2015年2月GMAT閱讀機經(火山熔漿的形成)

In the early 1970s scientists proposed an answer: the melt must make the last leg of its upward journey along enormous cracks. Open cracks would allow the melt to rise so rapidly that it would not have time to interact with the surrounding rock, nor would melt in the core of the crack ever touch the sides. Although open cracks are not a natural feature of the upper mantle— the pressure is simply too great—some investigators suggested that the buoyant force of migrating melt might sometimes be enough to fracture the solid rock above, like an icebreaker ship forcing its way through polar pack ice. Adolphe Nicolas of the University of Montpellier in France and his colleagues discovered tantalizing evidence for such cracks while examining unusual rock formations called ophiolites. Typically, when oceanic crust gets old and cold, it becomes so dense that it sinks back into the mantle along deep trenches known as subduction zones, such as those that encircle the Pacific Ocean. Ophiolites, on the other hand, are thick sections of old seafloor and adjacent, underlying mantle that are thrust up onto continents when two of the planet’s tectonic plates collide. A famous example, located in the Sultanate of Oman, was exposed during the ongoing collision of the Arabian and Eurasian plates. In this and other ophiolites, Nicolas’s team found unusual, light-colored veins called dikes, which they interpreted as cracks in which melt had crystallized before reaching the seafloor. The problem with this interpretation was that the dikes are filled with rock that crystallized from a melt that formed in the uppermost reaches of the mantle, not below 45 kilometers, where most mid-ocean ridge lavas originate. In addition, the icebreaker scenario may not work well for the melting region under mid-ocean ridges: below about 10 kilometers, the hot mantle tends to flow like caramel left too long in the sun, rather than cracking easily.

To explain the ongoing mystery, I began working on an alternative hypothesis for lava transport in the melting region. In my dissertation in the late 1980s, I developed a chemical theory proposing that as rising melt dissolves orthopyroxene crystals, it precipitates a smaller amount of olivine, so that the net result is a greater volume of melt. Our calculations revealed how this dissolution process gradually enlarges the open spaces at the edges of solid crystals, creating larger pores and carving a more favorable pathway through which melt can flow. As the pores grow, they connect to form elongate channels. In turn, similar feedbacks drive the coalescence of several small tributaries to form larger channels. Indeed, our numerical models suggested that more than 90 percent of the melt is concentrated into less than 10 percent of the available area. That means millions of microscopic threads of flowing melt may eventually feed into only a few dozen, high porosity channels 100 meters or more wide. Even in the widest channels, many crystals of the original mantle rock remain intact, congesting the channels and inhibiting movement of the fluid. That is why melt flows slowly, at only a few centimeters a year. Over time, however, so much melt passes through the channels that all the soluble orthopyroxene crystals dissolve away, leaving only crystals of olivine and other minerals that the melt is unable to dissolve. As a result, the composition of the melt within such channels can no longer adjust to decreasing pressure and instead records the depth at which it last “saw” an orthopyroxene crystal. One of the most important implications of this process, called focused porous flow, is that only the melt at the edges of channels dissolves orthopyroxene from the surrounding rock; melt within the inner part of the conduit can rise unadulterated.

【考古2】

V1

第一段：說的是火山爆發中熔巖(lava)是來自地幔的(mantle)，說是由地下升起來的，在此過程中成分變化,一種物質O1(O打頭)增加，另一個O2(也是O打頭)減少。

第二段：海底火山樣本大概都證明了，但是奇怪的是45KM時成分就不變了，這是怎么回事呢?若是不變它如何上來的呢?第一種解釋說是沿著裂縫上來的(crack)，然后說不可能啊，因為45KM還很深東西都很軟，沒有裂縫。第二種解釋又說，到45KM時O2就用完了 - 題目實在記不起來記得都挺好找的

V2

還有lava的那個

第一段是一個試驗說從mantle 怎樣就變成了melt 然后說越靠近surface越多 Ose。。很長一個單詞 然后越少 Olive之類的一個單詞。

第二段說在可是ocean land里面 45km以下就沒有這種substance的exchange 然后就分析原因 有一個是rock太軟不會裂開怎么流不出去還有幾個解釋忘記了

文言文 nowwsy & XYXB 未確認

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