評論
打印
Listen to part of a lecture in a geology class.
請聽地質(zhì)學(xué)課上的講座的部分內(nèi)容。
Alaska is fascinating to geologists because of its incredible landscapes. Permafrost has a lot to do with this.
阿拉斯加因其令人驚嘆的地貌令地質(zhì)學(xué)家著迷。這與永久凍土密切相關(guān)。
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.
所謂永久凍土區(qū)域，是指地表以下的土壤常年凍結(jié)，只有最表層（我們稱之為活動層）會在夏季融化，冬季重新凍結(jié)。
The northern part of Alaska is covered in lakes, thousands of them. And most of these are what we call thaw lakes, T-H-A-W, thaw lakes.
阿拉斯加北部遍布湖泊，成千上萬的湖泊中絕大多數(shù)屬于“熱融湖”。
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.
稍后我將展示幾張示意圖，幫助大家更直觀地理解我的講解。
So how these thaw lakes are formed has to do with... OK, it starts with ice wedges. The top part of the ice wedge melts. Should I back up?
這些熱融湖的形成與什么有關(guān)呢，整個(gè)過程始于冰楔。冰楔頂部融化。需要我補(bǔ)充一下嗎？
Ice wedges form when water runs into cracks in the ground, the permafrost, then freezes.
當(dāng)水分滲入永久凍土的裂縫并凍結(jié)時(shí)，就會形成冰楔。
You ever see mud after it dries? Dried mud has cracks because when it dries it contracts, it shrinks.
你們見過干涸的泥地嗎？干涸的泥地會出現(xiàn)裂縫，因?yàn)楦稍飼r(shí)會發(fā)生收縮。
Well, in winter permafrost behaves similarly. It shrinks in winter because it freezes even more thoroughly then. And as it shrinks, it forms deep, deep cracks.
同理，永久凍土在冬季也會收縮。極寒使其凍結(jié)程度加深，導(dǎo)致形成深邃的裂縫。
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.
到了夏季，當(dāng)永久凍土頂部的活動層融化時(shí)，融水會注入這些裂縫，隨后再次凍結(jié)，因?yàn)榛顒訉酉路降挠谰脙鐾寥蕴幱诒c(diǎn)以下。
So you have wedges of ice in the permafrost. Now, the ice wedges widen the original cracks in the permafrost because water expands when it freezes.
如此便在凍土中形成了楔形冰體。由于水結(jié)冰時(shí)體積膨脹，這些冰楔會不斷撐大原有裂縫。
All right, OK. Then in autumn, the active layer on top freezes again.
秋季來臨時(shí)，頂部的活動層重新凍結(jié)。
Then in winter the permafrost starts contracting again and the cracks open up even wider.
然后到了冬季，永久凍土再次收縮，裂縫進(jìn)一步擴(kuò)大。
So the next summer when the active layer melts again and flows into the widened cracks and freezes, it makes the cracks even wider.
次年夏季，融水注入擴(kuò)大的裂縫并凍結(jié)，使裂縫愈加擴(kuò)張。
So it's sort of a cycle through which the cracks and the wedges grow wider and wider.
如此循環(huán)往復(fù)，裂縫與冰楔在年復(fù)一年的循環(huán)中持續(xù)擴(kuò)大。
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.
所以當(dāng)冰楔達(dá)到一定大小時(shí)，其位于活動層的頂部在夏季融化形成小水塘。
And that's the beginning of your thaw lake. There are thousands of them in Northern Alaska.
這便是熱融湖的雛形。阿拉斯加北部有成千上萬這樣的湖泊。
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.
這些湖泊最為顯著的一個(gè)特征就是，它們大多呈現(xiàn)相同的狹長橢圓形（或稱卵形），且所有橢圓的長軸方向完全一致。
Here is an idea of what they look like, what the landscape looks like from an aerial view with the lakes side by side.
這就是它們看起來的樣子，從航拍圖可見，這些湖泊如同整齊排列的卵形圖案鋪展在大地上。
There's been considerable research done to try to figure out what causes them to be shaped and oriented this way.
為此，有大量研究展開，試圖揭示其形態(tài)與排列方向的成因。
We know that the shape and orientation are caused by the way the lakes grow once they are formed. But the question is what makes them grow this way?
我們已知湖泊的形態(tài)、方向性，與其形成后的擴(kuò)張方式相關(guān)。但問題在于是什么決定了這種擴(kuò)張模式？
One theory sees winds as the cause. This region of Alaska has strong winds that blow perpendicular to the lakes. What happens is wind blows straight into the longer side of the lakes.
有一種理論認(rèn)為風(fēng)力是主導(dǎo)因素。阿拉斯加該區(qū)域盛行與湖泊長軸垂直的強(qiáng)風(fēng)。強(qiáng)風(fēng)會直接吹襲湖泊的長邊方向。
Now, wouldn't that erode the lake bank in that direction? Same direction as the wind?
這難道不會導(dǎo)致湖岸沿風(fēng)向發(fā)生侵蝕嗎？
Well, no. Actually, what happens is that the waves caused by the winds build a sort of protective layer of sediment.
并不會。實(shí)際上，風(fēng)浪會在迎風(fēng)岸形成沉積物構(gòu)成的保護(hù)層。
It's called a protective shelf, along the bank of the lake directly in front of them.
這種被稱為“防護(hù)臺”的結(jié)構(gòu)，能有效保護(hù)正對風(fēng)向的湖岸。
So that bank is shielded from erosion and the waves are diverted to the sides, to the left and to the right. And that's why the left and the right banks start eroding.
因此正面湖岸免受侵蝕，而波浪能量被導(dǎo)向左右兩側(cè)。這正是兩側(cè)湖岸開始被侵蝕的原因。
Get it? The bank straight ahead is protected. But the lake currents, the waves erode the banks to the sides. That's the current model, the wind erosion model, which is generally accepted.
理解了嗎？正面湖岸受到保護(hù)，但湖的水流，浪花侵蝕兩側(cè)湖岸。這就是目前廣為接受的風(fēng)蝕理論模型。
But there is a new theory that says that thaw slumping, not wind, is what shapes the thaw lakes. Thaw slumping... Ok.
但有一種新興理論認(rèn)為，真正塑造熱融湖形態(tài)的是熱融滑塌而非風(fēng)力。熱融滑塌現(xiàn)象，好的。
Sometimes in the summer the temperature rises pretty quickly. So the active layer of permafrost thaws faster than the water can drain from the soil.
夏季氣溫驟升時(shí)，永久凍土活動層的融化速度可能快于水分滲出速度。
So the sides of the thaw lakes get like mushy and slump or slide into the lake. Then the lake water spreads out more and the lake gets bigger. OK?
這導(dǎo)致融湖的邊緣變成糊狀，發(fā)生滑塌墜入湖中。湖水因此向外擴(kuò)張，湖面逐漸擴(kuò)大。
Also in that part of Alaska, the terrain is gently sloped, so the lakes are all on an incline.
此外阿拉斯加該區(qū)域地勢平緩傾斜，所有湖泊都位于斜坡上。
Here, now, this is an exaggeration of the angle. The hill is not this steep.
圖示為夸張化的坡度，實(shí)際山體遠(yuǎn)沒有如此陡峭
But see how with the lake's banks, the side that is farther downhill, it's smaller, lower.
但注意觀察湖岸形態(tài)：位于下坡方向的湖岸較為低矮。
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.
這種低矮湖岸比高岸更快融化，因此更易也更頻繁地發(fā)生滑塌。
When the short banks of many lakes slump, they move farther downhill and the lakes grow all in the same downhill direction.
當(dāng)眾多湖泊的下坡岸持續(xù)滑塌，湖泊整體就會朝同一坡向擴(kuò)張。
This is a new theory, so it hasn't been tested much yet. In field studies, when we've looked at the banks of these thaw lakes, there is not much evidence of slumping.
這是一個(gè)新的理論，目前尚待驗(yàn)證。在實(shí)地考察中，我們在湖岸區(qū)域并未發(fā)現(xiàn)明顯的滑塌證據(jù)。
We'd expect to see cliff-like formation there from the slumping. But we haven't really found any of those.
我們認(rèn)為會在那里看到懸崖狀的滑塌構(gòu)造。但我們還沒有真正觀測到。
