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
Why does the professor mention a meteor that struck Earth during the time of the dinosaurs?
Why does the professor mention a meteor that struck Earth during the time of the dinosaurs?
admin
2018-08-16
46
问题
Why does the professor mention a meteor that struck Earth during the time of the dinosaurs?
Listen to part of a lecture in an Earth Science class.
Professor: Now, one of the things I like to do from time to time in this class is look at how knowledge we’ve gained from studying earth’s geology has been applied to questions outside our field. Take the mass extinction that occurred around 13,000 years ago, when most of the giant mammal species of North America vanished from the geological record, creatures like wholly mammoths, saber-toothed cats, giant sloths, beavers, and camels. So what caused these animals to suddenly disappear? One possible answer lies at the start of a period of sudden climate change called "the Younger Dryas."
Let me rewind for a moment. Just before this extinction, Earth was coming out of a long ice age. Glaciers were beginning to recede, but then temperatures in North America suddenly plummeted again, setting off this frigid thousand-year period known as "the Younger Dryas." North America became so cold, that glaciers started expanding again. And one theory is that this sudden change in climate would have made it difficult for these large beasts to survive, Of course there’s been other climate changes, some even more extreme and longer lasting, yet with no evidence that they triggered any extinction events, so maybe not the strongest theory, which means we need to look elsewhere, perhaps space.
I mean that we’re all familiar with how impact events can affect life on Earth, like it’s now generally agreed that a meteor triggered the extinction of the dinosaurs. Of course, since that theory’s been widely accepted, it’s tempting to look to space to explain all extinction events. A large meteor crashing into Earth would scatter cosmic debris and cause massive firestorms, and that’s the theory proposed recently as an explanation for the large mammal extinction.
Researchers who support this theory claim to have found evidence of a meteor impact at a site in the state of Arizona in a layer of sediment called "the Younger Dryas Boundary." The Younger Dryas Boundary, or YOB, is a very thin layer of sediment that was laid down across North America at the beginning of the Younger Dryas period, and what’s especially significant about this 13,000-year-old layer is that under it, we find lots of fossils of these large mammals, but above it, that is after the YOB was laid down, well, not a single one. You may be wondering, what happened here?
Although the researchers suspect a meteor event, they didn’t find any evidence of an impact crater, but they say they did find evidence of several types of particles commonly associated with meteor impacts, including nanodiamonds and high concentrations of certain magnetic particles. Nanodiamonds are small particles that can either originate in space or be formed in the extreme pressure of an intense explosion, like the impact of a meteor, for example. The magnetic particles can also come from space, and the ones found in the YOB are very similar to particles associated with other meteor impacts. Recently, however, a new group of researchers tried to replicate these findings, and while they also found nanodiamonds and high concentrations of magnetic particles at the Arizona site, these researchers wondered whether the presence of such particles might be accounted for in other ways. In order to test whether the magnetic particles were unique to the YDB, they analyzed dirt from the rooftop of a researcher’s house and in fact, they found magnetic particles in the rooftop sample, too. In actual fact, these particles are not just the result of impact events. They can have many origins, including the ash from nearby coal burning, electrical power plants, or even cosmic dust that originates in space and then falls to earth.
Ok, but then how to explain the elevated concentrations of the magnetic particles in the sample from the YOB? Well, these samples had come from a riverbed where rainwater would have carried and deposited the particles, and in fact, additional samples taken from outside the riverbed contained only normal concentrations of the magnetic particles in the YOB. As for the nanodiamonds, while a group of meteorite or comet fragments colliding with Earth would certainly provide the high temperatures and pressures needed to create such nanodiamonds, there might be other ways to explain their existence. It turns out, they’re also commonly found in cosmic dust, which could explain their presence not only in the YDB, but also in the rooftop sample. This means that if there’s no strong evidence for a meteor impact or for a sudden climate change, then we need some other explanation for the disappearance of so many large North American mammal species during the Younger Dryas.
选项
A、To explain why it is to hypothesize that a similar impact could have caused a later extinction event
B、To challenge a commonly accepted claim about dinosaur extinction
C、To imply that that meteor impact also affects other large animals in North America
D、To point out that Earth was often struck by meteors in the past
答案
A
解析
组织结构题。线索词为教授所说:...since that theory’s been widely accepted,it’s tempting to look to space to explain all extinction events.教授举流星撞击地球导致恐龙灭绝的例子是为了解释:一次类似的撞击事件也有可能导致北美大型哺乳动物灭绝。B选项与原文表述相悖,题干中的例子不是为了挑战一个关于恐龙灭绝的公认说法,而是为了利用这个说法去支持另一个类似的灭绝理论。C选项错误,因为举例的作用不是为了探讨陨石撞击所影响的动物种类。D选项错误,因为原文未提及流星撞击地球的频率。
转载请注明原文地址:https://kaotiyun.com/show/cjfO777K
0
托福(TOEFL)
相关试题推荐
Choosethecorrectletter,A,BorC.The"ReturningStudentsAdvisor"ConsultationWhatdoesthemanthinkofchildrennowaday
Completethetablebelow.WriteNOMORETHANTWOWORDSAND/ORANUMBERforeachanswer.
Completetheformbelow.WriteNOMORETHANTWOWORDSAND/ORANUMBERforeachanswer.MovingCompanyServiceReportExampleAns
Completethesentencesbelow.WriteNOMORETHANTWOWORDSAND/ORANUMBERforeachanswer.Localpeopledonot______crocodiles
Completethenotesbelow.WriteNOMORETHANTHREEWORDSforeachanswer.GIVINGASPEECHReasonsfornervousnessLecturers
Completethenotesbelow.WriteNOMORETHANTWOWORDSforeachanswer.AdvertisingEffectTheimportantfactortoconsider
Whatfeaturedothespeakersidentifyforeachofthefollowingcourses?ChooseFIVEanswersfromtheboxandwritethecorrect
Completethenotesbelow.WriteNOMORETHANTWOWORDSforeachanswer.ROBERTSCOMPANYFoundingThemostimportantprincipa
Whichadvantageismentionedforeachofthefollowingrestaurants?ChooseSIXanswersfromtheboxandwritethecorrectletter
Whatproblemsdothespeakersidentifyforthisproject?ChooseSEVENanswersfromtheboxandwritetheletters,A-H,nexttoq
随机试题
最容易导致股骨头无菌性坏死的药物是
根据病变发生的快慢,中毒分急性中毒、慢性中毒、亚急性中毒。()
可转让大额定期存单与定期存款的区别体现在()。
C语言规定,简单变量做实参时,它和对应形参之间的数据传递方式为()。
苏俄实施新经济政策的根本目的是()。
下列程序中的函数stropy2()实现字符串两次复制,即将t所指字符串复制两次到s所指内存空间中,合并形成一个新字符串。例如,若t所指字符串为:efgh,调用strcpy2后,s所指字符串为:efghefgh。请填空。#include<stdi
单击“演示文稿”按钮,然后按照题目要求完成下面的操作。注意:以下的文件必须保存在考生文件夹下。“天河二号超级计算机”是我国独立自主研制的超级计算机系统,2014年6月再登“全球超算500强”榜首,为祖国再次争得荣誉。作为北京市第××中学
下列叙述中,错误的是()。
Mostpeoplehavehadadogorwantedoneastheircompanionatsometimeintheirlives.Ifyouarethinkingofbuyingadog,ho
A、Mariebenefitedalotfromthepracticallessons.B、Mariewasabletoreadstorieswiththehelpofherson.C、Mariedecidedt
最新回复
(
0
)