How the Grand Canyon (大峡谷) was created remains one of the geology’s greatest mysteries. Some evidence suggests that the process was a gradual one in which the Colorado River (which runs through the canyon) slowly cut deeper and deeper into the ground over millions of years. But volcanic rock samples taken from the canyon now suggest that the canyon was down-cut instead.
Down-cutting is when a flood of water rushes over a landscape (地形) with enough force to cut deeply into the ground and leaves behind a canyon. Such a flood is usually released when a natural or man-made dam (堤坝) bursts.
Robert Webb, a research geologist, says natural dams seem to have formed and broken across the Colorado River several times during the last million years. The dams were built when lava(熔岩) from the eruptions of nearby volcanoes flowed into the river. The lava hardened into hard rocks and blocked the river, causing it to back up and form a lake. Each time the lake grew so huge that it broke the rock dam, releasing a flash flood that furthered the down-cutting process and deepened the canyon.
Down-cutting is not just an earthly event. Satellite photos sent back from Mars suggest that the process has happened there, too, say many other researchers.
The photos, taken by the Mars Global Surveyor, indicate that an enormous lake existed on Mars 3.5 billion years ago. The lake spilled into a large nearby hole. One edge of the hole broke, releasing a flash flood that quickly carved out a grand canyon.
The existence of down-cutting on Mars is just one more piece of evidence that the cold, dry planet was once warm and wet.
【小题1】In the past, deep canyons were believed to have formed .A．as a result of a sudden break of volcanoesB．due to river flows over millions of yearsC．owing to the burst of artificial damsD．thanks to our ancestors’ creative work【小题2】According to the passage, which of the following statements is TURE?A．Several volcanoes broke out on Mars directly creating grand canyons.B．Several great lakes existed on the upper parts of the Colorado River.C．People built high dams on the upper parts of the Colorado River.D．The Colorado River crossed the Grand Canyon to form down-cutting.【小题3】From the last three paragraphs we learn that .A．there are great lakes on MarsB．there are active volcanoes on MarsC．there might be cities on MarsD．there might be life on Mars【小题4】The passage can be titled as .A．Lakes on Earth and MarsB．The Cause of Lake FormationC．The Force of Dam BreakingD．Canyons—Results of Flood Cutting B
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(答案→)B
解析：和以往人们的想法不同的是，这篇文章的作者认为大峡谷的形成是洪水下切侵蚀造成的，主要是上游的湖泊泛滥强大的力量造成了峡谷，文章还介绍在火星也有这种现象。
【小题1】细节题：从文章第一段的句子：Some evidence suggests that the process was a gradual one in which the Colorado River (which runs through the canyon) slowly cut deeper and deeper into the ground over millions of years.可知过去深深的峡谷据相信是由于多年的河流造成的，选B
【小题2】细节题：从文章第三段的句子：The lava hardened into hard rocks and blocked the river, causing it to back up and form a lake.可知在Colorado 河的上游有几个大湖，选B
【小题3】细节题：从文章最后一段的句子：The existence of down-cutting on Mars is just one more piece of evidence that the cold, dry planet was once warm and wet.可知在火星上可能有生命，选D
【小题4】标题确定题：从文章的内容和第二段的句子：Down-cutting is when a flood of water rushes over a landscape (地形) with enough force to cut deeply into the ground and leaves behind a canyon.可知这篇文章讲的是峡谷是由洪水下切侵蚀造成的，选D