Click for a transcript of the course review lecture.
Well, it's just about time to run. We have been through a lot together. We've been to some of the most beautiful places in the world. We've talked to some of the most interesting people. We've seen wonderful things. You've had a chance to learn how the world works in a really fundamental way. We have crossed over a whole lot of the Earth sciences in a very short period of time.
And now you're going to go off and take that knowledge, and I hope you're going to go on a road trip, you're going to go see something really tremendous, go to some of these national parks, learn some of the things in real life that you've seen in our slides, and have a great time. I hope you know that Sridhar and I are really excited about this subject, that we really believe it matters to you. You don't have these little things that occasionally there's a little grading, occasionally there's a little exam. And so, we thought it would be useful to walk through a little bit of a review with you.
And so, I'm going to start you off on some reviewing things here. This is actually a picture that I took in Alaska of some stuff that was scraped off of an ongoing subduction zone slab. And then, there's been landslides. And there's been glacier erosion. And there's a whole bunch of beautiful things in this national park shown right here on the glacier bay.
And to make it easier, I hope, I'm going to try to sing you the course. I'm going to walk us through a couple of verses, sort of the first half of the course, what Sridhar taught and a little of what I did. Then we'll stop, and we'll write some overheads. And then, I'll sing you through the rest of the course, the stuff that I did. I'm throwing in a little more than we did in the course. One song we could do the whole course and a little extra. And then, we'll stop and write some more things. And then, I will bid you a pleasant good life and hope that we'll meet somewhere in a park down the road.
And so, will walk you through a little singing here. And so, I'm going to try desperately to sing this somewhere vaguely in tune and see what we come up with. The chorus is an opinion, OK. I'm giving you science in here, but when we get down into the chorus, this is an opinion. You don't have to parrot back what I say but I really believe this. So, let's see if we can get somewhere interesting here and see what we can do.
[SINGING] Mountains scrape the rainfall from out the azure sky, and they send it down the river so the valleys aren't dry. They send the silt that grows the crops that feed the folks in town, but in doing this must wear the mountains down. And deep within the planet, 200 miles below all the stately majesty the mantle currents flow. Upon their backs, the continents go slowly drifting by and they raise the mountains back up to the sky. And we're learning how to live with our mother on this fruitful globe beneath the playful sun. The tale is ours to tell if we learn our lesson well. We can prosper, if we don't, our best is done.
Now we'll get you one more verse because Sridhar did a lot of this with you. I hope you remember--
[SINGING] Earthquakes and volcanoes ride the currents of the deep. They're the growing pains of mountains and they keep the rivers steep. They tell us where they'll happen and they even tell us when. If we listen, we'll be ready there and then. The rocks that make the mountains and the muds beneath the sea and the fossils pressed inside them tell of Earth's deep history. Formation and bombardment, blazing heat, and bitter cold, smoothed by living as the new builds on the old. And we're learning how to live with our mother on this fruitful globe underneath the playful sun. The tale is ours to tell if we learn our lesson well. We can prosper, if we don't, our best is done.
So there is about half of the course in a very short order. And I'm going to try to flip over just a little bit. And we're going to change speakers on you. We have this wonderful AV set up from the really good-- Work? And I think we actually will have made this work. And so, now let's see if we can do a little bit of review on things that we've looked at. So, we'll switch over here, and now this is going to be more familiar.
In the course we started sort of back at the beginning. We said, hey, the universe is older than the Earth is. There were some stars. They blew up. They made very interesting things that floated around in the solar nebula. And, eventually, it all fell together to make the Earth. And so, the Earth formed by stuff falling down, under gravity stuff fell together. You know this is really technical. I hope you're with me on this.
So, stuff fell in from here. And when stuff falls together it gets hot. And there was a lot of radioactivity then. And so, the stuff got hot. And so, it heated. And the heat, mostly from radioactivity, and the heat led to melting. And the melting led to separation into layers. And this one you sort of remember, if you've ever driven in the north in the winter, and you drive along and, you get snow and ice on the bottom of the car. And there's rocks in it, and there's salt in it, and there's little dead squirrel parts in it and all that kind of stuff. And you drive in the garage, and it melts, it separates.
And so, in the same way, as the planet melted, it separated into layers, a dense core in the middle, a little scuzz that floats on top that is the crust in the atmosphere, and a big thick mantle in between. So, we looked at the different layers. And then, we said, OK, there is still radioactivity, and that is still making it hot. And so, there's some heat left from a long ago, but mostly there's radioactivity which makes it hot. And when you make things hot, whether it be the Hershey bar in your pocket or whether it be the center of the Earth, they get soft. And so, it makes it hot, and the hot makes it soft, and the soft makes it flow.
And, in fact, down there stirring in the mantle, the mantle currents flow, they have convection cells down there. And so, we have convection down deep in the mantle moving things around. And you may remember that convection sort of looks like this. And on top of that there are big slabs of rock. And on top of that is your house, big house. So, you can possibly remember going over some of this a little bit back.
And so, we have soft flowing down below, and we have hard breaking up above. And we live on the hard breaking so we sort of talk about the hard breaking most of the time rather than the soft flowing. Because we're sort of us-centric most of the time. So, when we looked at this we found that the upper layers of the planet are broken into a few big chunks which we call plates. And so, there are plates that are rafting around on the convection cells.
And you know that if you're on a raft going down the river, if you sit in the middle of the raft, it's sort of boring And if you hang your fingers over the edge they'll get nibbled on by the catfish, or they'll get crushed when you run into another raft. The action is at the edge. That's the bumpering sort of happens.
And so, we looked at the interactions of these plates at their edges. And we said there's really sort of three things they can do. They can slide past each other. They're not coming towards or away, they're just sliding past. And we said in a slide past sort of world you don't get a lot of mountain building but you do get earthquakes. And so slide past you can get quakes.
And the example that we talked about, there aren't many national parks devoted to sliding past. And the reason is it doesn't make big pretty valleys, and it doesn't make big pretty mountains. But the San Andreas fault that tries to knock down LA and San Francisco is a good slide past boundary. And so, we talked about the San Andreas as the big fault that is a slide past in the west. And it serves as a good example of this.
We get national parks at the other two kinds of boundaries. We looked at pull apart boundaries. In fact we started, way back when, at a pull apart boundary. That was Death Valley. And Death Valley, remember, is sort of unzipping California, moving away from the mainland a little bit, not as fast as they think they are but nonetheless they are. And south from there we went out into the Gulf of California, which was unzipping, and out into the great mid-ocean ridges that are unzipping.
And so. what we found at pull apart boundaries is if you start one in a continent you get pull apart faults, you get earthquakes, you get Death Valley. And if it keeps unzipping the rift valleys of Death Valley will make sea floor. And so, eventually, you get sea floor. And we saw that a little bit of melted rock leaks up the crack. You get a little basalt coming up the cracks.
And so, you get basaltic volcanoes that are leaking up from the mantle. And they cool and harden and make the sea floor. And then, there's sea floor spreading. Stuff is moving away from the center of this as things leak up.
And so, you may possibly remember diagrams that sort of look something like this with this going this way, and this going this way, and a red volcano, let's make a red volcano, a red volcano coming up like this from below, and this stuff freezing. And so, they're very interesting boundaries. There's fun stuff that happens at them. And they do give us some national parks.
The biggest deal for getting national parks turns out to be at the pushed together boundaries. And so, we have pushed together boundaries. And this one, there's a couple types. And you will possibly recall that when old sea floor gets really cold it gets dense enough to sink back into the mantle. And so, old cold sea floor will sink into the mantle. And when that happens, you get what we call a subduction zone. So, this is at a subduction zone. And in this subduction zone, we had all kinds of national parks.
We had stuff scraped off the down going slab. And when you scrape stuff off you get the hills of San Francisco, you get the hills that the redwoods are growing on, you get Olympic National Park. So, there's a huge number of fun things that happened out there. So, we have scrape off, and scrape off gives us Olympic. And that was a really fun national park to visit. I hope you remember that one. It was a pretty one.
And we also have a little bit of sediment and water go down. So a little of the sediment doesn't get scraped off, it goes down. Sediment goes all the way down, and water goes down. And what we found is that, when you put water in the system, water lowers the melting point. And so you're taking this slab down. There's friction. There's heating. And you do some melting down there.
And that melting is a very special melt. It has water in it and, it has a lot of silica in it. And so, this makes melt. And the melt comes up, but the melt has water in it. And when the water gets up to the surface either it sort of bubbles off nicely and you get really slow gooey lava flows, or it bubbles off not nicely and you get things that blast into the stratosphere and change the climate and kill everybody around. And you make strato volcanoes, and you make all sorts of interesting things.
So this gave us andesite as the rock. And this was one with more silica than basalt. You sweated the andesite out of the basalt essentially. And you get strato volcanoes. And the strato volcanoes, you will remember, are things such as Mount St. Helens or Crater Lake. And Crater Lake is a big hole. That mountain is spread out across the badlands and out across Yellowstone, and it's spread around the world. And so, we had Mount St. Helens, and we had Crater Lake and other sorts of interesting things that were sitting there like that.
We also know that there are earthquakes at subduction zones. Most of them are the same as other kinds of earthquakes, it sticks and it slips and it sticks and it slips. And there may be some really deep ones that if you squeeze the down going rock it just sort of collapses or implodes. And so, we saw there are earthquakes.
And at subduction zones the earthquakes include normal, or stick slip sort of motion, elastic rebound. We talked about there where they get stuck. They're like a rubber band, and then, boom, they let go. And then there were special implosion earthquakes as the stuff was going way down to great depth, and it's getting squeezed and squeezed and squeezed. And finally, it just caves in. So, we'll call those implosion quakes.
So, there is all kinds of activity going on at subduction zones. There are trenches off shore. And there's volcanic arcs. And so, we have trenches, and we have arcs, volcanic arcs. The Cascades are a wonderful example. The Ring of Fire, the whole Ring of Fire is basically this. So, if you're a Johnny Cash fan, you may remember the Ring of Fire, the Ring of Fire, anyway.
So, that was subduction, pushed together with one side going under the other. We also noted that there is a different kind of pushed together. As long as one side says, yeah, I'll go under, you over, it's fine. They come together, and one goes under. But what happens when two of them come together and no one will go down. They're both too buoyant. They stay on top. And then you get a big collision.
And so, we noted that there is pushed together obduction. Obduction is against, obdurate, obstinate, ob is running up against. And so, pushed together obductions and this is the big collision. Neither side goes down, so neither side down, and so things run into each other. And they get squeezed and squoozed and folded and bent, and you get this kind of stuff going on, whee, and it's a real mess.
The Appalachians are an obduction, the Himalaya today, the biggest mountain ranges are this. And so the Great Smokies are a wonderful example of what happens in obduction. And the Himalaya today are the same thing. And there are many other examples. Around the Urals are another example and so on.
And so, you get pushed together faults. You get folds. So, you end up with pushed together faulting and folding. You get earthquakes, all sorts of things happening, faults, folds, quakes, lots of activity.
A thing that often happens, if you take something that's long and skinny, and you squeeze it, it gets short and fat. And the mountains go up, but the root goes down. And so, we talked about how as you squeeze things in obduction it makes a big root. And the bottom of that root gets really hot. And then erosion takes the top off, and the bottom of the root bobs up. And it brings very interesting metamorphic rocks, and it brings ores and gems and all sorts of stuff from up there.
And so, the collision thickens the rock. And the mountains get a root. And so, you might look at it as something like this, if one side is coming in, and the other side is coming in, and they're headed towards each other, and then you have a giant collision, what do you end up with? So, this is, at some time, this is early, and then when you see this later, what do you end up with? You end up with the top that's been pushed up and the bottom has been pushed way down. In fact, it goes farther down than it comes up.
And so, you thicken it, and you get a mountain, and you get a root. And then when you come in and erode the top, the bottom bobs up. And that brings hot stuff up to the surface. And so, erosion brings rocks from way down, allows the root to bob up the way an iceberg bobs up if you cut the top off. And it brings very interesting things from depth to the surface. And so, you can find rocks that are right at the surface that have been way the heck down earlier.
And that was sort of our complete picture of the big stuff. Now there's volcanoes, and there's earthquakes, and there's tsunamis and all kinds of dangerous things that go with this. And we talked about those a little bit. We also talked about one other thing that was floating around here which is, this was the interactions of the plates.
But the mantle is still down there churning away deeper. And occasionally there are these things called hot spots. And the hot spots are coming from below the plates. They're coming from way down. They're like giant thunderstorms or giant atom bomb explosions that are coming from below, and they're pushing up. And they will poke through the plates. So, we also noted that there are hot spots, hot spots, and they come from below the plates. They may come from the core mantle boundary, probably some of them are deep and some of them are not quite so deep.
Normally they're sort of like sea floor rocks. They're basalt. They erupt quietly at volcanoes. So, they usually make quiet basaltic volcanoes. And quiet means that it'll throw a rock a mile but it won't throw it 100 miles, so, quiet basaltic volcanoes. You still don't want to go have your picnic right next to one unless you're fairly careful.
The best example of this is the hot spot of Hawaii, the giant island of Hawaii. The volcanoes there have all come from a hot spot feeding up from below. It makes a mountain. The mountain drifts away on the moving plate. It makes a new mountain. It drifts away and so on. And so, Hawaii is the best example of this.
Occasionally a hot spot comes up below a continent. And in trying to wiggle its way through the continent made of all that andesite with all that silica, it gains silica. And as it gains silica it may get water, and then it becomes explosive. And so, Yellowstone is also a hot spot. But Yellowstone, when it blows up, blows up really big. And so, occasionally, a hot spot will get wet and silica rich coming through a continent. It gets water and silica from a continent and silica as it's trying to come through.
Usually, you're out in the ocean because there is mostly ocean, but if you're coming through the continent when you get the silica in the water, then you blow up. And Yellowstone is the best example of such a hot spot that we have today. It may not come from as deep, but there's this long trail, the craters of the moon is out along the trail. And the pot is still under there, it's sort of under the northeast part of the park. And it's still simmering under there getting ready to have the next big one.
So, that is sort of how you make mountains. That's all the big stuff drifting around, that's sort of the whole story of what we did so far. The next thing we did is to say, OK, mountains are not forever. You put one up and something's going to beat up on it eventually. And so, we said, hey, what beats on it? We have the sun.
The Earth's heat comes mostly from radioactivity. The climate's heat comes mostly from the sun. And so, we said sun drives climate. And climate makes rain and snow. And we talked a little bit about how lifting air cools it and makes rain so that you can have the redwoods. So, lifting cools air because it expands and it's doing work. And that cooling makes rain. And that rain can give you things such as the redwoods which we enjoy or I hope you enjoy. I love going to the redwoods. So, that was very nice.
But rain also beats up on rocks. You put water in the cracks, it freezes, it expands. You put water through the air, and it picks up a little bit of acid. You put acid on the rock, and it changes it chemically. And so, the rain, and snow, and temperature, and all the stuff of weather, changing temperature, and what have you, ends up giving us beat up rocks. It changes rocks. And we call those changes weathering because they're done by weather. This is really not rocket science.
So, there's changes in the rock, there's physical and chemical changes. And what they do is they make little pieces or they make stuff that dissolves and washes away. And so, we looked at this is to give little pieces. The little pieces give you soil. You can grow crops because things get broken up like this. And they give you things that dissolve and wash away. And the little pieces sit around for a while in that soil and it's good. But then they sort of slide downhill. And that's mass movement. And if your house is on top of that when it happens or your house gets buried by that, then that's bad.
And so, eventually, you get gravity pulls the little pieces down. And so, we called this mass wasting because it's mass and then you're sort of wasting it, you're not keeping it on your hill or your farm. And so, mass wasting is moving things down. And more or less naturally new little pieces are made about as fast as the old ones slide off.
And the sliding off may be very slow, soil creep, a rock at a time or a hill at a time or something like that. It can be really fast, a whole mountainside falls off, and it buries the town, and it kills everybody, and it's not good, and you call it a landslide or something like that. But this goes from a fraction of an inch a year to 100 miles, 200, 300 miles an hour or more. So, there's this great, great range of things. And, eventually, you get to the bottom of the hill but there's a stream there or a glacier. And so, eventually, the mass wasting feeds rivers.
And we looked a lot at the very clear thing that a river is not a water pipe. It is a water and sediment pipe. It has to move the water it receives, but it also has to move the rocks that are sliding into it or are falling into it or creeping into it. And if you tweak with a river it makes a difference.
You put a dam across the river and make a lake, the lake fills with rocks. And the river below the dam comes out clean, and it picks up more rocks so it erodes. And so, you go down the Grand Canyon and the sand bars are washing away because the dam's trapping the sediment, the clean water comes out, it picks up more rocks. And so, the rivers move rocks as well as water.
And you forget that at your peril. Some people have forgotten that on occasion, and then they've ended up being very unhappy about that. And so, if you build a dam to make a reservoir, the lake fills with rocks. It may take a while. It may be very fast. Some of them, in one landslide comes in and boom it's full already, and it's not good with much. And the clean water that comes out tends to wash away sand below. And so, clean water coming out will pick up more rocks.
And so, there is a lot of effort going on in the Grand Canyon now. How do we get the sandbars back? Because the sandbars are where the cottonwood root that grow up that the birds live in, you need these for the ecosystem. It's really hard if you're a deer to stand on the rock wall and get a drink. You sort of want a sandbar there. And so, because the dam is trapping the sand, the river comes out below the dam, it washes the sandbars out of the Grand Canyon, and so, that's a big deal.
Now if you get cold, you don't get rivers, you get glaciers. And they pretty much do the same job. They move rocks and they move water from high to low. And so, the river is taking the rocks and the water downhill, and the glacier is taking the rocks and the water downhill, too.
There are some differences. Glaciers do get to the ocean in Antarctica. They do get to the ocean in Greenland. They don't get to the ocean in many other places.
Let me back up for just a second. Sorry, I should have made a point about New Orleans. So, back up for a moment. If you go back to rivers, when you make a pile of rocks and mud it's sort of squishy. And so, when the river, when the Mississippi, gets down to the ocean it makes a big pile of mud. And so, New Orleans is sitting on some miles of mud.
The deepest piece of the Mississippi Delta is about seven miles thick. And so, when the river reaches the ocean, it may make a big pile of mud called the delta unless it all washes away. If the river is really weak and the ocean's really strong, it just washes it all away to the beaches. But if the river's strong and the ocean's weak, you get a big pile of mud.
So, it makes a mud pile, and we call that mud pile a delta. And the delta of the Mississippi is very deep, and it's very long, and it sinks under its own weight. You make mud and watch it and it sort of goes squoosh. And so the mud squooshes. And if you put a city on top of that, this is a technical term too, mud squishes, you put a city on that, it squishes too.
And so, for years we taught students about what was going to happen in New Orleans. Everybody knew it. It was getting lower and lower and lower as the mud squished. And everybody knew that eventually the hurricane was going to get it. It did.
And now, they're taking your tax dollars and they're rebuilding it so it can happen all over again. And then you can pay for it again. And so, New Orleans has been sinking, sinking New Orleans, and it will sink more. And if they raise it up, that will buy time. It's only sinking sort of that much per year.
But, eventually, it gets there as the mud squishes. There's some other things going on down there as well. You pump oil and gas out from underneath and it squishes some more and you've loaded up. And so, there's all sorts of things there.
So, that was back to rivers. Now let's go around and go back to glaciers. Glaciers are the cold rivers. They grab stuff. They move it down. They go from where there's snow to where there's melt. And there are lots of places across 1/3 of the world that had glaciers fairly recently. And so when we go looking at glaciers what we find is that they have been a lot bigger and a lot smaller. We have ice ages. So we see the history of ice ages.
And the ice ages were paced by the Earth's orbit. The Earth's orbit has wiggles. The spin axis, if this is the North Pole, the spin axis is tilted over a little bit. And it wiggles, it goes a little farther over and a little less over. And if you can think of my bald spot up here as being the North Pole if it stuck straight up and the sun was shining in on my nose, the North Pole would never get a sunburn. Because it's tipped over it gets a sunburn, and if it tips over more, it gets more of a sunburn. It doesn't change the amount of sun reaching the planet, but it changes whether it's on my nose that's getting sunburned or it's on my bald spot.
And so this and a couple of other wobbles, this one and this one, end up changing the sunshine, and those have paced ice ages. And so changes in the orbit paced ice ages. These things take tens of thousands of years. They don't matter for next year. They don't matter for next century. They do matter for 100,000 years from now. And so this is slow, over tens of thousands of years.
It was a very interesting thing, though, that if my bald spot is getting more sun, my nose isn't. And there's some wiggles that mean when my bald spot is getting more sun that the south pole is not getting more sun. But what's happened is the whole world has an ice age together, and the whole world comes out of an ice age together. And the remarkable, remarkable result has been that as the sun changes in the north, it changes dust [? fluxes ?] to the ocean and some other things and that has affected CO2.
And so what we've actually been able to see, and this is a very interesting, very useful result, is that the north, where most of the land is, has controlled the world when it comes to ice ages. So the world has gone with northern sun, has followed sun in the north. The south is coldest when it gets the most sun. And the reason is that northern sun controlled CO2. And CO2 controlled the world's climate.
Now how did that work? We told you a story in the textbook which isn't complete. The ocean is blue and it's not green because there aren't a lot of plants in some places. And there aren't a lot of plants because there isn't enough fertilizer. And sometimes the fertilizer is iron. And dust supplies iron. And when there's an ice age in the north, there's lots of dust supplying iron to fertilize the ocean to grow plants, which take CO2 and make plant which get eaten by animals, get pooped into the deep ocean, and that lowers CO2 in the atmosphere.
And that's part of the story, but that's not the whole story. Don't get too excited about that particular one. The point is is that northern sun has controlled CO2, CO2 controlled the world climate. And this is one of many indications as we look at history to say, hey, CO2 matters. If you tweak CO2, you're going to know about it someday.
So, we looked about that far. And at that point, Sridhar bid you a cheerful good day, and he passed things over to me to chat with you. And so, at that point I'm going to stop for a moment. I'm going to try to switch from one microphone to another. I'm going to try to switch to our words here and give you the rest of the story, the rest of the song.
This will include some things that are extra that we haven't talked about in class. But just give me a moment here. The magic of technology, the guys that set this up are really good, in case you're curious, wonderful people working for Penn State, the e-Education Institute and other places here. And so, if we can get back on tune. I'm terrible on tune.
[SINGING] A little sun and water and CO2 can grow. Oxygen up in the sky, and plants down here below. We burn the plants with oxygen, for sunlight's energy, add nitrogen to plants, that's you and me. All the plants have been recycled through four billion years. Just a little bit was buried deep but nothing disappears. Wheels of industry are driven by this fossil fuel. CO2 may blow the planets cool. And we're learning how to live with our mother on this fruitful globe beneath the faithful sun. The tale is ours to tell if we learn our lesson well. We can prosper, if we don't, our best is done. Computer chips begin their lives as sand upon the shore, the gold that makes the contacts: humans' sweat and ancient ore.
And I will give you the other words that I'm sorry there, yep, there we go.
[SINGING] The salmon and the brown bear are the plankton of the wave. Together we can waste or we can save. She gives enough for all of us to live in harmony, from the tundra to the prairie to the forest to the sea. Pachyderms and people, clams and cedars, moose and mice. We can learn to get along or pay the price. And we're learning how to live with our mother on this fruitful globe beneath the glowing sun. The tale is ours to tell if we learn our lesson well. We can prosper, if we don't our best is done. The tale is ours to tell, let us learn our lesson well. So we prosper and our best is still to come.
So there's a little editorializing at the end there which I know that you are not responsible for. We will switch mikes. And I think we're now on and ready to roll. We'll go back to the writing on things and something maybe I'm a little more familiar with here and see if we can roll through the rest. We're getting closer and closer to recent times. So I'm not going to put quite as much detail on that because you probably are more familiar with it at this point.
What we did next was to notice that that sediment goes down to the beach. And so we chatted a little bit about beaches which are a lot of fun. And we noted that a beach is really sort of a river. There's water moving along it, and it's carrying sediment from here, from the river, down to fall into deep water, to go in the subduction zone, and come back out. And so we looked at beaches as sort of being like rivers in terms of getting sediment from somewhere, sediment from the river, and taking it somewhere dumping it into deep water. And so this is the sediment from the river and to deep water.
And while it's in transport, we enjoy it because we like to go down and see it. If it loses more than it gains, the beach gets narrow. And when the beach gets narrow, the waves come over and they tear apart your house. And so, if it too much loss or too little gain, then you get erosion. The waves are coming across the beach because there isn't enough beach there. As we build dams on rivers, we promote beach erosion. The rising of the sea levels at the end of the last ice age traps sediment way up in bays. It gives you beach erosion.
A lot of things-- the sea level is also rising, and that's drowning the beach, and that narrows the beach, and then you get beach erosion. And that may be the biggest factor right now is that sea level is rising because mountain glaciers are melting, and the ocean is warming and expanding, and sea level is rising, and that in turn gives you narrower beaches because you're flooding things. And as you get a narrower beach, you get erosion of the beach. And about 3/4 of the coast of the US are being pushed back inland right now because of various things but sea level rise may be the biggest one going on there.
So then we said, fine, that closes a loop. We had our subduction zone. We made mountains. We washed them or glaciered them down along the rivers. We took them to the beach. We go along the beach. They fall in deep ocean. What happens? They go down this subduction zone, or they get squeezed in an obduction zone, and you get mountains again.
And so this ended up, once you get to deep water, the sediment can end up either in a subduction zone or eventually getting squeezed in an obduction zone. And after you have been subducted or obducted it goes back to mountains and you can start over again, which is really sort of cool. A lot of these things work, and they work very nicely.
Then we said, fine, while we're waiting for the sediments to all get subducted or obducted, can we learn anything from them? Is there's something interesting going on in here? And so at this point in the game we started looking at those sediments, the mud in the lake, the mud next to the beach, in various places.
And we noted something, that when we look at sediment, we can do a couple things with the sediment. We can tell the environment in which it was deposited. Is this deposit from a lake? Is it from a river? Is it from deep ocean? Is it from a desert? And so it reveals the environment in which it formed.
And this is based on fossils, and it's based on the characteristic of the sediment itself. Glaciers make different things than wind dust. And so the sediment reveals the environment. It also gives us clues to time. And we looked first at relative time, which one came first, which one came later. So we looked at relative age, older or younger.
And we found that we could put things in order. It has to be there before you can cut it, the younger ones are on top. And if they get flipped upside down we saw that we could tell that nature had flipped it over.
And so we looked a good bit at up indicators. And if you didn't get that, it's a cool thing, it'd be nice to know. So we looked at up indicators, the things that will tell you whether nature actually went and turned something upside down by folding it. So whether you had a layer and then, whoa, it got rolled over, and some of it was upside down. We happen to know that there was something on top. And if you find that something that was on top on the bottom it got turned over.
And so it was almost as simple as that. And we looked at mud cracks and other things like that. And so we found that we could tell histories, what happened at a place when. Because if you can tell who's first, who's later, and what the environment was, this is a history.
We then looked at age real seriously. We said, OK, we know older and younger. Can we put years on it? And so we looked at age, and we looked for the number of years. And we did that in three ways. First we counted annual layers. And this takes a lot of pain and agony, and being really careful that you're not fooling yourself, and cross checking, and comparing the written histories and all sorts of things like that.
And what we found is it's simply counting three rings, simply counting lake sediments, simply counting ice core layers. There are more annual layers than all of recorded history. And so the first thing was that there's more years in sediments than all of recorded history. And that is in the mud. That's in the trees. That's in the ice.
We're not even down in the rocks yet. There's a lot of rocks down there. So then we started looking at the rocks. And we found that they are not catastrophes in general. We sort of recognize them. They're from rivers and lakes and sand dunes and things like this. And we tried to add up how much time was in the rocks. And we found an immense amount of time, more than 100 million years, probably a lot more than 100 million years.
So we did these uniformitarian things where we said, at sort of vaguely modern rates, how long to make this? And we saw vast deep time in the rocks. Deep time is a term we love. It just mean old, old, old, old, old. So to explain the rocks we see, no one has come up with any plausible explanation that does not involve deep time.
Then we finally looked at the radioactive clocks. And we were able to put number of years on this. And so we finally ended up using radioactive clocks to get real numbers. And we found the world that's about 4.6 billion years old. And we've sort of recognized it for the last half billion or so. And so this went to really old.
So we came up with something that looked vaguely like this. There's a lot of stories out there, a lot of changes have happened. You could take another course and actually learn more about the details of this history. We did not have time to tell the whole story.
One thing that we did note is that in this story if you put the rocks in order, if you date them, you find that it puts the fossils in order. And so what we saw was a canal engineer who had noted that ordering the rocks orders the fossils, puts them in order from oldest to youngest. And this immediately suggested evolution. It doesn't require it.
Then we found that people learned mechanisms. We can't conceive of how evolution could not occur given the age of the Earth and given what we know about genetics. And we found the record of that in transitional fossils. And so this ordering of the fossils, the law of faunal succession, plus the mechanism and the transitional forms led us to evolution. And it is a very well founded scientific theory. There's no serious problems floating around here with this.
It is something that matters day to day, if you're worried about disease organisms. Because it takes lots, and lots, and lots, and lots, and lots of generations to see much change. If you're looking for change in big critters that reproduce slowly, you have to look at fossils. Because big critters don't breed fast enough to have enough generations to make enough change that you could see it in your lifetime.
On the other hand, if you're interested in some disease that's inside of you and is reproducing like crazy, they can change fast enough that you really need help to keep them in control. And if you don't have that help then periodically lots of people die. And so we saw that evolution is a practical thing. It's something that when you start talking about disease organisms, it is so fast that we really have to know about it. When you start talking about big critters it is so slow that you don't see much change over a human lifetime. So we saw sort of hundreds or thousands of generations to make much change, more generations to see even a little bit of change.
And what that means is sort of two-fold which is, again, when you're looking at disease organisms, you better know evolution or somebody that you know better know it. And so, for diseases, this is fast compared to us. The diseases are outracing us, and we have to really work hard to keep up with them. So this is faster than us. For other things, if you kill off the big critters, if you have a mass extinction, suppose that we didn't have national parks, suppose we didn't save the big critters, evolution will make new ones. But it will take millions of years.
And so essentially, if we lose the biodiversity we have, will nature bring it back? Yes. Will nature bring it back fast enough that it will help us? No, it is very, very slow compared to us. And so for sort of big critters it is very slow compared to human time spans.
So we got through evolution just fine. Then we had a quick look at fossil fuels and biodiversity. And we noted that, yes, there are fossil fuels out there. You can make a lot of money digging them up or pumping them up and selling them to other people. But we noted that eventually the fossil fuels will run out. And so the fossil fuels are large but finite.
We noted that if we burn them all there is very high scientific confidence that we will change the world, big time. So burning all will lead to big changes. And this is very strongly believed scientifically. The evidence for this is very strong. There's no serious argument about this to be honest.
And among those changes will be impacts on biodiversity. If you have Yellowstone and then just people all around it and the critters in Yellowstone need to migrate and they can't get out of there, they're likely to be fairly unhappy. And so this has impacts on biodiversity. And that's where we ended up last time, and so I won't go into great detail on that. But this is something that matters. That gets us to the end.
I hope you know they Sridhar and I and the wonderful people, Eric and others who have helped put this together for you, care very deeply about this. We actually believe in the future. We believe in students. We believe in the inherent cleverness of people. We think that you're going to solve the problems. We think you're going to keep the national parks happy and healthy. We have every intention when we're old of not being feeble and being out in Yellowstone in September, and we hope to see you there. It's going to be a great thing to go do that.
I think we're worried, I know I personally am worried, if we don't use your cleverness, if we don't keep an eye on the future, there's so many of us now that we really can screw it up. And it is possible. Right now people have tried very hard to sort of weigh how much are humans using, and how much do we leave for the elk, how much do we leave for the orchids. And in round numbers now humans and our immediate friends, cats and cows and soybeans and corn, are sort of using half of everything that the world makes available. And we're leaving half to everything else.
And if we keep on the track we're on right now we humans will double our numbers. Now if we double our numbers and keep using as much as we're using right now that means we will be using all. And that in turn means it may be very difficult if you're not a cat, a cow, a corn, or a soybean, or a human. We think you'll figure this out. But we're reasonably confident that if you go blindly into the future and say somebody else will fix it, that it won't get figured out.
Now this is editorial. You don't have to do what I say. I'm not going to tell that you have to put this on a final exam so you can pass. We're staying with science on the science class. But we put our lives into this. We care about it very deeply. And so I hope you will forgive me for five minutes or three minutes of actually selling what I believe.
And what I believe is that this matters, that you matter, and that we need to keep an eye on the future. Because if we don't we'll be very unhappy. And if we do our best is still to come. And so I thank you for joining us for a semester. And I hope to see you at the bottom of the canyon or next to Old Faithful sometime down the road. Thank you and take care of yourselves.