入海洋。向前的移动，在进入海洋的时候冰川底部的融化，和波浪与潮汐变化造成了冰块的断裂从而漂浮在海上。
冰山的颜色一般是从蓝到白，虽然有时会因为他们带有砂砾和石块而显得颜色很深或不透明。在不同情况的光和云量下，它们的颜色呈现可能会随之不同，如在早晨和傍晚的阳光下所呈现的耀眼的粉色或金色，但这个颜色变化大致与太阳位于海平面上的低角度有关。不管怎样，总会有到南极洲的旅游者们报告说在Weddell Sea看到了绿冰山，南极洲东部Amery Ice Shelf的附近则更为常见。
一个在Amery Ice Shelf西部滞留的绿冰山呈现出了两个明显的层：含有气泡的白蓝色冰，和没有气泡的绿色冰，它们中间是由1米长的带有沉积物的冰分隔开的。海水的侵蚀决定了绿色冰的质地。由于光的分散，裂痕处的颜色是浅绿的；而没有裂痕的地方是深绿色。绿色冰中是没有气泡的，因为它是由冻结的海水所构成，而不是压缩的雪。沿着这个地区冰架的边缘，可以发现，带有绿色色素的单细胞生物非常多，而且海水里面含有它们丰富的分解有机物质。绿冰山虽没有包含很多这些生物体的微粒，但从海水中所积累的分解有机物质还是很多的。不同于盐，分解有机物质并没有在结冰过程中被排除掉。分析表明，分解的有机物质会从太阳光中吸收足够的蓝波段，从而使冰呈现出绿色。
The word “penetrate” in the passage is closest in meaning to