As many of the photos we have posted recently deal with glacial processes, we figured it was only fair to bring the non-geologists up to speed. So, let's start with glaciers. Everyone has heard the word, but what really is the difference between a glacier and the piles of snow in the Blacksburg parking lots this winter?
Simply put, quantity and longevity. You have to get enough snow that hangs around long enough, more than just one winter. It also has to be of a certain size, but this seems to vary with who you talk to. Yet....
...it moves.
Glaciers flow. As snow accumulates at the upper elevations of a glacier, it will tend to flow downward. Not as rapid as say, a river, but surprisingly fast nonetheless. So imagine you have this giant sheet of ice flowing out across the surrounding rock and soil. Just the fact that it is moving guarantees some erosion will take place, oh, but the weight! If you have ever purchased block ice at the grocery store (a la Cassie), you know that a ten pound block of ice isn't that big. So hundreds of feet of ice grinding down on the ground is surely going to do some damage.
Most of this grinding is actually done by rocks that are picked up by the glacier and dragged along the ground, pulverizing and striating the bedrock below. Occasionally, these stones melt out or are dropped out into glacial lakes, explaining the curious presence of large rounded "dropstones" within the fine, muddy, lake sediments in the rock record.
(source: University of Maryland)
All this pulverized rock has its place, too. The "rock flour" is washed out into the glacial lakes. Often, it is so fine that it doesn't settle to the lake bottom as rapidly as it is washed into the lake. This suspended sediment is what gives glacial lakes their milky-aqua color. It's kind of like the classic question, why is the sky blue? The rock flour tends to reflect back a little more green light than clear water. So, to recap, these glaciers are flowing and eroding.
Which brings me to the big U. Glacial landscapes are easily recognized by the huge U-shaped valleys. In the absence of active processes, geologists can look at a landscape and determine if it was a river or glacier valley. Imagine that you have a river, about the size of the New, slowly cutting its way down through bedrock. This is a relatively slow process. What if I handed you a piece of sandstone and told you that you couldn't eat until you sawed through it... the only catch? You could either use a water hose or another rock. Your chances of grinding through the sandstone are a lot better if you act like a glacier, and grind it down with the other rock (and once again, this doesn't even take into account the forces from the sheer weight of a glacier!). So, river erosion is quite slow. So slow in fact, that as the river cuts down, other erosive processes have a chance to act. Rain and landslides will take out the sides, ideally bringing the sides of the river to a nice angle of repose. This results in a classic V-shaped valley.
However, when a glacier comes through, all of this erosion happens pretty quickly. Additionally, the glacier is still present and holds up the sides of the valley, preventing erosion by rain or landslides.
Suddenly, there is a change in climate, and the glacier starts to melt. Often, all of the rocks and gravel that had flowed down to the bottom of the glacier melt out at the edges, helping to form moraines (the terminal moraine at the toe of the glacier and the lateral moraines along the sides). When the glacier is gone, that's when the real fun starts. Suddenly you have these huge steep valleys exposed to gravity, weather, and all the erosive forces start to attack. Depending on how resistant the rock is, this might take a while, but eventually even these massive glacial valleys will strive to attain equilibrium. Although it will take millions of years, our favorite glaciated regions will also succumb to the erosion that eventually brings down the grandest of mountain ranges.
Suddenly, there is a change in climate, and the glacier starts to melt. Often, all of the rocks and gravel that had flowed down to the bottom of the glacier melt out at the edges, helping to form moraines (the terminal moraine at the toe of the glacier and the lateral moraines along the sides). When the glacier is gone, that's when the real fun starts. Suddenly you have these huge steep valleys exposed to gravity, weather, and all the erosive forces start to attack. Depending on how resistant the rock is, this might take a while, but eventually even these massive glacial valleys will strive to attain equilibrium. Although it will take millions of years, our favorite glaciated regions will also succumb to the erosion that eventually brings down the grandest of mountain ranges.
Okay, so the only thing missing in your post is the fact that the glacial till or rock flour (I like that term as I always tell students that the consistency is just like flour.) is sometimes picked up by the wind and deposited in certain locations in the northern hemisphere. This stuff called loess (once it is deposited) is very fertile (all those rock minerals ground up by that glacier, remember?). The presence of loess leads to high denisities of humans trying to take advantage of the fertile soils.
ReplyDeleteI actually saw this stuff on the Loess Plateu just north of Xi'an, China. It is so deep there that when the farmers have exhausted the nutrients in the soil, they just haul the stuff away and begin with new fertile soil in the next layer. There is also some physical property of the loess that enables the Chinese to carve out rooms in which they live or store agricultural tools. I never could figure out why these rooms didn't collapse. I was in one and there really isn't any support.
Darn geologists never consider people. Love you guys, even if you stop before the next chapter.