Listen to part of lecture in a geology class.
Professor: Um, beginning in the late 1960s, geologists began to uncover some evidence of a rather surprising kind when they looked...um..at various places around the word. What they found out when they examined rocks from abou a... the period from about 750 milion years ago to about 580 million years ago, they found that ..it seemed that glaciers covered the entire surface of the Earth-from pole to pole, including the tropics.
Um...how did they come to this astonishing conclusion? What was the evidence for this? Especally when glaciers today are found ony at the poles...or in the mountains.
Well, uh ..basically when glaciers grow and move they leave behind a distinctive deposit consisting of primarily. ..of at least on the toplevel of ground up little bits of rock...almost...they almost look like rocks that have been deposited by streams, if you've ever seen those.
And that's caused because, although the glacier is ice , it is actually flowing very slowly and as it moves it grinds the top layer of rock, it breaks off pieces and carries them away. So when you have glaciation you have a distinctive pattern of these pieces of rock which arecalled "erratics". Erratics are rocks..they're the stones that are often carried long distances by glaciers.
So, in the 1960s and onward up through the 1990s, we keep finding evidence for glaciation, no matter what the latitude...even in tropical latitudes. Now, today there are glaciers in the tropics but only at very high elevations. But 750 million years ago, apparently there were glaciers even at sea level in the tropics. How could this have happened?
Well first..the growth of glaciers, uh, benefits, if you will from a kind of a positve feedback loop called the "ice-albedo effect".with the ice-albedo effect, glaciers-cause they're white-refiect light and heat more..much more than does liquid water ..or solil and rock, which are dark and absorb heat. So, the more glaciers there are, the more heat is reflected, so the climate gets cooler, and glaciers grow even more.
However...normally, on a global scale, there is a major process that functions to curb the growth of glaciers. And, that process involves carbon dioxide.
Now, we're ll familiar with the notion that carbon dioxide is what we call a "green-house gas". The more carbon dioxide there is in the atmosphere, the more heat the atmosphere retains. That's what a greenhouse gas does. So, the greenhouse-gas effect is kind a the opposite of the albedo effect.
Um..now as it happens..when silicate rocks, which is a very common class of rock, when they're exposed to the air and to normal weathering, they erode. Carbon dioxide is attracted to these eroding rocks and binds to them, forming calcium carbonate. Calcium carbonate is eventually washed into the ocean where it settles to the bottom. This process ,this forming of calcium carbonate,has the effect of sucking the carbon dioxide out of the air and storing it at the bottom of the ocean.
Now, follow me here. The process that's sucking carbon dioxide out of the air, keeping the greenhouse gas levels low, cannot happen if the rock is covered with ice.
So, while glaciers reflect light and heat...cooling the Earth, they at the same time cover rocks so there's less calcium carbonate formed..which leaves more carbon dioxide in the atmosphere. Higher levels of carbon dioxide keep the atmosphere warm..which slows the growth of glaciers. So, it's a balance, and the glacier growth remains pretty much under control.
Now, what happened 750 million years ago to upset that balance ? lt seems a relatively simple explanation actualy...
750 million years ago...all the major continents are rocky, bare, and pretty much lined up along the equator, they hadn't yet moved to where they are today. So, what happened was, perhaps a slight cooling of... the very slight and temporary cooling of the Sun-which stil happens from time to time-and the Earth starts to cool, the ice starts to spread on the oceans...starting at the poles.
Now, by the time the ice reaches about two-thirds of the way to the equator its too late. See...because the continents are the last things to be covered by glaciers, they continue weathering; the rocks keep eroding and the carbon dioxide levels keep falling.
So, the ice-abedo effect from the glaciers is increasing in strength while the atmosphere continues to lose its abilily to retain heat making glacier growth unstoppable. Now you have what's called a "runaway freeze". And for perhaps as long as 50 milion years, possibly withsome interludes, the Earth was frozen from pole to pole, like a giant snowball.
这个geology class难度并不小，特别是其中有一条，“什么促成/促进了冰川化”，教授提供的一条线索是一个假设，太阳可能会时不时的温度变小，同时一个更重要的原因是750 million years ago，大多数板块都是在赤道的，而且是整块的、裸露的，这才是真正的促因——裸露的岩石风化，硅酸盐和二氧化碳结合生成碳酸钙沉淀到海底。这样彻底导致了温室效应减弱，冰川的扩展更加肆无忌惮......
有关于750 million years ago 整个地球都是覆盖在冰川下面的假说，证据之一就是冰川融化后板块移动产生的碎岩。
- uncover 揭露
- glaciers 冰川
- astonishing 惊人
- distinctive 独特的
- streams 流
- grinds 磨
- glaciation 冰川化
- erratics 漂砾
- elevations 海拔;高地;高处
- ice-albedo effect 冰反照率效应
- curb 抑制
- retains 保留
- albedo 反射率；漫反射系数
- albedo effect 反射效应;反照效应
- silicate 硅酸盐
- erode 侵蚀;腐蚀;风化;
- calcium carbonate 碳酸钙
- settles 定居
- equator 赤道
- interludes 插曲
- giant snowball 巨大的雪球
- onward up through 向上穿过
- upset that balance 破坏平衡
- runaway freeze 失控的冻结