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SSS 2011-08-24

时间:2011-09-05 06:21来源:互联网 提供网友:gmeng   字体: [ ]
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    (单词翻译:双击或拖选)

 This is Scientific American’s 60-Second Science. I’m Christopher Intagliata. Got a minute? 

 
Rivers today have high muddy banks, sandbars and bends. But they didn’t always look that way because it wasn’t until the evolution of tree-like plants, some 330 million years ago that rivers were corralled into their current form. Before that, ancient waters flowed wide and shallow over the land, with little to constrain1 them other than mountains. So says a study in the journal Nature Geoscience.
 
Researchers looked through over 400 studies of the Earth’s rock record, and visited nearly 70 field sites. And they found that channel formations in the rock—a signature of modern rivers—didn’t appear until the Carboniferous period, when tree-like plants evolved. That’s because larger plants needed deeper roots, which stabilized2 river banks and forced rivers into narrower paths. And deep roots helped form sticky clays, which are harder to erode3.
 
All this engineering was to the trees’ advantage, the researchers say, because river banks provide trees with easy access to water, without the constant risk of flooding. Pretty much what we humans want. Many of our greatest cities formed along river banks—for which we might have trees to thank.
 
Thanks for the minute. For Scientific American’s 60-Second Science, I’m Christopher Intagliata.

点击收听单词发音收听单词发音  

1 constrain xpCzL     
vt.限制,约束;克制,抑制
参考例句:
  • She tried to constrain herself from a cough in class.上课时她竭力忍住不咳嗽。
  • The study will examine the factors which constrain local economic growth.这项研究将考查抑制当地经济发展的因素。
2 stabilized 02f3efdac3635abcf70576f3b5d20e56     
v.(使)稳定, (使)稳固( stabilize的过去式和过去分词 )
参考例句:
  • The patient's condition stabilized. 患者的病情稳定下来。
  • His blood pressure has stabilized. 他的血压已经稳定下来了。 来自《现代英汉综合大词典》
3 erode NmUyX     
v.侵蚀,腐蚀,使...减少、减弱或消失
参考例句:
  • Once exposed,soil is quickly eroded by wind and rain.一旦暴露在外,土壤很快就会被风雨侵蚀。
  • Competition in the financial marketplace has eroded profits.金融市场的竞争降低了利润。
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