Geology of the National Parks

Video Lecture


The Unit 5 lecture features Dr. Sridhar Anandakrishnan and is 57:43 minutes long.

Unit 5 Lecture
Click for a transcript of the Unit 5 Lecture.

Good morning. Welcome to GSCI 10, Geology of the National Parks. Now for the next two or three classes, we're going to be looking at how we tear down mountains. For the first part of this class, for the last four lectures, we've been building these mountains.

And just to do a real quick recap, if you remember, it's all driven by heat inside the earth. And where does that heat come from? From radioactive decay. All right, so radioactive decay inside the earth drives the heating or produces the heat, which then drives these convection cycles inside the asthenosphere. So the hot rocks rise up, get to the surface, cool off, sink back down again. And these big cycles go on inside the asthenosphere.

Up at the surface, in the lithosphere, that's broken up into 8 or 10 lithospheric plates. And those plates move around on top of the convection cells. When they move around, they interact with each other because the surface of the earth is a finite space. And so when one plate moves, it's got to do something relative to another plate. It's either moving away from another plate, or it's moving towards another plate, or it's moving parallel to another plate.

Most of the action of building mountains goes on at those places, where the plates are moving apart, like Death Valley. Where the plates are coming together, like here in the Appalachians, like the Himalayan Mountains in India and Tibet. Like the Pacific Northwest, where the subduction zones are producing those beautiful volcanoes and mountains. Like the San Gabriel mountains in Los Angeles where there the plates are sliding past each other, but because of that small kink in the San Andreas fault, we get mountains being built over there.

All right, so all the action is at these plate boundaries, 30 second review of the last four sections.

Now we're going to take the next step. We've built these mountains. They're sitting out there. We know they're not static. We know they're not just sitting there and nothing happening to them. We know that they change. We know they erode over time. They get torn down. They get lowered further and further down.

We actually saw hints of that when we were talking about how the Appalachian Mountains used to be much higher and now they're much lower. They're not as low as they could be because of isostasy, because they're being floated up by these deep roots. Like that iceberg, if you remember that cartoon, the iceberg and the alien embedded in it, and that alien slowly rose up from within the iceberg.

So, we got a little bit of a hint of this when we talked about how the Appalachians were lowered down, how the tops of them were chopped off. That's what we're going to spend the next few minutes talking about, is how that chopping off process goes on.

Here we are in the badlands. Badlands in the Dakotas are a really beautiful place, but there's a reason they're called the badlands. Not very much grows there, except jackalopes. There's a lot of tourist trade through there because that's all they can do. They have a very hard time growing crops. It's one of the poorest parts of the country. The Pine Ridge Indian Reservation that's near here is the poorest county in the country.

Nevertheless, it is a dramatic place. All of these lovely gullies, and beautiful, stark moonscape-like surfaces, they're lovely for us to look at as tourists, not so nice for the people that live there.

Most of the rocks that you see here are sediments that have been washed down from the ancestral Rocky Mountains. The Dakotas are far away from Colorado, and Wyoming, and Montana, but nevertheless, transport of this material can take place over long, long distances. Mix in some volcanic ashes from, for example, the big Yellowstone volcano and you have the soil of the badlands.

Here's another picture of it. It's a very clean, rich soil. And so, as these soils get wet and dry, they expand and contract. And the trees and plants have a really hard time living in that kind of environment. Their roots get ripped up and torn to pieces. The water comes along and washes off the surface layer or cuts these deep gullies. It's a hard place to make a living.

And if you don't have plants, if you don't have grasslands, or you have very minimal grasslands, this is another shot showing some of the lovely landscape, then there aren't that many animals there. And so eventually, it's hard for people to make a living there as well.

Here's an example of erosion. This tree didn't grow that way to begin with. Those roots were deep inside the soil, but the soil got washed away from around them. You can see to the left of that tree there's that deep gully. And as the rains come along, that soil gets washed off and carried down in there.

This is really commonsense stuff. You've seen it. You've seen it in your backyard. It has enormous implications. If you can do it for year after year, after year, after year for millions of years, then you can even take a huge mountain like the ancestral Appalachians or the Rocky Mountains, and you can tear them down to nothing.

Here's an example of the rate at which this goes on. This as a carving that the ancestral Pueblo people did on rock varnish in the desert. This black glaze gets on to these surfaces in the desert where it's really dry. And the ancestral Pueblo people would go and make these carvings in it.

And what you can see in the corner over there is where one of those designs, that spiral design, has broken off. This ancient artist didn't come along and simply end his design at the edge of his canvas, his design continued on to the left and there was probably even more to the left of that. But it broke off, and that front part fell down.

And so that is weathering. The rate of it is slow. This design is from perhaps 1,000 or 2,000 years ago. And you can see some of it is still there and a very little bit of it has torn away. Here's another example of that same thing. At the lower end, you have some designs of animals, some deer and perhaps horses. And those are being torn down.

Here's an example from far afield, from Greenland. This is a rock that has been split open by one of the most powerful processes known on earth for erosion, a simple act of freezing. You take water, you put it into something, you let it freeze, it expands, and it will crack open just about anything. It'll crack open that rock.

It'll certainly crack open your water bottle. Your mom told you when you were a kid, fill up that water bottle and put in the freezer. You didn't fill it right to the top. Because if you filled it right to the top, when the water froze, it would crack that water bottle open. So, you know that.

And the same thing happened here. The water got down into some little, tiny, minute crack and eventually it broke that big rock. It's the size of your head, that nice big rock.

All right, so we've taken some pictures. We've had a look at some of the weathering that goes on in the badlands, and a little bit of it in the desert southwest, a little bit of it in Greenland. Let's take another quick field trip to where the source material is. I said that these rocks, these big mountains get torn down, and then the material gets spread out in the valleys, in the badlands, and then turns into soil. Well let's go take a look at one of those mountains because they are really beautiful places.

This is Grand Teton National Park in Wyoming, near Jackson Hole, Wyoming. Jackson Hole is very famous. Lots of famous and rich people have their ranches there. It has become quite a popular destination. But before that, it was a simple ranching town. And one of the reasons that people went there was for tourism, for some of the really just magnificent scenery that's available there.

These high, sharp peaks, we're looking from the hole, from Jackson Hole Lake, up towards the mountain to the west. You have this relatively low spot and then these mountains rise, soar up, thousands of feet next to you.

There's a large active fault in the earth and these mountains are actually growing along them. As they grow up into the sky, weathering processes, erosion processes work at them and tear them down producing these jagged cliffs, and these sharp edges, and these very steep sides, and these very dramatic looking valleys and gullies. It's because of that process that you have those sharp edges, because of the erosion process. Here's another beautiful shot of it with the sun setting in the background.

There's lots of rain up there. You get these thunderstorms. And those thunderstorms will go up there and there's these big rocks that are lodged up in these gullies. And the water will come down and then these rocks will all cascade down. Gravity is a very irresistible force. Everything wants to head downhill.

It might stay up there for a little while, it might stay jammed into the side for a little while, but eventually gravity will win and things will come down. Now, if you've got a big rock and it's solid, it won't just simply fall apart by itself. It has to be helped along. That's what we're going to talk about there.

Here's a beautiful photograph of Mount Moran with Jackson Lake in the foreground. Another beautiful view of it. As I said, all of these slides are available online.

Back about 20,000 years ago, there used to be huge glacier that came down from the mountains, down into the lake and probably carved out that lake. And now that glacier is gone. We'll talk about that in a couple of sessions.

Here's the Gros Ventre slide in the Grand Tetons. This was a landslide that came down and dammed a river and ended in a catastrophic flood.

So, let's go to the videotape now. Let's move to the drawing and we'll do a quick summary of where we're going to head for the next class.

Tearing down mountains, weathering, wind, and mass movement. What is erosion? Erosion consists of three things. Weathering, all weathering means is taking a rock and breaking it down, either physically breaking it into smaller pieces or chemically breaking it down into some of its constituent minerals. That's all it is, taking a big thing, making it smaller.

Transport is moving those weathered materials, the smaller rocks, the dissolved chemicals, moving them from here to there. Deposition is the end process of that transport. You've weathered something over here, you've transported it over there, and you've deposited. That whole cycle is known as erosion.

If you have enough time, even very slow erosion processes can dramatically modify the landscape. You can take a huge mountain and you can tear it down to nothing if you give it enough time. This was one of the keys that the early geologists used to say that the world must be really old. Because they looked at these mountains and they said, but look, we know that erosion is really slow here, but we've torn down this huge mountain. It must have taken hundreds of millions of years to do this, to produce all of this deposited sediment pile and so the earth must be old.

That's one of the keys that the early geologists used to say that the earth was old. And then, we got more evidence as time went along and we've found newer and better techniques to date the earth. And we'll talk about that later on.

The most important agents for erosion are water, ice, and temperature. You take water, you put it into a rock, you freeze it, that will break the rock apart. You take that same water, you turn it into a river, it will carry the broken-up pieces down river. Where that river gets to a spot where it slows down, where it gets to the plains, where it starts to meander and heads to the ocean, that material will be deposited.

Ice is the same way. Glaciers come along. Glaciers can tear rocks apart. They can move the rocks to a new place and then they'll deposit those rocks and build these beautiful big moraines like Cape Cod. And we'll learn about Cape Cod in a little while.

So, the atmosphere is involved because that's where the rain water and the snowfall comes from for the glaciers. All of that material, that snow or rain, lands on the earth and it works on the rocks to tear them apart.

The erosion agents are, as I said, water in glaciers and simple, old downhill flow of stuff. You've got a hill, you pile up rocks on it, eventually that will start to head downhill. Especially if you break it up and you help it to move down by raining on it or something like that. So, let's take a look at mass wasting first and then we'll talk about the others in a second.

When we talk about mass wasting, think of Teton National Park, those beautiful, big, sharp, jagged peaks. The reason they're like that is that the edges have been sharply torn down and head downhill. That's mass wasting. We'll talk about soils. Think of Badlands National Park with all of this big soil layer that was produced by weathering. And when we talk about rain, we'll talk about Redwood National Park.

Mass wasting is defined as simply masses of debris or bedrock moving downhill. It can be something fast and dramatic and disastrous like a landslide, or it can be something as slow and innocuous, but ultimately far more important, like simple mass creep, it's called. And we'll talk about that.

All of these things are driven by gravity. Gravity is a force that is pulling material at the surface, mass at the surface, towards the center of the earth. And if you give a small slope or a big slope, there is a component of gravity that's pulling it along that slope. And anything on this slope will want to head downhill.

This is the most basic, simple, common sense thing in the world. You live it every day. You live it when you take a slide down a children's plastic slide in the playground. The children go up to the top, they jump on to it, and they slide whooshing down these plastic slides. They don't know it, but what they're doing is they're responding to the force of gravity. But you do know it because now you've grown up, and you've studied this, and we're talking about it here.

This map here is a map of the landslide potential in this country. How much in danger of having a landslide are we in Ohio, or Wisconsin, or Florida? Not very much. How much of a danger are we to having a landslide in Colorado, or Idaho, or Washington state? Quite a bit.

That's what the colors are. The dark greens are high severity of landslides and the white and tans are low or moderate risk of landslides. Why? It's very simple.

You've got these high mountains in Colorado, or Washington, or Idaho, or Montana. And you don't have them in Florida. In Florida, it's all flat. You're not going to have a landslide. Common sense, it's just plain old gravity that's going to grab things and shove them down. So, this is where gravity comes into play with mass movement downhill.

We can classify this mass movement into a couple of different ways. And really, all it comes down to is how fast this stuff is going to swoosh downhill. If it comes really, really fast, then it's a landslide. If it comes really, really slow, then it's called creep. And it can be everything from in between. From 100 kilometers per hour, the stuff comes shooting down the mountain and wiping out houses, and roads, and bridges, and people that are in the way, and it really is quite disastrous and dramatic. Or it can be really slow, this slow creep of soil where just little bits and pieces of soil will head down the hill, year after year. And after 100 years, maybe it's come down a foot, or half a foot, or something like that. In the end, soil creep is the most important because it happens everywhere.

Here's a picture showing the gravity pulling down towards the center of the Earth. And if you've got a slope, then you have a component of gravity that's pointing downhill along that slope. And you put something on that slope, and it will slowly make its way down the slope. If the slope is really steep, maybe it will go shooting down really fast. If the slope is really gentle, then it will move down much, much more slowly. But in the end, it will move down.

And there's a few things that control that rate. If you've got lots of trees and grass on this hillside, then the soil gets bound together by the roots of these trees and grass, and so the soil doesn't want to slowly go creeping down the hill. Whereas, if you don't have any trees holding it back in place, if you don't have any grass holding the soil in place, the first rain that comes along will wash this soil downhill.

You've seen that. You've seen where there's a gully with nothing planted in it, it will erode faster and faster. But if the farmer goes and plants some crops on it, then it will not waste away quite so fast.

The most important of the controlling factors is water. Water inside the soil will bind the grains together. Unless there's too much of it, at which point the grains will be floated apart, and then it will swoosh away.

When you were a kid, you went to the beach, I know you did. And you were given a bucket and a little shovel, and you were told to build a little sand castle. And away you went, and you started playing in the sand and building your sand castle. And you found that if the sand was completely dry and you tried to build a sand castle with it, whoosh, it would just swoosh away, slump away, and you wouldn't get a sand castle. All it would be would be a pile of dry sand.

You knew that if you went down into the surf and you picked up a bucket full of sand that was filled with water from the ocean waves and you tried to make a sand castle out lot of that, flump, it would just slump down and you would just have this pile of wet sand. But if you got the water mixture just right, if you mixed the right amount of sand with a little bit of water, you got these lovely clumps of sand that you could then build your walls, and your turrets, and your towers, and you could put a little flag in it, and put seashells on it for decoration, and all was well.

So, a little bit of water bound the sand together. If there wasn't enough water, then the sand fell apart. If there was too much water, then the sand fell apart. The same thing happens in soils. And the same thing happens with mass wasting. Ice and vegetation can control that. If you've got vegetation that is holding the soil together, then it will help to stabilize the slopes.

We're going to go to the drawing tablet, and we'll go over these controlling factors once again. I have to have this thing come up.

When you have a mountain as we have over here, gravity wants to pull everything towards the center of the earth. But if you've got a rock sitting on the side of this mountain, it can't shoot towards the center of the earth because the hill here is preventing it from doing that. But what it can do is roll along that slope.

And if you have a very shallow slope like this, it's not going to roll very fast. But if you have a steep slope as we have up in here, then these rocks will flow downhill, or roll downhill, more rapidly.

If we zoom in on the soil surface right over here, what we will find is that the soil is made up of lots and lots of small particles or grains. And inside the pore spaces we have water. That water can bind the grains together unless there's too much of it, unless there's too much water. If there's too much water in there then the grains will be floated apart. Or float them apart.

And it really depends on the amount of water in there. And that's why after a rainfall, you will get much more erosion, much more movement of soils downhill. Because all of that water in the rainfall goes down into the soil and it will float the grains apart. And then those grains will simply wash downhill.

Grasses and trees can stabilize a slope because the roots hold the grains in place. And the roots will suck excess water out. That's what trees and grasses do really well, they drink water. So it rains and a lot of that water gets sucked up by the grass and trees, and so it doesn't float them apart.

The way that these smaller grains do move down hill is helped a lot by ice. And let's take a look at that next. Ice plays a major role in erosion in a couple of ways. The most important is in freeze-thaw splitting of rocks.

If we have a rock with a small crack in it, and we fill it with water, and then we let that water freeze, it will expand. You know that. You've seen that many times.

When it expands, here it is down in this crack. It's filled up this crack, the water has, and it starts to freeze. And it's got to expand, it needs more room. What's it going to do? It's going to push against the sides of the rock. I know rocks are hard, I know rocks are strong, but it turns out the ice can be even stronger. Rocks just cannot resist this. When the water freezes, and turns to ice, and starts to jack the sides of this rock apart, that crack gets deeper and deeper until finally the rock splits in two.

Now you have two smaller rocks. And you do it again, and you do it again, and you do it again, and you keep breaking these enormous boulders down into smaller and smaller pieces. Eventually, the pieces are small enough to be washed away by streams and rivers.

Ice also helps this erosion process in a second way. So the first and most important is breaking large rocks into smaller ones. A second way in which ice does its thing is when you've got a slope, and you've got a bunch of grains piled up on this slope, and you have water in the pores in between the grains. When this material freezes in the wintertime, what happens is, when the water freezes in winter, the grains are pushed apart.

And so, you can imagine that we have frozen this material. And I'm just going to erase these pictures and redraw it after it's been frozen. And we'll exaggerate it a little. Now, we have these rocks pushed apart after freezing.

And then, finally, spring comes around, that ice starts to melt and thaw, and it goes back to water, and the water shrinks back down again. And what happens? And we'll erase it again and redraw it a third time. And now, in spring, the ice thaws and the grains fall down. But not to their original position. They have moved a little downhill.

These grains that used to be piled up, nicely pile together uphill. They froze, got pushed apart a little bit. When they thawed, they didn't simply go back to where they were before. The one that got pushed up fell downhill a little bit.

I know it seems like such a minor, tiny process. But you do this over, and over, and over again, and eventually this grain can be moved here, and then here, and then here, and then here, and slowly creep down the hill. This process is known quite descriptively as soil creep. It is the most effective way of eventually tearing down these mountains.

So, you have this big dramatic action of the rocks breaking off into smaller pieces and then breaking up into even smaller pieces. But eventually, when you get down to small grains that are just the size of a grain of sand, for example, or even smaller, that's when this process takes over. And even on very gentle slopes, you can start to move these grains of sand or grains of soil downhill.

We're going to go back to the presentation here and look at some pictures of this action. Here's one of those dramatic examples. This is a hill slope. You can see there's all those tarps that are lying there. The reason there are tarps there is because it was raining. And after the rain, that hill slope failed, fell apart, fell to the bottom of that hill slope there on the right, and these poor houses are left high and dry.

Here's one of my favorite pictures. Here's this poor policeman that took shelter under a highway overpass. And he was sitting there, maybe eating some doughnuts, I don't know, waiting for the storm to pass. And what happened? This big mud slide came by very rapidly and covered up his cruiser. Poor fellow.

Here's one of those dramatic examples. You've got this rock face off on the side of the road. And probably because of freeze-thaw action, one of these big rocks that was held up against the side of the mountain got water in it. It froze, it thawed, it froze, it thawed, it froze, it thawed. And it just jacked those cracks apart until eventually the whole hillside failed.

Here is a picture of a hill slope in Los Angeles. And we have that white scarp or wall that you're looking at. It didn't used to be white like that, it used to be nice, and gentle, and grass covered. And it has failed, probably after a rainstorm. You get these amazing downpours of rain in Los Angeles. And it has just slumped down. And all that material at the bottom is what used to be a big hill slope.

Here's another picture of some dramatic destruction along the coastline in California. The bottom one, you have all these houses that have been torn apart when the hill slope failed.

This is a very dramatic and tragic example from Wales where it was a mining town. And they used to mine up on the hill just above this little town of Aberfan and drop all of the mine tailings. The tailings are the waste products that are left over from the coal mining. They would bring them up to the mouth of the mine and simply pile them up in these big piles. And after a big rainstorm, you had this huge landslide that came down and killed a number of people in this town. And you can see where that landslide has come through and destroyed the houses.

This is in South America and two enormous landslides that have come through and destroyed part of this village.

All of those were the dramatic examples. The most common one, the one that you see around here, even on gentle slopes, are soil creep. Very, very slow flow, even after a year if you were to sit there and very patiently watch one particular piece of soil, after a year it might have moved half an inch. So it doesn't seem like a lot, but you do that for 100 years, you do that for 1,000 years, you do that for 100,000 years, and eventually you've moved a lot of material downhill.

With these very dramatic things, those happen once every 10 years, or 20 years, or 30 years. And you move a lot of material down with those, but this other process of soil creep happens everywhere, all the time, it doesn't matter how steep the slope is. You've got to have some slope, but as long as you've got a little bit of slope, you're going to get some of this process. If you have vegetation, it happens even slower, but it does happen. It's facilitated by water in the soil and it's facilitated by freeze-thaw in colder climates.

Here's that picture that I showed you, again. You have all of the grains that are piled up, it freezes, they all spread apart. When it thaws, some of the grains fall down, farther downhill than they started out. And so the one in the bottom right is the grains after a winter and a summer. And the grains at the top left are before all of that began.

And here's an example of that soil creep. We have this beautiful stone wall along the side of this house. And the person that built this didn't build it with that big dip in it. They built a nice flat stone wall that went all the way along there. And over time, that very gentle slope above that stone wall has managed to slowly slump down and deform that wall that way.

So we've seen mass wasting and the pictures that we saw were in the Teton National Park. Now let's take a look at soils and what's going on with those soils.

Badlands are erosion from the Rockies transported from the Rockies all the way across to the Dakotas and deposited over there. And this soil is very clay rich. Clay minerals are ones that create a really hard surface that channels the water away and washes away plants. You just can't grow stuff in this. When you mix in lots and lots of clays into the soil, you turn what used to be very friable, very workable soil into this hard layer.

You know that. Pots are made out of clay. Clay pots, you go out into your garden and you'll find all of these lovely reddish clay pots there. Those are hard. They're just soil. They're a type of soil called clay. They just happen to be really, really hard.

Now those are obviously put into a potter's kiln, and fired, and so on, and so those become really, really hard. But the same process goes on even in nature, even without a kiln, even without taking those soils up to high temperature. You take these clays, and you spread them through the soil, and they make a really hard layer.

And that's why the badlands are bad lands. Beautiful places, nice jackalopes in them, but very, very hard to grow plants because that hard layer channels the water. You don't get the water sinking down into it. It just runs along the surface and washes away any trees that are in the way.

Weathering is the breakdown of the rocks at the surface. Remember, I said water controls it to a great extent. Changing temperatures are the next thing that we're going to talk about and biological organisms, plants, trees, and earthworms and other burrowing insects.

There are materials removed by erosion. I defined erosion earlier, which was taking big rocks, breaking them up, transporting them somehow, by water, or wind, or glaciers, however you want to do it, and then depositing them someplace. Now, if you just take a nice low, flat spot, and you start working on that soil, you get what's known as in place weathering, without the transport, without the final deposition. And that's how you make soil.

If you just take a chunk of real estate right around here in State College or wherever it is it you live, and you start working on that soil, you start heating it up, and cooling it, and washing water through it, and you'll eventually change it enough to produce soil. And if you have earthworms, they help, if you have insects, they help. All these things go into making soil.

Mechanical disintegration was the one that just tore these rocks apart. And then, we'll talk about soil. OK, I'll have to delete some of those.

Biological organisms, tree roots and trunks, burrowing insects, earthworms, all of these burrow down into the soil and they bring material up. One thing they do is they bring deeper layers up to the surface. But more importantly, they let water trickle down to those deeper layers. And water is one of those really important materials they can operate on these soils to weather them and to break them down even more. So, all of these critters have a huge impact on them. And when we sterilize the soil, when we don't allow the soil to have vegetation and sources of nutrients for these insects, then we make these soils less productive.

Here's a beautiful example of how these biological organisms work. There's a tree growing in a rock. There's just a little crack in that rock and that tree has managed to get its roots down into it. And those roots will allow water to get down in there, and they will allow soil to get down in there, and they will allow organisms to get down in there. And over time, that tree and all the associated processes that go along with it will break that rock down into smaller pieces.

Chemical weathering, or in place weathering, is helped most strongly by these very weak acids that are formed when rainwater falls through the atmosphere, picks up carbon dioxide, and creates something called carbonic acid. And these acids fall down onto the rocks or onto the soil and they basically dissolve little bits of the soil. And the water carries away some of those, and it leaves some things behind.

The acid attacks the rock. It depends on the type of rock, temperature, acidity, so on and so forth. But at the end of the day, if you spend enough time doing this, even this really, really weak acid can break down the rocks. And we'll look at one example that produced the soils in the badlands. And we'll go through it quickly, but it's all in your textbook, and you can take a look at it.

In the rocks of the badlands you have all of these small grains that are made up of pieces of granite that have washed down from the Rockies. The rain falls down on them. This rain falls through a CO2 layer, that's what we have in the air around us. It makes this acid, this really weak acid called carbonic acid, and it attacks the granite.

The granite is made up of the number of things and one of them is iron. And as you know, iron simply rusts. You take a chunk of iron, you leave it out in the air, it's going to rust. And that's what will happen. The iron parts of the granite will oxidize, will rust, they will fall out, and they'll stay in the soil. That's one of the reasons that some of these soils have this reddish tint to them. It's because all of the iron in the rock has been pulled out and it just lies there within the rock.

The aluminum, potassium, and silica are clays and they remain in the soil as well. These don't get washed away. These are minerals that spread out through the soil. And so what you're left with are iron and these clays in the soil of the badlands. There's a few other parts of the granite, they all get washed away. They all disappear and go off with the rainfall.

Calcium, sodium, and magnesium all get dissolved into the water. And they wash away to the ocean. The calcium is used by critters in the ocean to make seashells. And the sodium washes into the sea to make it salty. And the magnesium eventually gets deposited at the bottom of the ocean and gets recycled at spreading ridges.

So, this is one process we've gone through. And we'll draw it out here in a second. Granite comes down from the Rocky Mountains, gets deposited in the badlands, the rainwater falls on it, and then these pieces of granite get weathered or altered in place. It's not physically tearing it apart, it's chemically tearing it apart, pulling out the differences constituent parts of it. The irons and the clays stay there in the soil. The calcium, and the sodium, and the magnesium all get dissolved into the water and get washed away. I'm going to go to the drawing tablet here and we'll draw that process.

Rocky Mountains, badlands, mechanical disintegration. that's that freeze-thaw. That's taking rocks, breaking them up into smaller pieces, all of the things, the trees growing in the cracks, breaking them apart, all these other things, and breaking the rocks into the smaller pieces. Gravity and streams carry material downhill and deposit them in badlands.

All right, so that's the beginning of the process. We've taken granite up in the Rocky Mountains, broken them into pieces, washed them down in the streams, and deposited them in the badlands. Then we have chemical disintegration in the badlands that makes soil. In the case of the badlands, it isn't very good soil, but it's soil nonetheless.

You have chunks of granite in the soil. You have rainfall. And in the air, you have carbon dioxide or CO2. These two mix together to make a weak acid called carbonic acid that attacks these granite grains and pulls iron and clays out and leaves them in the soil. The water dissolves calcium, sodium, and magnesium and washes it away to the ocean. So, you're left with this iron and these clays over there because all the rest have been washed away.

In areas with lots and lots of rainfall, at high temperatures, this process happens really fast. And so a lot of the material gets washed away and the soils are very poor. In areas with moderate rainfall and moderate temperatures, this process is not quite so fast and so you end up with good soils, better soils, ones that have more of these natural products, of these chemical still left in them, and so you have richer soils.

Let's go back to the presentation. And we will finish up this process with a review. More heat and more water, more stuff is washed away and the soil has fewer minerals in it. In very, very dry areas, even the calcium, and the magnesium, and the sodium stay in the soil and you get these really, really salty soils, which are really bad to grow anything in. So, paradoxically, you could have either too much water, in which case all of these things will be washed away, or too little water, in which case all of this material will stay in the soil. And you've got to have just the right amount to make it all happen.

Next time, we're going to look at the rainfall. Where did this rain come from? Rain seems to be so important for both producing the rivers that wash all this material away and for producing this acid that can eat away the water and for washing away all of these other materials into the sea. And that's what we're going to look at next time.

Credit: Dr. Sridhar Anandakrishnan

Want another look?

Check out the Unit 5 Overview Presentation used in the online lecture here.