首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
"Four Stages of Planetary Development" Planetary Development → In our study of the planet Earth, we will find a four-sta
"Four Stages of Planetary Development" Planetary Development → In our study of the planet Earth, we will find a four-sta
admin
2012-01-14
25
问题
"Four Stages of Planetary Development"
Planetary Development
→ In our study of the planet Earth, we will find a four-stage history of planetary development. The moon and all the terrestrial planets have passed through these stages, although differences in the way the planets were altered by these stages have produced dramatically different worlds. The moon, for example, is much like Earth, but its evolution has been dramatically altered by its smaller size. As we explore the solar system, we will discover not entirely new processes but rather familiar effects working in slightly different ways.
The Four Stages
The first stage of planetary evolution is differentiation, the separation of material according to density. Earth now has a dense core and a lower-density crust, and that structure must have originated very early.
Differentiation would have occurred easily if Earth were molten when it was young. Two sources of heat could have heated Earth. First, heat of formation would be created by in-falling material. A
meteorite
hitting Earth at high velocity converts most of its energy of motion into heat, and the in-falling of a large number of meteorites could release tremendous heat.
If Earth formed rapidly, this heat would have accumulated much more rapidly than it could leak away, and Earth may have been molten when it formed.
A second source of heat requires more time to develop. The decay of radioactive elements trapped in the Earth releases heat gradually; but, as soon as Earth formed, that heat would have begun to accumulate and could have helped melt Earth to facilitate differentiation. Most of Earth’s radioactive elements are now concentrated in the crust, where they continue to warm and soften the rock layers.
Earth formed by material falling together, but meteorites could have left no trace until a crust solidified. Once Earth had a hard surface, the meteorites could form craters. This second stage in planetary evolution, cratering, was violent. The heavy bombardment was intense because the solar nebula was filled with rocky and icy debris, and the young Earth was battered by meteorites that pulverized the newly forming crust. The largest meteorites blasted out crater basins hundreds of kilometers in diameter. As the solar nebula cleared, the amount of debris decreased, and the level of cratering fell to its present low level. Although meteorites still occasionally strike Earth and dig craters, cratering is no longer the
dominant
influence on Earth’s geology. As we compare other worlds with Earth, we will discover traces of this intense period of cratering, the heavy bombardment, on every old surface in the solar system.
→ The third stage, flooding, no doubt began while cratering was still intense. The fracturing of the crust and the heating caused by radioactive decay allowed molten rock just below the crust to well up through fissures and flood the deeper basins. We will discuss such flooded basins on other worlds, such as the moon, but all traces of this early lava flooding have been destroyed by later geological activity in Earth’s crust. On Earth, flooding continued as the atmosphere cooled and water fell as rain, filling the deepest basins to produce the first oceans. A Notice that on Earth flooding involves both lava and water, a circumstance that we will not find on most worlds.B
The fourth stage, slow surface evolution, has continued for the last 3.5 billion years or more. C Earth’s surface is constantly changing as sections of crust slide over each other, push up mountains, and shift continents. D Almost all traces of the first billion years of Earth’s geology have been destroyed by the activecrust and erosion.
Earth as a Planet
All terrestrial planets pass through these four stages, so in that respect, Earth is a good basic reference planet for comparative planetology. Some planets have emphasized one stage over another, and some planets have failed to progress fully through the four stages. Nevertheless, Earth is a good standard of comparison. Every major process on any rocky world in our solar system is represented in some form on Earth.
On the other hand, Earth is
peculiar
in two ways. First, it has large amounts of liquid water on its surface. Fully 75 percent of its surface is covered by this liquid and no other planet in our solar system is known to have such extensive liquid water on its surface. Furthermore, some of the matter on the surface of this world is alive, and a small part of that living matter is aware. We do not know how the presence of living matter has affected the evolution of Earth, but this process seems to be totally missing from other worlds in our solar system.
Glossary
meteorite: a mass that falls to the surface of a planet from space
planetology: the study of planets
The word dominant in the passage is closest in meaning to
选项
A、most limited
B、most likely
C、most rapid
D、most important
答案
D
解析
In this passage, the phrase most important describes the word "dominant." Context comes from the contrast with the phrase "still occasionally" earlier in the sentence.
转载请注明原文地址:https://kaotiyun.com/show/bNyO777K
0
托福(TOEFL)
相关试题推荐
Lookatthefollowingstatements(Questions18-23)andthelistofpeoplebelow.Matcheachstatementwiththecorrectperson,A,
ClassifythefollowinginformationasreferringtoAadoptedthewritingsystemfromanothercountryBusedorganicmaterialst
DothefollowingstatementsagreewiththeviewsofthewriterinReadingPassage3?Inboxes33-37onyouranswersheet,write
DothefollowingstatementsagreewiththeviewsofthewriterinReadingPassage3?Inboxes33-37onyouranswersheet,write
Moreonlineusersofsocialnetworkingsitesovertheageof45allowstrangerstoseetheirphotosthanothergroups.
Completethenotesbelow.WriteONEWORDONLYforeachanswer.EngineeringforsustainabledevelopmentTheGreenhou
Completethenotesbelow.WriteONEWORDONLYforeachanswer.TheTawnyOwlMost【L31】______owlspeciesinUKStronglynocturna
Completethenotesbelow.WriteONEWORDONLYforeachanswer.TheTawnyOwlMost【L31】______owlspeciesinUKStronglynocturna
Choosethecorrectletter,A,BorC.MayorJohnC.WileydecidedtheClocktowerwouldbeusedasa______intheearlystageso
ChooseTWOletters,A-E.WhichTWOrecentdevelopmentsinroof-gardenbuildingarementioned?AwaterproofbarriermaterialsBd
随机试题
患者,男性,79岁,患外套细胞淋巴瘤第Ⅳ期,无输血史,因继发性贫血而就诊。血液常规检查:Hb70g/L,HCT0.23。医生申请输注RBC,血库对患者标本进行常规血型鉴定(试管法)和抗体筛选试验(抗人球蛋白法),结果见下表:根据表中各项试验反应格局
下列叙述中,不属于寸口诊法原理的是
患者,女性,25岁。经过装修工地后出现干咳、气急、胸闷、呼吸困难。经治疗后好转。对该患者的健康教育内容,不正确的是
施工用电分配箱与开关箱、开关箱与用电设备间的距离最大限值是()m。
若某投资者在2017年10月31日,买人1股A公司股票,价格为100元,2018年10月31日,A公司发放3己分红,同时其股价为105元。那么该区间内总持有区间的收益率为()。
由受托方代收代缴增值税、消费税的单位和个人,其代收代缴城建税适用的税率是()。
某医院近5年新录用员工信息如下表所示,其中学历要求必须是硕士研究生及以上,在关系数据库中,对学历的这一要求属于数据完整性约束的()约束。
通过公有派生类的对象,只能访问基类成员中的()。
Reading________themindonlywithmaterialsofknowledge;itisthinkingthatmakeswhatwereadours.
Students’pressuresometimescomesfromtheirparents.Mostparentsare【B1】______,butsomeofthemaren’tveryhelpfulwiththe
最新回复
(
0
)