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第一段
1 .Listen to part of a lecture in a geology class.
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第二段
1 .Alaska is fascinating to geologists because of its incredible landscapes.
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2 .Permafrost has a lot to do with this.
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3 .That is the areas where the ground,the soil,is always frozen,except for the very top layer,
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4 .what we call the active layer of permafrost,which melts in the summer and refreezes again in the winter.
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第三段
1 .The northern part of Alaska is covered in lakes,thousands of them.
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2 .And most of these are what we call thaw lakes,T-H-A-W,thaw lakes.
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3 .I'm going to show you a few sketches of them in a minute,
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4 .so you'll have a good idea of what I'm talking about.
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第四段
1 .So how these thaw lakes are formed has to do with...
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第五段
1 .OK.It start with ice wedges.
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2 .The top part of the ice wedge melts.
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3 .Should I back up?
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第六段
1 .Ice wedges form when water runs into cracks in the ground,the permafrost,then freezes.
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2 .You ever see mud after it dries?
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3 .Dried mud has cracks because when it dries it contracts,it shrinks.
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第七段
1 .Well,in winter permafrost behaves similarly.
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2 .It shrinks in winter because it freezes even more thoroughly then.
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3 .And as it shrinks,it froms deep,deep cracks.
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4 .Then in the summer,when the active layer,the top layer of the permafrost,melts,
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5 .the melt water runs into those cracks in the permafrost,
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6 .then freezes again,because that ground,the ground beneath the active layer is still below freezing.
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7 .So you have wedges of ice in the permafrost.
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第八段
1 .Now,the ice wedges widen the original cracks in the permafrost because water expands when it freezes.
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2 .All right,OK.
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3 .Then in the autumn,the active layer on top freezes again.
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4 .Then in winter the permafrost starts contracting again and the cracks open up even wider.
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5 .So the next summer when the active layer melts again and flows into the widened cracks and freezes,
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6 .it makes the cracks even wider.
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7 .So it's sort of a cycle through which the cracks and the wedges grow wider and wider.
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第九段
1 .So when the ice wedge reaches a certain size,
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2 .its top part in the active layer turns into a little pond when it melts in the summer.
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3 .And that's the beginning of your thaw lake.
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4 .There are thousands of them in Northern Alaska.
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5 .One of the most fascinating things about these lakes and this is important,
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6 .is that they mostly have the same shape like an elongated oval or egg shape,
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7 .and what's more,
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8 .all the ovals are oriented in the same way.
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9 .Here is an idea of what they look like,
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10 .what the landscape looks like from an aerial view with the lakes side by side.
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第十段
1 .There's been considerable research done to try to figure out what causes them to be shaped and oriented this way.
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2 .We know that the shape and orientation are caused by the way the lakes grow once they are formed.
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3 .But the question is what makes them grow this way?
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第十一段
1 .One theory sees winds as the cause.
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2 .This region of Alaska has strong winds that blow perpendicular to the lakes.
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3 .What happens is wind blows straight into the longer side of the lakes.
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第十二段
1 .Now,wouldn't that erode the lake bank in that direction?
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2 .Same direction as the wind?
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第十三段
1 .Well,no.
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2 .Actually,what happens is that the waves caused by the winds build a sort of protective layer of sediment.
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3 .It's called a protective shelf along the bank of the lake directly in front of them.
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4 .So that bank is shielded from erosion and the waves are diverted to the sides,to the left and to the right.
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5 .And that's why the left and the right banks start eroding.
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第十四段
1 .Get it?The bank straight ahead is protected.
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2 .But the lake currents,the waves erode the banks to the sides.
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3 .That's the current model,the wind erosion model,which is generally accepted.
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4 .But there is a new theory that says that thaw slumping,not wind,is what shapes the thaw lakes.
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第十五段
1 .Thaw slumping...
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2 .Ok.Sometimes in the summer the temperature rises pretty quickly.
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3 .So the active layer of permafrost thaws faster than the water can drain from the soil.
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4 .So the sides of the thaw lakes get like mushy and slump or slide into the lake.
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5 .Then the lake water spreads out more and the lake gets bigger.OK?
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第十六段
1 .Also in that part of Alaska the terrain is gently sloped,so the lakes are all on an incline.
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2 .Here,now,this is an exaggeration of the angle.The hill is not this steep.
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3 .But see how with the lake's banks,the side that is farther downhill,it's smaller,lower.
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4 .This short bank thaws faster than the tall one does,so it falls into the lake,it slumps much more and much faster than the other bank.
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5 .When short banks of many lakes slump,
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6 .they move farther downhill and the lakes grow all in the same downhill direction.
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