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How Do Glaciers Move?

有些事 我们认为它理所应当:液体的流动得像液体 固体的
Here’s something we all take for granted: Liquid things flow like liquids, and solid
流动得……呃 固体不流动 对吧?
things flow like, well… solid things don’t flow, right?
Not so fast.
冰川很酷(冷) 并不是因为它由冰构成
Glaciers are cool, and not just cuz they’re made of ice.
我现在在门登霍尔冰川 它是著名的朱诺冰原上40座冰川中的一座
I’m here at Mendenhall Glacier, one of about 40 glaciers in the famed Juneau Icefield.
冰川很酷 因为它们移动的方式
Glaciers are cool, because the way that they move, even the very fact that they move seems
to defy physics.
固体结构 像液体一样流动 怎么做到的呢?
A solid structure, that flows like liquid. How?
THAT is a question of glacial proportions.
为了了解冰川是如何移动的 首先让我们回到最初的时候
To figure out how a glacier moves, first let’s go back to the beginning.
<宇宙大爆炸> 啊 不是这个开始啦
Ahh, not that beginning.
冰川形成之初的样子 首先 你得去现场看看
The beginning of the glacier, and first you’ve got to get up there.
我太激动了 我之前从未坐过直升飞机
I’m super excited, I’ve never been in a helicopter before
我们得坐这个 到冰川的顶部去看看
and we’re gonna take this thing up to the top of a glacier.
This ice is born in the ocean.
来自太平洋的 温暖潮湿的气流沿着海岸山脉上升 然后 冷却 凝结
Warm, moist air from the Pacific rises up coastal mountains, where it cools, condenses,
and falls as snow and rain.
30米厚的积雪 有时 每年会有更多的雪覆盖其上
30 meters of snow, sometimes more, falls on this icefield every year.
But you need more than a mountain of snow to make a glacier: you also need the right climate.
即使是在夏天 这个地区的温度也会接近冰点
Even in summer, temperatures in this region dip near freezing.
What this means is the accumulation of snow in winter exceeds snowmelt in summer.
所以 经年往复 每一层雪都会被另一层所覆盖
So snow builds over time, with each layer landing on top of the one before it.
Now, a cubic meter of fresh snow typically weighs 70 to 150 kilograms.
这和一个成人的体重相当 或者两个成人
That’s about as much as an adult human or two.
Most of that volume is air.
但随着它的不断积累 在松软物质上的集体力量开始转变
But as it continues to pile up, collective forces on the fluffy stuff begin a transformation.
首先 这些有美丽形状的雪花完全被粉碎成冰晶
*First, those pretty snowflake shapes are totally shattered into smaller crystals; about
as big as grains of sugar.
随着不断的挤压 中间的空气开始缩小
And as they get squished, the air pockets between them start to shrink.
The snow gets denser.
大约两年后 地面积雪形成新的结构 叫做“粒雪” 一种中间产物
*After about two years, ground snow takes a new form, called “firn,” an intermediate
state between snow and glacial ice.
粒雪的密度是水的三分之二 完成最后的形式可能需要
Firn is about two-thirds the density of water, and it can take decades to complete the transition
into its final form.
最后的形式是密集的 无气泡的冰
That final form is a big mass of dense, bubble-free ice.
The ice in Mendenhall is flowing forward more than a half meter every day.
冰川常被比作“冰河” 这没错
Glaciers are often compared to “rivers of ice,” and that’s not wrong.
These giant solid structures behave with liquid-like tendencies.
*I know what you’re thinking: “ice flows because it melts.”
但是 冰川冰没有融化就流动了
But glacier ice flows without melting.
门登霍尔冰川符合对固体的定义:我可以站在上面 甚至跳跃
Mendenhall fits the definition of a solid: I can stand – even jump – on it, and it
will hold its shape.
Short term stress doesn’t affect that.
但是长期的压力 如承受自身巨大的重量 会使它变形 弯曲
But long-term stress, like bearing its own extraordinary weight, will cause it to deform and bend.
我们被告知物体是固体而不是液体 是因为
What we’re told makes something a solid, rather than a liquid, is that its atoms and
它的原子和分子是紧密结合的 它们彼此无法分离
molecules are so tightly bonded, they can’t move past each other.
但 冰川冰并非如此
But this isn’t the case with glacial ice.
它的水分子排列有序 但在某些条件下
Its water molecules are arranged in an orderly pattern, but under certain conditions they
can still flow.
Much of this has to do with the pressure melting point.
随着压力的增加 冰的融点降低
As pressure increases, the melting point of ice decreases.
当冰川冰接近 但还在融点之下时 会具有可塑性
When glacier ice stays close to – but just below – that point, it becomes malleable,
很像接近融点 可以随意弯曲和重塑的固体金属
much like how you can bend and deform solid metal when it’s heated near its melting point.
*The deepest layers of a glacier are exposed to the most pressure.
这就是所谓的“塑性流动区” 因为冰晶体之间的键
This is known as the “zone of plastic flow,” because the bonds between the ice crystals
can be stretched rather than broken.
这里 冰晶体之间的分子键 实际上是伸展和滑过彼此
Here, the molecular bonds between ice crystals actually stretch and slide past each other,
而不是打破 就像一副牌 随着卡片的相互滑动而变形
rather than break, like how a deck of cards deforms as cards slide past each other.
在巨大障碍之下 冰川的底部需要移动时 像岩石
When the bottom of the glacier needs to move around large obstacles, like boulders, even
甚至造成冰融化的上坡更高的压力 会围着障碍流动
higher pressures on the uphill side cause the ice to melt, flow around the obstacle,
and refreeze on the other side.
随着这些过程的不断重复 冰川在重力的推动下 会慢慢蔓延
*As these processes repeat, the glacier creeps along, propelled by gravity, like a giant,
就像一个巨大的 粘糊糊的传送带
gooey conveyor belt.
在冰川的顶部 事情就有点古怪了
*Up in the top of the glacier, things get a bit cranky.
The upper 150 feet or so – “the zone of brittle flow” – the ice’s under much pressure.
这使得冰川易于因重压而破裂 这正是你现在可以看到美丽裂缝
That makes it prone to cracking under stress,which is why you’ll find beautiful crevasses
and things like this.
Imagine a candy bar warping around a curved surface.
粘稠的糖果底部弯曲伸展 导致糖果顶部不得不伸展得更剧烈 更快速 所以它断裂了
The top has to stretch farther, faster, so it cracks, while the gooey bottom bends and stretches.
Some glacier movement comes from slipping on sediment or a thin layer of water, but
most glaciers not at Earth’s poles move by this process of deforming.
但冰川是气候的产物 会随着气候的变化而变化
But glaciers are a product of climate, and they change *with* the climate.
门登霍尔冰川正在沿一条13公里的路径滑向该地最低点 门登霍尔湖
Mendenhall Glacier is currently slinking along a 13-mile journey to its lowest point, Mendenhall Lake.
The terminal edge of a glacier is one of the easiest places to see its movement in action,
but it’s also where we can see how much things are changing.
冰川从不向反方向移动 并且一直在融化
Glaciers never move backwards, and they are always melting.
但当底部的大冰块比顶部新形成的大冰块融化得更快时 冰川将会后退
But when mass melts away at the bottom faster than new mass is added up top, they can recede.
It takes about 200 years for new ice on Mendenhall to move from the icefield to the lake.
这是一个缓慢的过程 但逐渐变暖的夏天加上降雪减少的冬天 加快了融化速度
It’s a slow process, but warmer summers combined with less snow in winter are speeding things up.
The glacier is melting faster than it’s growing – and it’s already retreated miles.
仅几十年前 我现在所站的地方还曾覆盖着冰川
Just a few decades ago, where I’m standing was covered in ice.
冰川的移动确实是难以想象的酷 但若冰川想要继续移动 它们需要 保持原样
How glaciers move is incredibly cool, but for that to continue, they have to stay that way.
感谢收看 保持好奇心!
Thanks watching and stay curious!
如果你想观看更多阿拉斯加迷人的野生鸟兽 敬请收看“狂野阿拉斯加现场版”节目
If you want to see more of Alaska’s incredible wild life, watch Wild Alaska Live, a special
3-night live event brought to you by PBS and BBC.
Check the description for more info.