第一篇：

　　洋流

　　海水在季风和地转偏向力(Coriolis Force)的作用下产生洋流，洋流分为上下两层。上层温暖密度小，下层含盐多密度大，这两层各自流动不混合。

　　上层带着温暖海水向两极流动，在极地地区变冷，结冰使得盐分析出，密度变大而下沉成为下层洋流。下层洋流流回赤道。

　　两层混合的地方发生在风与海岸平行的地方，这里的风在地转偏向力的作用下把上层洋流推离海岸，于是下层洋流向上补充。洋流混合对生态有很大作用，因为把海洋生物死亡后沉到下层然后被分解的营养带到上层。滋养植物-浮游生物-更高级消费者。

　　洋流对流经的地区的气候有重要英雄。

　　洋流还与全球气候变暖联系在一起，因为二氧化碳可以溶在海水里然后沉淀到海底。

　　解析：本文属于地理地质类文章，讲述的是洋流的形成过程。地理地质类文章属于托福阅读中不太陌生的话题，但是从机经判断，本文属于流程类讲解方式，流程类文章在托福阅读里不是很常见，而且出现时，往往对学员的逻辑性考核较强，在备考时需要额外关注。类似流程类文章可以参考TPO18的Lightning.

　　参考阅读：

　　Lightning is a brilliant flash of light produced by an electrical discharge from a storm cloud. The electrical discharge takes place when the attractive tension between a region of negatively charged particles and a region of positively charged particles becomes so great that the charged particles suddenly rush together. The coming together of the oppositely charged particles neutralizes the electrical tension and releases a tremendous amount of energy, which we see as lightning. The separation of positively and negatively charged particles takes place during the development of the storm cloud.

　　The separation of charged particles that forms in a storm cloud has a sandwich-like structure. Concentrations of positively charged particles develop at the top and bottom of the cloud, but the middle region becomes negatively charged. Recent measurements made in the field together with laboratory simulations offer a promising explanation of how this structure of charged particles forms. What happens is that small (millimeter-to centimeter-size) pellets of ice form in the cold upper regions of the cloud. When these ice pellets fall, some of them strike much smaller ice crystals in the center of the cloud. The temperature at the center of the cloud is about -15℃ or lower. At such temperatures, the collision between the ice pellets and the ice crystals causes electrical charges to shift so that the ice pellets acquire a negative charge and the ice crystals become positively charged. Then updraft wind currents carry the light, positively charged ice crystals up to the top of the cloud. The heavier negatively charged ice pellets are left to concentrate in the center. This process explains why the top of the cloud becomes positively charged, while the center becomes negatively charged. The negatively charged region is large: several hundred meters thick and several kilometers in diameter. Below this large, cold, negatively charged region, the cloud is warmer than -15℃, and at these temperatures, collisions between ice crystals and falling ice pellets produce positively charged ice pellets that then populate a small region at the base of the cloud.

　　Most lightning takes place within a cloud when the charge separation within the cloud collapses. However, as the storm cloud develops, the ground beneath the cloud becomes positively charged and lightning can take place in the form of an electrical discharge between the negative charge of the cloud and the positively charged ground. Lightning that strikes the ground is the most likely to be destructive, so even though it represents only 20 percent of all lightning, it has received a lot of scientific attention.

　　Using high-speed photography, scientists have determined that there are two steps to the occurrence of lightning from a cloud to the ground. First, a channel, or path, is formed that connects the cloud and the ground. Then a strong current of electrons follows that path from the cloud to the ground, and it is that current that illuminates the channel as the lightning we see.

　　The formation of the channel is initiated when electrons surge from the cloud base toward the ground. When a stream of these negatively charged electrons comes within 100 meters of the ground it is met by a stream of positively charged particles that comes up from the ground. When the negatively and positively charged streams meet, a complete channel connecting the cloud and the ground is formed. The channel is only a few centimeters in diameter, but that is wide enough for electrons to follow the channel to the ground in the visible form of a flash of lightning. The stream of positive particles that meets the surge of electrons from the cloud often arises from a tall pointed structure such as a metal flagpole or a tower. That is why the subsequent lightning that follows the completed channel often strikes a tall structure.

　　Once a channel has been formed, it is usually used by several lightning discharges, each of them consisting of a stream of electrons from the cloud meeting a stream of positive particles along the established path. Sometimes, however, a stream of electrons following an established channel is met by a positive stream making a new path up from the ground. The result is a forked lightning that strikes the ground in two places.

　　第二篇：

　　地球大气

　　太阳大气反映了太阳系形成初期的成份构成，但地球现在的大气与那时的显著不同。原始大气在太阳风和陨石的作用下被剥离地球，然后地球的火山排气作用(outgassing)形成了现在大气的各种主要成分，但不包括氧气。

　　相较现在20%的氧气，一开始地球只有很少量的氧，这些氧主要试大气中的水发生光解作用(photo dissociation)产生的而那些大量的氧是由植物光合作用(photosynthesis)发生的。氧气和海洋里的铁发生氧化，会沉淀到海底，氧气多的时期形成红色的氧化铁，少的时候会形成黑色的，于是海底有些时候就呈现带状(这里有年份具体忘了)

　　地球上的氧气还会变成臭氧，臭氧可以阻挡紫外线保护地球上的生命。臭氧在某个时间段内增加，在差不多结束的那段时间有个生命形式的爆发。

　　解析：本文同第一篇，属于地理地质类文章，讲述的是地球大气的构成。地理地质类文章是属于托福阅读中不太陌生的话题，但是因为讲解内容较为晦涩，也是学员理解不是太好的内容，在备考前需要额外关注，并且仔细分析，避免因为特殊背景或者学科词汇不熟悉导致阅读速度及理解程度受影响。

　　参考阅读：

　　Outgassing from volcanism, supplemented by gases produced during the late heavy bombardment of Earth by huge asteroids, produced the next atmosphere, consisting largely of nitrogen plus carbon dioxide and inert gases. A major part of carbon-dioxide emissions soon dissolved in water and built up carbonate sediments.

　　Researchers have found water-related sediments dating from as early as 3.8 billion years ago. About 3.4 billion years ago, nitrogen formed the major part of the then stable "second atmosphere". An influence of life has to be taken into account rather soon in the history of the atmosphere, because hints of early life-forms appear as early as 3.5 billion years ago. How Earth at that time maintained a climate warm enough for liquid water and life, if the early Sun put out 30% lower solar radiance than today, is a puzzle known as the "faint young Sun paradox".

　　The geological record however shows a continually relatively warm surface during the complete early temperature record of Earth - with the exception of one cold glacial phase about 2.4 billion years ago. In the late Archean eon an oxygen-containing atmosphere began to develop, apparently produced by photosynthesizing cyanobacteria (see Great Oxygenation Event), which have been found as stromatolite fossils from 2.7 billion years ago. The early basic carbon isotopy (isotope ratio proportions) very much approximates current conditions, suggesting that the fundamental features of the carbon cycle became established as early as 4 billion years ago.

　　Ancient sediments in the Republic of Gabon dating from between about 2,150 and 2,080 million years ago provide a record of Earth's dynamic oxygenation evolution. These fluctuations in oxygenation were likely driven by the Lomagundi carbon isotope excursion.

　　The constant re-arrangement of continents by plate tectonics influences the long-term evolution of the atmosphere by transferring carbon dioxide to and from large continental carbonate stores. Free oxygen did not exist in the atmosphere until about 2.4 billion years ago during the Great Oxygenation Event and its appearance is indicated by the end of the banded iron formations. Before this time, any oxygen produced by photosynthesis was consumed by oxidation of reduced materials, notably iron. Molecules of free oxygen did not start to accumulate in the atmosphere until the rate of production of oxygen began to exceed the availability of reducing materials. This point signifies a shift from a reducing atmosphere to an oxidizing atmosphere. O2 showed major variations until reaching a steady state of more than 15% by the end of the Precambrian. The following time span from 541 million years ago to the present day is the Phanerozoic eon, during the earliest period of which, the Cambrian, oxygen-requiring metazoan life forms began to appear.

　　The amount of oxygen in the atmosphere has fluctuated over the last 600 million years, reaching a peak of about 30% around 280 million years ago, significantly higher than today's 21%. Two main processes govern changes in the atmosphere: Plants use carbon dioxide from the atmosphere, releasing oxygen. Breakdown of pyrite and volcanic eruptions release sulfur into the atmosphere, which oxidizes and hence reduces the amount of oxygen in the atmosphere. However, volcanic eruptions also release carbon dioxide, which plants can convert to oxygen. The exact cause of the variation of the amount of oxygen in the atmosphere is not known. Periods with much oxygen in the atmosphere are associated with rapid development of animals. Today's atmosphere contains 21% oxygen, which is high enough for this rapid development of animals.

　　第三篇：

　　石器时代

　　介绍了一个人类学家的看法。他认为石器时代的人比现代人更富裕，好像是因为石器时代的人欲望更少更易满足。他们限制人口，除了吃的比大的部落饱还可以避免一些疾病。因为石器时代的人到处探索，除了个别太恶劣的环境，吃的都是富足的。(这篇各个点比较散乱还集中在一个段落，记不住)

　　石器时代的人工作比较少只有4、5小时，他们有很多休闲时间。然后比较各个时期的工作时间，逐渐增加，现代之前一个最高9.5h，现代稍微回落一点。因为社会发展需要一个比较安定的生活，因此家庭花在家政(household)方面的时间增多，但男人花在家政烦吗比例有所下降。

　　最后说了一下学界的评论。忘了是作者的观点还是学界的观点，说他是overstatement。但文章最后一句话我理解是：认为石器时代的人富足不是因为他们生产比现代更多的东西，而且因为他们的欲望更少。

　　解析：本文属于人文类文章，关注的是石器时代人的生活。人文类文章属于托福阅读的常考内容，从机经来看本文讲解的具体内容，涉及到的专业词汇量比较小，在理解时词汇方面的难度应该不大，但是一般人文类文章因为句子偏长而给学生带来理解障碍，在备考时需要做专门的长难句准备。

　　参考阅读：

　　The Stone Age was a time thousands of years ago, when humans lived in caves and jungles. Life was simple, and there were only two main things to do – to protect themselves from the wild animals and to gather food. It started almost with the evolution of mankind.

　　For both purposes, people made tools from stone. The oldest stone tool that we have as an evidence is almost 3.4 million years old . It was found in Lower Awash Valley in Ethiopia. Stones were also used to make fire. Since in those times, humans used stone for almost everything they did, hence the name Stone Age. To look at a timeline of this period click here.

　　The Stone Age went on for a long time. Obviously in the beginning of the Stone Age rock shelters were the way to go. Any signs of trouble and the humans would go hiding in the caves. In India, the Bhimbetka rock shelters show the earliest signs of human life in the region. They are almost 30,000 years old. Some researchers have discovered Stone Age art in European caves. The inside walls of the cave are adorned with paintings of animals like horses, deer, and mammoths.

　　One of the most important advancements in human history was the development and use of tools. Tools allowed hominids to become the masters of their environments, to hunt, to build, and to perform important tasks that made life easier for them. The first tools were made out of stone. Thus, historians refer to the period of time before written history as the Stone Age.

　　Hunter gatherers often painted and engraved the inside of caves and huts.

　　The Palaeolithic people decorated their cave walls either by carving or engraving them with sharp stones, or by painting them with pigment made from minerals. Red (rust) colours were obtained from the stones and black came from charcoal. The colours were mixed with water to make paint. They were applied to the walls with either their fingers, fur or brushes made from twigs. The paintings often represented daily life such as men hunting, animals, women gathering crops.