Why the Great Smokies Are Still So High

Why the Great Smokies Are Still So High

Remember that crustal rocks are the low-density “scum” that floats on the denser mantle. When obduction occurs, this crustal scum is crunched—it goes from long and thin to short and thick, in the same way, that the front end of a car is changed when it runs into a brick wall, or a carpet is changed if you shove the ends together and rumple it in the middle. Then, much like an iceberg floating in the water, a mountain range is a thick block of crust floating in the mantle, with most of the thickness of the mountain range projecting down and only a little bit sticking up.

Notice something else fascinating; when a mountain range is being eroded, the top is taken off, and rocks below bob up almost as high as before. Erosion continues to remove those almost-as-high rocks, allowing more rocks from below to rise. Pretty soon, the rocks at the surface have come from far down in the Earth, where temperatures and pressures are high. And as you might imagine, those rocks were changed by the high temperatures and pressures. The rocks around Penn State’s University Park campus have not been “pressure-cooked” much, but the rocks around Philadelphia have been - they tell the story of a great mountain range that fell apart, leaving the remnant that we know as the Appalachians. The rocks in Rocky Mountain National Park are like those in Philadelphia, in the sense that they once were deep in the Earth and now are at the surface. This is similar to how icebergs work. See the animation below about icebergs to learn more about how isostasy works.

With an iceberg, about 9/10 of the thickness is below the water and 1/10 above. As shown in the narrated diagram below, if you could instantly cut off the 1/10 that is above water, the iceberg would bob up to almost as high as before. A 100-foot-high berg would have 10 feet above the water and 90 feet below. Cut off the top 10 feet, and it is a 90-foot berg with 9 feet, or 1/10, above the water and 81 feet below. So, removing 10 feet from the top shortens the ice above the water by 1 foot and the ice below the water by 9 feet. With mountain ranges, the density contrast between crust and mantle is larger than that between ice and water—only about 6/7 of a mountain range projects down to form the root, and 1/7 projects up to form the range. 

Video: Icebergs (3:02)

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0
Click here for a transcript of the Icebergs video.

Dr. Richard B. Alley: Out in the ocean, there's a giant iceberg sitting out here. Big thing threatening the oil platforms floating around. And in the bottom of this very special iceberg, there is a really strange looking, googly-eyed space alien. And you have been given the task to go out there in your little boat sitting here in the water to get the space alien out.

Now, you're a good geoscience 10 student, and you know that all icebergs have about 1/10 above the surface, and they have about 9/10 below the surface. So you know immediately what to do. You take your gimongous chainsaw, and you chainsaw off the top of the iceberg and throw it away, because you know what this will cause is that the iceberg will come bobbing up, carrying the space alien with it.

And so after it gets done bouncing up and down for a little bit, you find that the iceberg is almost as tall as it was before. It still has, sitting way down in the bottom of it, the space alien that you're trying to get to. So there's a space alien down here. And it still has about 1/10 of its height above the surface and about 9/10 of its height below the surface.

But what you find is it's just a little bit shorter than it was, and it doesn't stick down quite as far as it did. Now you wump the top off again, and you keep wumping the top off, and you keep wumping the top off. And after a long time, you get down to a little iceberg that doesn't stick very far down.

Now it still is the same picture, that it has 1/10 above, and it has 9/10 below. And if you're not careful, you're sitting there admiring this lovely fact of science, and the space alien sticks out a giant tentacle, and it grabs your ship and throws it to the bottom of the sea. And so you'd better not do that.

However, there is a scientific piece to this. Suppose, instead of space aliens, that we wanted to talk about mountain ranges. Now, we know that mountain ranges stick up above the plains. But you might not have known that they also stick down.

They have a root in the same way that an iceberg has a root that it's sitting on. There's a slight difference in that about 1/7 of a mountain range is up and about 6/7 of a mountain range is down, way down. The rocks have been heated. They've been squeezed. There are all sorts of interesting things going on and new minerals being grown.

And at the surface, the streams are sitting here, busily trying to grind away the mountain range. As the streams grind away the mountain range, why, the deep stuff will come bobbing up towards the surface. And if you come much later and look at it, you'll find that the rocks have barely any mountains left.

There's still a little bit of root with 1/7 up and 6/7 down. But now what you'll find is that the rocks that had been cooked way down, and bent way down, are very near the surface. And you can go see them.

Still, if rivers or glaciers erode a mountain range (something we’ll study in modules 5, 6, and 7), some of the root is freed to float upward. Only by eroding the equivalent of 7 mountain ranges can you eliminate the mountain range entirely. So, the Appalachians, despite having been deeply eroded, are still high because they still have a root.

The idea that things on the surface of the Earth float in softer, denser material below is called  isostasy, which means “equal standing”—each column of rocks on Earth has the same weight or standing. Lower-density columns then must be thicker to weigh as much as thinner, higher-density columns. The continents stand above the oceans because the silica-rich continental crust is lower in density than the silica-poor sea-floor crust. The mountain ranges stand above the plains because the thick, low-density roots of the mountains have displaced some of the high-density mantle that is found beneath the plains, or because the rocks beneath the mountains are especially hot and so low in density. Look back at the animation about icebergs to learn more about how isostasy works.

Put a big weight on a piece of crust (say, an ice sheet, or the Mississippi Delta, or a mountain range) and that piece of crust sinks, pushing up material around it in the same way that the surface of a waterbed sinks beneath your posterior when you sit down, while the surface is pushed up around you by the water that is shoved sideways. The rising and sinking of the land are slower than for a waterbed—thousands of years rather than seconds—because the hot, soft, deep mantle flows a lot slower than water does. But for a mountain range over 100 million years old, a few thousand years doesn’t mean much.