Narrator: Listen to part of a lecture in a geology class.

旁白：听一段地质学的讲座。

Professor: Alaska is fascinating to geologists because of its incredible landscapes.

教授：阿拉斯加对于地质学家很有吸引力，因为它的地形令人难以置信。

Permafrost has a lot to do with this.

永久冻土与这个有很大关系。

That is the areas where the ground, the soil, is always frozen, except for the very top layer, what we call the active layer of permafrost, which melts in the summer and refreezes again in the winter.

这些地方的地面、土壤始终是冻结的，除了最顶层，我们称之为活动层，它在夏天融化，在冬季再次冻结。

The northern part of Alaska is covered in lakes, thousands of them.

阿拉斯加北部被湖泊覆盖，数以千计的湖泊。

And most of these are what we call thaw lakes, T-H-A-W, thaw lakes.

其中大多数的湖泊是融湖，T-H-A-W，融湖。

I'm going to show you a few sketches of them in a minute, so you'll have a good idea of what I'm talking about.

我马上给你看几张草图，这样你就可以很清楚地知道我在说什么了。

So how these thaw lakes are formed has to do with... OK, it starts with ice wedges.

那么这些融湖是如何形成的呢……好的，它是由冰楔发展而来的。

The top part of the ice wedge melts.

冰楔顶部融化。

Should I back up?

需要我补充一下么？

Ice wedges form when water runs into cracks in the ground, the permafrost, then freezes.

当水流入地面的裂缝时，冰楔开始形成，然后永久冻土冻结。

You ever see mud after it dries?

你们看过干了的泥浆吧？

Dried mud has cracks because when it dries it contracts, it shrinks.

干泥浆会有裂缝，因为当它变干的时候，它会收缩。

Well, in winter permafrost behaves similarly.

在冬季，冻土有着和泥浆类似的情况。

It shrinks in winter because it freezes even more thoroughly then.

它在冬天收缩，因为它冻结得更彻底。

And as it shrinks, it forms deep, deep cracks.

当它收缩时，形成很深的裂缝。

Then in the summer, when the active layer, the top layer of the permafrost, melts, the melt water runs into those cracks in the permafrost, then freezes again, because that ground, the ground beneath the active layer is still below freezing.

然后在夏天，当活土层（顶层的永久冻土层）融化，融化的水流到这些永久冻土的裂缝中，然后再次冻结，因为在活土层以下的土地仍然低于冰点。

So you have wedges of ice in the permafrost.

所以冻土带中有冰楔。

Now, the ice wedges widen the original cracks in the permafrost because water expands when it freezes.

冰楔将永久冻土最初的裂缝扩大，因为当水结冰时会膨胀。

All right, OK.

好的，好的。

Then in autumn, the active layer on top freezes again.

然后在秋天，顶层的活土层再次冻结。

Then in winter the permafrost starts contracting again and the cracks open up even wider.

然后在冬季，永久冻土开始收缩，裂缝变得更大了。

So the next summer when the active layer melts again and flows into the widened cracks and freezes, it makes the cracks even wider.

因此，到下一个夏天，当活土层再次融化，流入扩大的裂缝并且冻结起来，它使裂缝变得宽。

So it's sort of a cycle through which the cracks and the wedges grow wider and wider.

所以它是一种循环，通过这种循环，裂缝和楔变得越来越宽。

So when the ice wedge reaches a certain size, its top part in the active layer turns into a little pond when it melts in the summer.

所以当冰楔达到一定的大小时，当它在夏天融化时，活土层的顶部变成一个小池塘。

And that's the beginning of your thaw lake.

那就是融湖的开端。

There are thousands of them in Northern Alaska.

阿拉斯加北部有成千上万这样的湖泊。

One of the most fascinating things about these lakes and this is important, is that they mostly have the same shape like an elongated oval or egg shape, and what's more, all the ovals are oriented in the same way.

这些湖泊最迷人最重要的特点就是，它们大多具有相同的形状，像一个拉长的椭圆形或卵状，而且，所有的椭圆形都朝着相同的方向。

Here is an idea of what they look like, what the landscape looks like from an aerial view with the lakes side by side.

这就是它们看起来的样子，也就是鸟瞰这些并排的湖泊时所看到的样子。

There's been considerable research done to try to figure out what causes them to be shaped and oriented this way.

已经有相当多的研究试图找出是什么原因使他们形成如此的朝向。

We know that the shape and orientation are caused by the way the lakes grow once they are formed.

我们知道，形状和方向取决于湖泊形成后的增长方式。

But the question is what makes them grow this way?

但问题是，是什么让它们这样成长？

One theory sees winds as the cause.

一种理论认为风是原因。

This region of Alaska has strong winds that blow perpendicular to the lakes.

阿拉斯加的这一地区有着垂直于湖泊的强风。

What happens is wind blows straight into the longer side of the lakes.

发生的情况是，风直接吹进湖泊较长的那边。

Now, wouldn't that erode the lake bank in that direction?

那会不会侵蚀那个方向的湖岸呢？

Same direction as the wind?

同风向一致（的那边）？

Well, no.

并不会。

Actually, what happens is that the waves caused by the winds build a sort of protective layer of sediment.

事实上，发生的是风引起的波浪形成了一种沉积物保护层。

It's called a protective shelf, along the bank of the lake directly in front of them.

这是所谓的保护架，沿着湖岸，直接在它们面前。

So that bank is shielded from erosion and the waves are diverted to the sides, to the left and to the right.

因此，湖岸得以免受侵蚀，波浪被转移到其它方向，到左边和右边。

And that's why the left and the right banks start eroding.

这就是为什么左右湖岸开始被侵蚀。

Get it? The bank straight ahead is protected.

明白了吗？正前方的湖岸受到保护。

But the lake currents, the waves erode the banks to the sides.

但湖的水流，浪花侵蚀两边的湖岸。

That's the current model, the wind erosion model, which is generally accepted.

这是目前的模型，风蚀模型，这是被普遍接受的。

But there is a new theory that says that thaw slumping, not wind, is what shapes the thaw lakes.

但有一个新的理论，这个理论认为热融滑塌是铸就融湖形态的原因，而不是风。

Thaw slumping... Ok.

热融滑塌……好的。

Sometimes in the summer the temperature rises pretty quickly.

有时在夏天，气温上升很快。

So the active layer of permafrost thaws faster than the water can drain from the soil.

所以，永久冻土的活土层融化的速度比土壤中流失的更快。

So the sides of the thaw lakes get like mushy and slump or slide into the lake.

所以融湖的边缘变成糊状，并且坍落或滑入湖中。

Then the lake water spreads out more and the lake gets bigger. OK?

随后，湖水继续扩张，湖泊越来越大。明白了吗？

Also in that part of Alaska, the terrain is gently sloped, so the lakes are all on an incline.

并且，在阿拉斯加的那片地区，地形稍有倾斜，所以湖泊都在斜坡上。

Here, now, this is an exaggeration of the angle.

来看这里，这个角度有些夸张。

The hill is not this steep.

山并不陡。

But see how with the lake's banks, the side that is farther downhill, it's smaller, lower.

但是，看看湖岸，往下坡的那一边更小、更低。

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.

更短的湖岸比高的那边解冻的快，所以它掉进湖中，这一边比其它湖岸掉落地更多更快。

When the short banks of many lakes slump, they move farther downhill and the lakes grow all in the same downhill direction.

当许多湖泊的短河岸塌下来时，它们向下坡进一步移动，湖泊就沿着同一个下坡方向生长。

This is a new theory, so it hasn't been tested much yet.

这是一个新的理论，因此还没有被很好地验证过。

In field studies, when we've looked at the banks of these thaw lakes, there is not much evidence of slumping.

在实地研究中，当我们看着这些融湖的岸边时，没有很多和滑落相关的证据。

We'd expect to see cliff-like formation there from the slumping. But we haven't really found any of those.

我们认为从那里可以看到由于坍塌而形成的悬崖。但我们没有类似的发现。