北美托福考试阅读回忆&名师解析

　　毕达留学指南小编为大家搜集整理了北美托福考试阅读回忆&名师解析。

　　第一篇

　　topic 地球形成

　　有一篇是讲地球的形成的,先讲了星球大概形成的一个过程。然后具体说了地球内部的硅浮上来,重物质下去。还讲了地幔和地壳的形成。然后还有什么一开始是一些神马物质,后来这些物质就走掉了,还有水汽,还有其他氢啊神马的进来,然后形成最终状态。

　　解析：以地球为代表的行星等的形成一直是托福阅读天文学话题部分的考察重点。与之对应的tpo文章为planets in our solar system。

　　the history of earth crust

　　the earths crust, its outermost layer, formed about 4.44 billion years ago, roughly 100 million years after the formation of the earth itself. prior to 4.44 billion years ago, the earths crust was entirely molten, due to residual heat from the planets initial collapse. evidence that the earths crust cooled within 100 million years comes from measurements of hafnium levels in the jack hills in western australia, one of the oldest areas of exposed crust today.

　　during the initial formation of the earths crust, an event known as the iron catastrophe occurred, where the denser elements of the earths composition, such as iron and nickel, sank to its core, while the lighter elements, like silicon, formed a crust at the top.

　　the crust began to cool when the earth was at least 40% of its current size, possessing enough gravity to hold down an atmosphere containing water vapor. much of this early water vapor would have come from comets. this era in the earths history, extending from the earths birth to about 3.8 billion years ago, is known as the hadean era, after the greek hell, hades, for the difficult conditions on the planet at the time. scientists believe the hadean era was lifeless.

　　around 4.0 to 3.8 billion years ago, towards the end of the hadean era, the planet underwent the late heavy bombardment, a period of time with many large asteroid impacts. these impacts may have literally shattered the virgin earths crust, preventing the creation of any long-lasting continents. in practice, looking back beyond 3.8 billion years ago is difficult, as the oldest rock formations are about this age. the only earthly minerals older than this are individual rocks and crystals, which give less information about the overall global state of the earths crust at the time.

　　about 2.7 billion years ago, photosynthetic life evolved. it released huge amounts of oxygen, isolating it from water in the process of photosynthesis. the earth built up an oxygen atmosphere, and the oxygen began to bond with most of the elements on the surface, creating huge amounts of oxides. today, most of the earths crust is composed of silicon oxide.