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2011年9-10月gmat阅读机经(至10.11)(十三)

所属:其他考试 作者:cwf1986 阅读:2237 次 评论:0 条 [我要评论]  [+我要收藏]

小编摘要:词为您收集GMAT机经,充足的准备是考G成功的最重要的一步。

  40、**科学家研究organism

  [版本1] by sallyonet

  第一段大概是说科学家研究organism(还是micro organism来着)发现他们XXX(这个就是文中要解释的我理解是运动路线?反正就是下文要解释的看了后面就明白了)是为了food然后给出一个例子一种A什么的东西它向右拐一次下一次向左拐的几率就两倍于向右拐(先向左还是右我忘了反正意思就是这玩意先向那边拐下一次它向另一边拐的几率就是两倍)

  注意:这里有道题大概好像是问这个东西什么时候向左拐的几率是右边的两倍(比这个叙述要绕反正我的理解是这样但也许是我被绕进去理解错了…反正注意下应该能选)我选得就是:向右拐以后

  然后科学家就发现这种东西拐弯时候会释放一种物质(好像是往哪边拐就那边释放吧我不记得了…)这种物质能帮他们…大概意思就是不走回头路这样就能在有限的时间里找到更多food还是怎么的

  第二段是讲另一种东西具体叫什么忘了…印象不是很深了反正也是科学家发现这种东西好像每次拐弯都是36度还是怎么的然后也能防止原地转圈之类的所以证明organism为了food可以XXXX(you know what I mean…)除了之前那道题还有个问主旨的我好像选得是用了两个research去说明一个观点还是什么的,记得有个选项干扰来着所以也不太确定,但我忘了…

  好像还有一题是问第二段的作用?我不确定可能跟别的月度混了…

  [考古]

  科学家研究organism

  by elaineliang 680,Q50, V31 3/7/2011

  讲两个物质如何更高效地去找食物。第一段先讲两个科学家研究表明a物质会记得自己转弯地方向,研究表明a物质转左的几率是转右的两倍有多,如果上一次是转右的话。同时a物质会用protein留下scar做记号。第二段讲b物质。也是一些科学家研究出b物质有跟找食物有关的特性,具体想不起来了~题目都不是很难,都是主旨加细节。

  40再补充一点

  2.2.12.两个物质如何更高效地去找食物

  V1 讲两个物质如何更高效地去找食物。第一段先讲两个科学家研究表明a物质会记得自己转弯地方向,研究表明a物质转左的几率是转右的两倍有多,如果上一次是转右的话。同时a物质会用protein留下scar做记号。第二段讲b物质。也是一些科学家研究出b物质有跟找食物有关的特性,具体想不起来了~题目都不是很难,都是主旨加细节。

  V2

  大概是说一个什么什么东西 然后如果先往右了的话 没有找到食物 下次就更可能往左 大概是这个意思 然后这里有个题 问什么情况下更可能往左

  第二段也是说什么zooplant吧 也是需找食物什么 有遇到什么珊瑚礁还是什么东西 然后又几个角度

  V3 是讲生物学家研究动物吃食范围的问题,举了两种动物,说A和B吧。第一段先陈述了这么一个hypothesis,然后就举例,举A,说A在向右转头吃草以后,下一次往左转的概率是往右转的二倍。这么个现象说明它有一种记忆功能,貌似是一种什么细胞造成的,又研究了这个细胞吧,其中的什么含量会变化。第二段,就开始举例说B,忘了具体讲什么了,总之还是为证明开始陈述的那个hypothesis,说动物吃食的方向选择是为扩大它们的食物范围。还有一篇忘了。。

  V4

  一个是2.2.12.两个物质如何更高效地去找食物

  狗主记得一道题问什么情况下往右是往走的两倍(可能相反),我选的是上次往左走了。

  V5

  阅读:2道和JJ一样,另外2道记起来一道,是说有一些小的器官可以使动物更好的猎食,然后有2个科学实验,第一个实验是一个A开头的动物,第一下左转,然后第二下右转的概率是左转的2倍,是因为这个动物脑子里有个什么S,一转弯就从一边移动到另一边了,然后就加强了另一边转弯的概率。第二段是另一个实验,一个水里游的动物P, 给湖里撒食物,发现这个P一次就转36度,科学家说这样可以最大程度上获取食物。 题目一个是主旨题,选claims一个conclusion,用了2个research。 另一个是第二段的作用,支持主旨。 第三个是个细节题。 总这这篇阅读灰常简单。

  41、personality trait 与职业选择

  [版本1] by symbolicsmile Q51 V40

  (1)2段,讲的是personality trait与职业选择关系。

  第一段讲了两类人,leaders和technicians,说某个学者研究出来leaders很outgoing,technicians很passive。(尼玛这不是废话了,是人都知道),然后开始装模作样的分析原因,说为什么leaders需要outgoing的品质啦,technician因为做的东西大部分是分析内容,不用和人打交道,所以passive没关系啦。(lz读完第一段就傻了,怀疑自己怎么这么快就进低分库了,这尼玛讲的是小学生的东西,而且整一段的单词那个简单)

  第二段更弱智,开始扯个人的职业选择了,又说了正常的outgoing的人都会选择去当manager,因为自己性格适合呀。(这尼玛又是废话。)然后最后扯到说虽然个人可以选择呀,但是最后要不要你还是employer自己来决定的。

  题目有点难,尼玛这么短的文章一共还4到题目,其中有一道问的是第二段的作用。我最后不知道是选了restate the argument in the first paragraph,还是选了provide new evidence to the explanation in the first paragraph,其他选项太不靠谱了。

  [考古]

  性格与职业选择

  第一段:研究人员发现outgoing的人做管理类工作的多,passive的人从事technical的多。然后解释原因:management position的 daily work 主要是和人打交道的工作等。Organized person需要lead,talk and organize staff。

  第二段:然而一种可能性是:人们倾向于根据自己的personality traits来选择occupation。就算outgoing的人去应聘manager,withdraw的人都去应聘tech工作,however, the employer has the authority to final say to decide 会不会聘用这个人从事某个工作。

  1)作者认为personality traits对人们的occupation有什么影响?

  2)什么决定人们从事某个occupation?(最后还是雇主说了算)

  3)什么人适合做管理工作? (outgoing and organized person)

  4)主旨题(比较difference)

  5)Inference: withdraw的人做管理类的工作会怎么样?(这些人tend to be less successful in leading than outgoing managers)。

  6)第二段的作用是什么? (一个选项是undermine第一段的内容,还有一个是restate 第一段)

  7) 文中说从事technology的人性格withdraw and passive,问以下哪个能weaken从事technology的人的passive性格?(从事technology的人也需要经常跟别人交流并且说服别人)

  [版本2] by finalef V30 M50

  第一篇就是personality trait 与职业选择 symbolicsmile Q51 V40 牛人考到的尼玛让我信心倍增的啊!

  的确很短 就是两段 第一段讲最近研究personality trait和 occ 啥的关系 然后一种就是outgoing的managers还有passive的technicians 接着具体介绍了点有的没的 第二段就是然而one problem about 上面两个还要被讨论一下啊 就和大牛写的一样还是解释了一番 最后说并不是每个选择manager的人 那个occ都会要你的 结束

  题目考了四题 尼玛又和大牛一样! 最后也是碰上那个主旨 我考虑了下选了restate 因为另一个provide anther 啥啥啥不太对

  [版本3] by bonbon_sj 760 V42 Q50

  personal trait和occupation关系,就两段。极简单。

  一题问第二段作用,因为第二段头两句话都是might,may,我选择propose new researh program to determine personal trait和occupation关系.

  还有一题问以下那句为真,我选employer's decision outweigh其他因素

  [版本4] by xbftx 720,M50,V37

  再有一篇阅读是《personal trait 与职业选择》。文章不长,也比较简单。outgoing的人适合做manager,因为manager要怎样怎样,passive的人适合做technicians,因为怎样怎样。第二段说虽然这些性格的人会选择相对的工作,但是最终录用权还是在employer的手上。题目有问第二段的作用。大家好好理解一下。我这题犹豫了很久。还有主旨题。

 


  42、建筑文化史

  见14 V9

  43、**英国 tenant 与landlord

  [版本1] by symbolicsmile Q51 V40

  英国的tenant与landlord,共3段

  第一段讲landlord何其残忍呀,剥削农民呀(周扒皮出现在了英国呀),导致农民没有多余的钱去investment,所以社会的经济发展特别缓慢,强调的是13世纪的英国,考古的童鞋们可以自己查一查。

  第二段开始反驳,说landlord没那么残忍,因为有free tenant的存在,然后开始形容这个free tenant,有哪些权利呀等等。

  第三段开始讲由于free tenant的要求何其高(天啊,农民们你们权利真大呀,中国农民太惨了啊),landlord为了yield to the favorable requirements to tenants,制定了某些某些政策,是这些政策导致了农村的congestion,所以社会发展停滞了。

  题目还清楚的记得一道,说最后一段这些landlord制定的政策long-term和short-term的影响。

  lz的选择是:long-term对tenant不好,short-term好(因为这些是favorable to the tenants的)。

  [版本2] by ayy90

  英国地主:说英国经济停滞原因的,寂静说的不错,生词很多,有点儿慌,就顺着寂静选的。对不起实在记不得了。

  [版本3] by haoli1991 V37

  2英国的landlord 和 tenant, 说tenant 很惨,受到landlord的压榨,后来有些人就成为什么free-tenant, 最后为了解决这个问题, landlord又用了什么招,但是该方法也没奏效,而且阻碍了经济的发展。

  (以上两篇都特别长,考试的时候还是要自己的理解,我能力有限,就理解到这个程度)

  [考古] by jackjack251 阅读43英国地主完美匹配原文(已确认)

  第七篇中世纪经济恶化探源:庄园主(lord)和佃农(tenant)的关系

  版本一acrophobia (V35)

  这题的主旨是讨论为什么英国十四世纪时候生产力下降了呢?

  p1有老观念说是因为以前人的认为是lord只关心自己享乐不关心生产导致的(还以为法律规定了佃农没有自己土地所有权)

  p2使用了一个free serf的例子说明以上观点是错误的

  p3说access to land 的变化 才是生产力下降的真正原因

  版本二 scratte (V42)

  第一段:传统的观点认为,lord给tenant,特别是serf tenant (隶属于lord的,给其交租纳贡的tenant),带来了极大的经济压力,因为tenant要交很高的税负,要孝敬lord,缺乏人身自由。但是,这一观点是不正确的。因为其一,有很大一部分是nonserf tenant,这些人只要交很低的钱,有很大的自由度,小日子还挺滋润;其二,即便是serf tenant,他们的生活也没有那么糟糕,他们还是有一定自主权的,诸如lord要增加税赋的时候,他们可以提出反对意见,并且有一定的力量。

  第二段(还是第三段?不是很清楚了):但是中世纪的经济还是恶化了,这是为什么呢?究其原因,是因为lord在表面上向tenant做了让步,使其获得短期经济利益,但是从长期来看,lord通过这些举措遏制了人口和经济的发展,并最终带来了社会环境的恶化。

  这是一篇长阅读,我记得最后一个题目好像是我上面讲到的第二段,问你lord的这些举措是serve了谁的长期or短期利益?我从文章的理解是,serve the short terminterest of tenant but not the long term。大家碰到的话可以再看看。

  问题有:7/6

  Q1: 有主题题

  Q2: 问作者在第二段提到free-tenant的作用

  Q3: 问你lord的这些举措是serve了谁的长期or短期利益?

  答:serve the short term interest of tenant but not the long term。也有答: 短期对领主有利,但不利于长期的经济发展。因为造成乡村的失业。大家小心。

  考古 nowwsy!!谢谢 7/1

  14世纪的有关英国封建领主的东西

  新老观点型:关于封建领主影响英国经济长期发展的真正原因

  1)传统观点。14世纪时人民的生活水平日益下降,甚至到了威胁生存的地步(开头句)。接着说了英国lord和tenant的关系,tenant的生活如何疾苦,如何受lord剥削控制,以至于他们的关系像是master和serf(农奴)。学者们还认为这些地主好吃懒做,表现在很重视眼前的享受和消费,而不在乎长远的发展,所以经济一直处于投资不足的状态(underinvestment),还说了一些其它因素,这些都妨碍了宏观经济的发展。

  2)新证据。虽然老观点有着很大的吸引力(作者用了compelling),但是它越来越不能回答近期发现的一些新证据(主题句)。老观点忽视了(overlook)另一个不同于lord和tenant的新人群:free tenant。他们不是serf,不受地主的人身控制,只用按一个较低的fixed rate缴纳租金。最后提到其实tenant也不是原来想的那样软弱无力,他们已经能够组织起来和地主negotiate rate。

  3)进一步的补充。为了应付tenant想多租地扩大自身利益的欲望,地主想出了一系列方法。。。,生词很多,看不太懂。。。结果就是造成巨大的 unemploymentin rural area. 地主的这种做法相当于对经济的一个“brake”,阻碍了14世纪的英国经济发展。(有题,说地主的做法对谁有利?选项里都分了从长期看和从短期看,两方面讨论)[推测:短期对领主有利,但不利于长期的经济发展。因为造成乡村的失业]感觉第三段可能是对第一段经济学家的反驳,说这才是阻碍经济发展的真正原因,当然这只是我个人的看法。

  超完美匹合原文版bycamelo777

  Any analysis ofwhy the early fourteenth-century agrarian economy was so predisposed to'crisis' necessarily requires careful consideration of class and propertyrelations on the land, for, as Robert Brenner and S. H. Rigby have bothemphasized, these could be of decisive importance.7At that time landlordsexercised feudal rights of lordship over their tenants, many of whom were ofservile status and therefore legally subordinate to their lords. This powerrelationship shaped the tenurial relationship between those who owned the landand those who occupied and worked it. Thus it was tenure, as regulated bylandlords, that determined the supply of holdings and the form and level ofrents that were charged. Until recently it was widely believed that feudaltenurial relationships sanctioned and facilitated the extra-economicexploitation of tenants by their lords. Together, the heaviness of rent chargesand the arbitrariness of lordship discouraged and depressed tenant investmentin agriculture.8Meanwhile, lords were more interested in pursuing a feudallifestyle of conspicuous consumption than in enhancing the productivity andprofitability of their estates.9The upshot, it has been claimed, was a viciouscircle of underinvestment, static technology, and low and decliningagricultural productivity.10

  Such pessimisticviews of lords and their relations with their tenants have long exercised acompelling appeal.15 Nevertheless, they have become increasingly difficult toreconcile with a growing body of historical evidence. The preoccupation withserfdom overlooks the numerous free tenants who were exempt from the mostcoercive aspects of lordship.16Free tenants mostly paid fixed and low rents andtheir property rights enjoyed the protection of the royal courts。The more substantial customarytenants were, in fact, relatively well off 。 Many of these tenants certainly paid a proportion of their rent inlabour, but historians, probably because of a modern abhorrence of theinstitution of forced labour, have exaggerated its economic significance. Inreality, only a minority of tenants actually performed labour services, and theaggregate value of rents in cash far exceeded that of rents in kind.21Notwithstanding the much-vaunted powers of lordship, tenants had long beenremarkably effective at opposing efforts by lords to raise rents and increaselabour services in line with rising land values and commodity prices.22They didso by countering seigniorial power with custom and denying that, as tenants,they were obliged to pay their lords anything more than a de facto ground rentfor the land. Tenant right, in fact, often proved more powerful than landlordright.

  As this articleargues, the fact that so many tenants were in such conspicuous economicdifficulties by the early fourteenth century had less to do with feudallordship per se and the supposed oppressions and inequalities of serfdom, thanwith the contradictions and inefficiencies inherent in the coexistence ofcustomary, contractual and commercial relationships. Herein lay the real sourceof the agrarian problem in the early fourteenth century. In so far as lordswere the inadvertent agents of this adverse state of affairs, it was becausetheir dealings with their tenants were typically more compliant than coercive.By yielding to tenant demands for access to land on terms that were sofavourable to the tenants, lords created the preconditions for the subdivisionand subletting that stoked population growth and thereby engendered the ruralcongestion that was the source of so much under- and unemployment, with all thenegative consequences that this implies for labour productivity, livingstandards and purchasing power. This deteriorating situation in the countrysideacted as a brake upon the continued growth of the economy and, from 1315, leftincreasing numbers ever more cruelly exposed to the heightened risk ofenvironmental hazard

  附上bale的中文版

  Para1:

  B:傳統的觀點認為,lord給tenant,特別是serf tenant (隸屬於lord的,給其交租納貢的tenant),帶來了極大的經濟壓力,因為tenant要交很高的稅負,要孝敬lord,缺乏人身自由。

  Para2:

  P:但是,這一觀點是不正確的,雖然老觀點有著很大的吸引力(作者用了compelling),但是它越來越不能回答近期發現的一些新證據(主題句)。

  S:因為其一,有很大一部分是non serf tenant,這些人只要交很低的錢,有很大的自由度,小日子還挺滋潤;其二,即便是serftenant,他們的生活也沒有那麼糟糕,他們還是有一定自主權的,諸如lord要增加稅賦的時候,他們可以提出反對意見,並且有一定的力量。

  Para3:

  S:但是中世紀的經濟還是惡化了,這是為什麼呢?究其原因,是因為,為了應付tenant想多租地擴大自身利益的欲望,地主想出了一系列方法。。。,生詞很多,看不太懂。。。結果就是造成巨大的 unemploymentin rural area. 地主的這種做法相當於對經濟的一個“brake”,阻礙了14世紀的英國經濟發展。lord在表面上向tenant做了讓步使tenant獲得短期經濟利益,但是從長期來看,lord通過這些舉措遏制了人口和經濟的發展,並最終帶來了社會環境的惡化。


  44、火山喷发

  [版本1] by raphael0827

  阅读有一个说的是lava,先说lava流的慢,会吸收一种矿物,释放另一种矿物,后来举个反例,提出两个假设,第一个是错的,第二个合理。

  [版本2] by leeern 750, Q 51, V 40

  火山喷发第一段主要说一个实验, 第二段说海底发现的lava, 生词较多, 建议大家时间充裕的话可以看几集Discovery讲火山的补补背景知识.

  [版本3] by laixiangeffort

  火山爆发的 一共2段

  就是那个lava jj上讲的很不全 但是他说这是高分库里面的 所以我郁闷了 因为我也遇到了 但是楼主表示前30道题正确率应该挺高 后来是被时间影响了应该。这是出现在前30到中的火山爆发 balabala 讲到了一个试验(有题目)题目:问什么melten的什么是哪个揭示的就在第一段 所以楼主选了这个试验讲到了melten lava在流动时候与周围exchange 一个含氧的什么物质

  第二段 因为到距离地面多少km以后却发现不再交换了 所以提出了2个假设 第一个假设根本没看懂说什么 好像是什么因为到地面了以后流速很快之类的 但是作者是否定这个的第二个假设 也没看懂 反正题目好像不要求你看懂

  题目2:问文章主旨是什么 忘了选项

  [版本4] by viper999 690 (V34 M49)

  补充火山的题:

  (1). 主旨

  (2). 问我们如何知道larva在上升的过程中会吸收一种元素排除另外一种(原文里找,是lab experiment shows....)

  (3).高亮部分在全文中起到的作用。

  [考古] lava

  2.2.1. 火山熔岩的来由

  V1 yueqianchen

  第一段:说的是火山爆发中熔岩(lava)是来自地幔的(mantle),说是由地下升起来的,在此过程中成分变化,一种物质O1(O打头)增加,另一个O2(也是O打头)减少。

  第二段:海底火山样本大概都证明了,但是奇怪的是45KM时成分就不变了,这是怎么回事呢?若是不变它如何上来的呢?第一种解释说是沿着裂缝上来的(crack),然后说不可能啊,因为45KM还很深东西都很软,没有裂缝。第二种解释又说,到45KM时O2就用完了 - 题目实在记不起来 记得都挺好找的

  V2 g20040636

  还有lava的那个

  第一段是一个试验说从mantle 怎样就变成了melt 然后说越靠近surface越多 Ose。。很长一个单词 然后越少 Olive之类的一个单词。

  第二段说在可是ocean land里面 45km以下就没有这种substance的exchange 然后就分析原因 有一个是rock太软不会裂开怎么流不出去 还有几个解释忘记了

  2.2.1. ★火山熔岩的来由

  V1 duke3d001 750, wade777, echosweet 700 & yueqianchen

  关键词:45KM, Olivine, Orthopyroxene (referenced by gitarrelieber)。这篇文章的题目不难,狗的骨架也很清晰。

  第一段讲火山爆发来源于Mantle中的Lava,而Lava来源于Melt ,Melt在向地表上升的过程中会与Mantle中的Rock反应并不断互相交换物质、变化结构,即吸收Orthopyroxene并排出Olivine。

  第二段说一个跟理论不太相符的事情,一种海底里的lava sample,在距离地表45千米突然发现已经停止这种物质交换,Melt的结构不变了。一种假设是那里的Mantle太松散了,使Melt无法与他们接触并交换物质,但立即被否定了(因为45KM还很深东西都很软,没有裂缝)。另一种假设是Melt在之前的上升过程中已经吸收了足够的Orthopyroxene, 并将能排出的Olivine都排了,无法继续反应。

  1 darkchoco 710是什么可以证明这种exchange的存在:熔岩的成分

  2 gyz12 740 一道文章最后句定位:Olivine的用完了,exchange就停止了

  3 gyz12 740 一道是选chemical composition为特征 sashimiyuki 720 V37 选“lab experiments” indicate 那个melt 的变化的,没有选chemical composition, 细节题定位后决定的,确认后到现在还没有深深后悔过

  4 tianmo0512 是什么发生反应:选melt

  5 feifeizoe 750 V39 文中什么情况下描述了那种正常的exchange:lab experiment中实现了那种现象

  6 The author mention “the melt to rise so rapidly” in order to:提出了一种hypothesis,这种hypothesis在后面被反驳

  (疑似)原文未缩减 gitarrelieber (sereneys 730 V40 基本确认)

  节选自The Origin of the Land under the Sea (Scientific American Magazine @ February 2009)

  Author: Peter B. Kelemen

  Knowledge of the intense heat and pressure in the mantle led researchers to hypothesize in the late 1960s that ocean crust originates as tiny amounts of liquid rock known as melt—almost as though the solid rocks were “sweating.” Even a minuscule release of pressure (because of material rising from its original position) causes melt to form in microscopic pores deep within the mantle rock. Explaining how the rock sweat gets to the surface was more difficult. Melt is less dense than the mantle rocks in which it forms, so it will constantly try to migrate upward, toward regions of lower pressure. But what laboratory experiments revealed about the chemical composition of melt did not seem to match up with the composition of rock samples collected from the mid-ocean ridges, where eruptedmelt hardens. Using specialized equipment to heat and squeeze crystals from mantle rocks in the laboratory, investigators learned that the chemical composition of melt in the mantle varies depending on the depth at which it forms; the composition is controlled by an exchange of atoms between the melt and the minerals that makeup the solid rock it passes through. The experiments revealed that as melt rises, it dissolves one kind of mineral, orthopyroxene, and precipitates, or leaves behind, another mineral, olivine. Researchers could thus infer that the higher in the mantle melt formed, the more orthopyroxene it would dissolve, and the more olivine it would leave behind.(melt上升时, 溶解Ort产生Oli, 所以melt higher, 溶解的Ort越多,产生的/留在身后的Oli也越多) Comparing these experimental findings with lava samples from the mid-ocean ridges revealed that almost all of them have the composition of melts that formed at depths greater than 45kilometers. This conclusion spurred a lively debate about how meltis able to rise through tens of kilometers of overlying rock while preserving the composition appropriate for a greater depth. If melt rose slowly in smallpores in the rock, as researchers suspected, it would be logical to assume that all melts would reflect the composition of the fashallowest part of the mantle,at 10 kilometers or less. Yet the composition of most mid-ocean ridge lavas amples suggests their source melt migrated through the uppermost 45 kilometers of the mantle without dissolving any orthopyroxene from the surrounding rock. But how? (疑大概为狗狗第一段的背景内容)

  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.

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2012-01-23 12:18 编辑:cwf1986
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