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How the Grand Canyon was created is one of the geology’s greatest mysteries(奥秘). Some geologists believed that it came into its shape slowly in the long history: the Colorado River went through the canyon and slowly cut deeper and deeper into the ground over millions of years. But some rocks of the volcanoes, which have been discovered in the canyon recently, show that the canyon was down-cut instead.

Down-cutting is when a flood of water rushes(冲) over a place with enough force to cut deeply into the ground and leaves behind a canyon. Such a flood usually comes when a natural or man-made dam(坝) breaks.

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 (火山岩浆) from the nearby volcanoes flowed into the river. The lava hardened into hard rocks and blocked(阻挡) the river, causing it to back up and form a lake. Each time the lake grew so huge that it broke the rock dam, then the flood of water caused the down-cutting and the canyon became deeper. Down-cutting does not only happen on the earth. Satellite photos sent back from Mars show that down-cutting has happened there, too, say many other scientists. The photos, taken by the Mars Global Surveyor, indicate(表明) that a huge 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 huge canyon. The finding of down-cutting on Mars is just one more piece of evidence(证据) that the formation of the Grand Canyon was not a gradual course.

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