Geology of the National Parks

Video Lecture


The Unit 8 lecture features Dr. Richard Alley and is 51 minutes long.

Unit 8 Lecture
Click for a transcript of the Unit 8 lecture.

Well, hello. It's a pleasure to be back and chatting with you. As you know, Sridhar and I have been working on this course together. And so, we're playing tag team. I won't actually throw him out of the ring. But we're going to do tag team. And so, now I get to chat with you for a while about some very interesting things. And I hope we have a good time with this.

As you've seen from Sridhar, we've been looking at a big story, a very interesting story, a story that goes a lot of ways. You've watched from Sridhar how the moving continents make big mountain ranges and how some of those mountain ranges explode and give you volcanoes, and how subduction works, and how obduction works, and what happens. You then saw that when you get these big mountain ranges sitting up there, that they have clouds over the top, and rain falls on them. And so, weathering happens and breaks down the rocks.

Then either glaciers come along or rivers come along, and they take the loose pieces of rocks and they head them down towards the ocean. You have seen that if you build a dam on one of those rivers, that you're going to end up trapping sediment behind the dam. And you'll get a big pile of sediment in there.

And if you don't build the dam, if you let the river do what it wants to do, eventually what you're going to find is that the river is headed down to the sea. And it's going to make a giant pile of mud such as the Mississippi Delta. And if you build your cities on top of the giant pile of mud, then they will sink into the mud. And you're going to have problems.

Now you can imagine that eventually we're going to have to close this loop. We don't just build infinite piles of mud that sit there forever. Somehow, that mud is going to make its way down into the deep sea where it can be taken in subduction zones, or it can be squashed in the giant collisions. And it can raise back up in the new mountains and start this over. And so, we're going to now try to close this loop. And we're going to close this loop by going and looking at some coasts for you.

So, let's switch over to some pictures for you. I happen to love coasts. I hope you do. They're very pretty places. They're very interesting to see. And as we're going to see, they are the extra piece that's going to get from the Mississippi Delta down into the deep ocean where eventually its subduction zone can grab something and take it back up.

Here we're going to start with a picture Cape Cod. You might possibly recognize this. It's a very nice coastal place. Cape Cod is going to be a lot of our story. Right now it is a beautiful place. It's a fun place. It is also a place that is disappearing, as we will see.

The magenta arrow sitting out here on the side is pointing at the outer beach. And the outer beach of Cape Cod is marching back. It is being eroded, slowing scratched away at a rate that's sort of that much a year. And if you had coastal property, that means well within your lifetime, it's gone.

Sediment from that, you can see if you look in this picture sort of the whitish stuff down under the sea, say down in here. This is sand that has been washed off of the great outer beach. It's being transported along. It is building the great island of Monomoy down at the bottom. It's also building the Province Lands up at the top. But then, some of it is ending up being dumped into deeper water either that way or that way. And so, eventually, what we're going to see is that Cape Cod is disappearing. It is a pile that is not sustainable. And, in thousands of years, it goes away.

Before it goes away, let's take some pretty picture looks at it because it is a beautiful place. We actually get to summer there sometimes. And we have kayaks. And so, we can go out kayaking and see what you can see. We also have daughters who like puns. So, one good tern does deserve another.

When we go to the Cape to visit, this is at Eastham. It's sort of about there. And when we go, this is the Salt Pond Visitor Center. And we like to stay right over here just out of the picture. And we take our kayaks, and we go out on the great Nauset Marsh which is sitting out here and goes for very long distances. And it's absolutely chock full of birds.

The wetlands are the productive, they're the nurseries of the ocean. They're the things that grow the fish. They grow all sorts of things. And they grow plenty of birds that can take care of them. You'll notice another bad pun, a fish that doesn't wash out may realize too late that he blew it. These are Great Blue Herons, of course, the big wading birds. There are lots of little wading birds around the marsh. Flocks of birds come through. You can just sit there in a kayak and watch for days and just keep seeing new things as they come along.

The marsh is a mixture of sand, a mixture of mud underneath, plants that are growing up, water that is sloshing in with the tides, and bringing in nutrients, and raising young things, and what have you. It's really a wonderful place.

The Cape is changing. This lighthouse, Nauset Light, almost fell into the sea because the outer beach was catching up with it. And they, just in time, hauled it back. They are hauling lights back all up and down the East Coast. Because wherever the lights are sitting on sand, the beach is heading towards them.

The Cape is there because of the glaciers. Sridhar talked to you about the glaciers. Most of the Cape is sand, and gravel, and mud. But it has little rocks like this one. This is Great Rock or Doane Rock. And it is 30 feet high, something like that. And it extends that far down below. The wind did not bring it. A glacier had to bring it there. It is just sitting there in the sand, the giant rock cliff, where the glaciers were. The glaciers made this big pile of sediment, and the ocean is slowly taking it apart. OK, a beautiful, beautiful place, a wonderful beach, a wonderful place to go visit.

Now, I'd like you remember that there's lots of different kinds of coasts. There's muddy coasts. There are rocky coasts. There are fjord coasts and so on. And so, I've got a picture here taken from space of Acadia, which is very, very different place up in Maine. And the yellow arrow that you can see pointing up there is pointing at a fjord. The US doesn't have many fjords on the East Coast. Although we have many on the left. And this sticks up into the heart of Acadia National Park.

Acadia National Park is renowned for the beautiful art that's been done of it. The Hudson River School painters love to go up and paint naturalizing sorts of things up there. The Rockefellers helped preserve this. I love the oil money going back into the public pot. There are these beautiful carriage roads that one can go ride your bicycles on if you ever get there.

Of course, the interest there is still the coast. It is a rocky coast now, not a sandy coast. The glaciers cleaned this off. They took the sand down to Cape Cod, and they left the rocky roots of the old mountain range sticking out where you can see them at Acadia.

Of course, they put the lighthouses on here. But this lighthouse is sitting on rock. And so, this one is actually not having to be moved back from the coast because the rock erodes a little slower than the sand does. And so, it's sitting there fairly happily at this point.

The glaciers carve these beautiful troughs through the mountains. And they're filled with lakes if you get inland and ocean if you get out to the coast. Here's a loon watching you right there. Wonderful, wonderful place.

Again, this is cleaned off by the glaciers. And so, the sand is down making Cape Cod. And you can see that hard rocks sticking out instead. And so, this would be granite sticking out there watching you very clearly right there.

You go down to the beach, there aren't many beaches. It's mostly rock. Where there are beaches they often are a little coarser. So, you'll see fairly large pieces down here. You would not go and sit quietly on the beach and play with the sand running through your fingers. These are rocks. And that's sort of one beach in that area. But there's not much.

And most of the beach at Acadia is more like this This is actually the bare roots of the mountain range, the granite hanging out where the waves are breaking against it and making great blow holes and spray flying when the storms come in. So, this is a very different sort of place.

You can see the tracks of the glaciers. You might remember that glaciers tend to streamline things as the ice goes over. And then, they pluck things loose from the end. And so, the glacier that went over this, and cleaned if off, and taking the sand down to Cape Cod, went the direction of the red arrow. So, you can see what the ice was doing as it came through.

OK. So, that is a little start of some of our coasts. They're beautiful places. They are mostly moving back at this point. Acadia is not. It's hard. It's sitting out there. But Cape Cod is moving back.

And so, let's talk a little bit about how coasts work, and what coasts do, why they tend to be moving back. And then, let's close that loop and show you how the mud of the Mississippi is eventually going to get down into the subduction zone where it can come back and make new volcanoes and come back over again.

To do this, we're going to start talking about waves. The main energy source for the coast now is not gravity. It's not glaciers or rivers going downhill. It's actually the wind-driven waves coming in.

So, let's imagine that you're way out in ocean somewhere, and you're in your boat, and that you have some waves going by. And so, let's do something like this. Here is a wave out in deep water. And you can tell which way is up because you have your boat sitting on top of it. There you go, wonderful boat. And let's say that the wind is blowing towards the shore. And so, the wave is moving this way. What happens when the wave gets to shallow water?

And the answer is that as you get a wave into shallower and shallower water, it starts to have friction with the bottom. It feels the bottom. And as it does that, whenever you run into friction, you sort of slow down. And so, as the wave moves in towards the shore, when it comes towards shallower and shallower water, what happens is the front slows down and the back catches up.

And so, if you were to check this wave later as it's gotten closer and closer to the shore, what you're going to find is it's doing something more like this. Here's a house over on the shore. So, you know you're getting there. And now, your boat is maybe in a more precarious position.

So, what happens is the front slows down, the back catches up, and they pile up. You can see this if you ever go down to the shore. You look out to sea, and you can see little waves coming in. But when they get close to shore, they make big breakers. Well, why? The front slowed, the back caught up. And if you take this long thing and you squeeze it, it gets high. And then eventually the top may fall over. And you get a breaking wave, and you can go surfing. So, at any rate, that's the first thing that one sees as the waves change that way.

Now, there's another important part that goes with this. Suppose instead of looking at the wave coming in and piling up, suppose that you had a helicopter, and you fly over the top in your helicopter. So, let's see if we can draw this. We're going to put the land over here. That's our beach. So, over here is land.

And you're sitting in your private helicopter, wop, wop, wopping over the top. And you're looking at the tops of the waves, or the crests of the waves as they come in. So, let's make waves in blue here.

And, typically, the wind will not be blowing exactly at the shore. It will sort of be coming in at an angle. And so, let's suppose that the top of a wave is sitting out here someplace, and the wind is blowing the top of that wave in. And so, if you had a whole line and surfers that were sitting on this particular wave, they'd be going in the direction of the arrow. They'd all be riding down the face of the wave going something like this. So, this is a top view of a surfer. And so, the surfers are all headed it in that way.

Now, what's going to happen? Remember the rule. Wave gets to shallow water, it slows down. And so, the water, as you might imagine, you're close to land up here, and you're far out down here. That's where the surfer is. OK, so, you're close to land at the top. You're far out down at the bottom. What happens when it's close in? The water is shallow. And when the water is shallow, the wave slows down.

So, on this end, when you look at the wave somewhat later, it's only moved a little. Now on this end, where it's far out, the water is deep. And where the water is deep, the wave is still going fast. So, when you look at this wave later, on this end the wave has moved a long distance. And if it moves a little bit on one end and a lot on the other end, what do you find? It's turned a little bit. And so, we have that.

And so, the wave it is now headed in. This side is still going slower. This side is going faster. If we look at this wave later, it keeps slowing down on the shallow end, and the other end catches up. And by the time it gets close to the land, it's coming in something like this. And the motion of the wave, now, is going almost towards the land, not quite, but almost straight in toward the land. And so, waves almost always turn until they're almost coming straight in.

And this is why you can always go down to the beach, and get on your surfboard, and ride the in and out. You never see anyone surfing down the coast. Because the wave is always coming in because it turns this way.

And so, what we find is that all the waves of the ocean turn. And the waves turn to come almost straight in, not quite though. And that's where this gets more interesting. The fast end never quite catches up because, as it catches up, it gets into shallower and shallower water. And it slows down itself.

And so, there's always a little bit of motion along the shore, not very much, but a little bit. And if you've ever gone down to the shore and sort of not paid too much attention. And you're just sort of riding the waves. And you come in, and you go out, and you come in, and you go out, and you come in, and you go out, and pretty soon the guard's chair is way back over that way. And so, you've got to go trucking across again. And so, there's always a little bit of motion along the shore.

So, you come almost right in. But there is a little bit of motion along the shore. Most of the motion is in and out. So, most in and out and a little bit along. So, that's an interesting thing that we can chat about.

I will show you a couple of pictures in a little bit when we switch over here. There are some seasonal effects in this, which is interesting. If you happen to go to the beach in the winter, you may not recognize it. Many beaches in the winter mostly wash away. The sand gets washed offshore a little bit. And then in the summer the sand gets brought inshore. And the reason is that the winter has bigger breaking waves. And the bigger the braking wave, the more in tends to take sediment offshore.

And this is an interesting thing. This one may not be the least bit obvious. But let's go to a new screen here. And let's see if we can do something like this. Here is now a beach face. Up here is the bathhouse where you go changing. And we have to have blue so that we can do water. And so, we have some water coming in here.

And now let's ask something. What happens if there's a giant breaking wave? OK. There's a giant breaking wave. You've got your surfer on this, crazy people out there. And what happens? The water is coming in through the air. It's not surging up the beach face, it's coming in through the air.

What's it going to do? Well, you know what it's going to do. It's going to turn over. And it's going to crash down. And there's going to be a giant BFFFT! And then the water is going to go out along the beach. Well, what does that mean? If the water comes in through the air, it's not bringing much sand in. When it goes out along the beach, it takes some sand out.

And so, at the end of a giant winter storm, the sand-- let's see if we can make some other interesting color for sand-- at the end of the giant winter storm, some of the sand has been transported out here. It's sitting down in there. And some of it may have gone off the edge into deep water.

In the summer when it's nice and quiet, and people are out there bobbing around with their kids and their little boogie boards, the waves are not so big and crashing. The waves come in more like this. And they'll actually bring some of this sand back on. But the really deep sand they won't. And so, the observation is very often in the winter, beaches will be moved offshore. In the summer, they'll be moved back onshore towards your beach. But some of it will get out to deep water and fall off.

So, just a couple of notes here is that big waves come in through the air. And if you're surfing on that big wave when it comes in through the air, it might be very interesting. And they go out along the surface. And because of that, because they're coming in, breaking through the air, they're going out along the surface, they tend to move sand. A little more sand goes out than comes in. Some comes in. If you stand on a beach facing that big wave, you'll get sand in your teeth. Don't worry. But a little more goes out than comes in. So, more sand out than in with big waves.

But with the little waves, it turns out, they come in just a little faster than they go out. They bring a little more in than they take out. And so, over time, they will build it back up. And so, the big waves coming in through the air out along the surface move more out than they bring in. The little waves bring in more than they take out, just a little bit. If you watch a wave as it surges up that beach towards your feet on a nice day, it'll come in just a little faster than it goes out. And as it comes in a little faster, it will bring in a little more than it takes out.

So, the little waves bring a little more sand than they leave with. And they build the beach back in the summer. So, they take a little more in than they take out. They build beaches. But they can't reach really deep.

Waves cannot go two miles down in the ocean, and grab some sand, and bring it up. So, if those big waves took it out to deep water, it's lost. It's gone over the edge. It's headed down to the deep water, headed down to the trench if there's a trench there. It's headed for the subduction zone. It's headed back to the volcanoes. And so, waves can't reach too deep. They can't reach to the bottom of the ocean. And so, when you get sand out to really deep places, it's lost. You need a subduction zone to bring it back. And so, eventually we're going to close this balance, and we're going to get somewhere.

When we think about these waves coming in, they bring it in on each wave. They take it out on each wave with the sand. They bring a little more in in the summer. They take a little more out in the winter. They lose a little bit more down. But you have this motion along the shore going on because the waves don't come quite straight in. So, there's always this little bit of in and out and drifting along the shore.

And so, we call that drifting along the shore a very technical name. We call it longshore drift. And so, the motion along the shore is longshore drift. Somebody lost the A sometime back in history. So, it is a longshore drift. And you're just drifting along the shore.

What you can think of now, the drift of sand along the shore is a little bit like the sand moving down a river. And so, now you can think of the beach as being a river of sand. So, let's call the beach a river of sand. It's very often used in geology. It gets sand where? From the Mississippi Delta, from lots of places that it can pick up sediment. If it's eroding a little bit, it's getting that sediment. It takes it along the shore, and then, eventually, in these big storms, or when it comes to a really deep spot, it loses that sand down into deep water.

And so, it's going to lose sand from somewhere. It has to pick up sand from somewhere. And so, you have to have a balance. Just like a river is getting sediment from mass wasting delivering it, it's losing the sediment down to the delta, or down to the reservoir, down to the ocean. The beach has to be getting sediment from somewhere. It has to be losing sediment to somewhere else.

And so, the beach loses sediment to deep water. Lose sand to deep water, we've seen that already. And it has to get the sand from somewhere. And it has various choices. It's either delivered by the rivers or the waves are going to take apart what ever is behind them to get that sand. So, it gets sand somewhere.

And so, we're going to look at some pictures here now to show you where you get the sand from. We'll come back. So, let's switch over to a pretty picture time. We're back to Cape Cod. And now, we're going to try to talk a little bit about how beaches work. I'm going to show you a bunch of pictures of different beaches, and what they've done, and how they behave. And we'll see if we end up somewhere.

So, this is a weird beach. You don't have to worry too much about this one. This is a very interesting beach. We are actually in Greenland at this point. And you may possibly remember when Sridhar was telling you about glaciers, that glaciers are very big, and they can be very thick, and they can be very heavy. And we know that the land down underneath, down in the upper mantle is soft.

And so, you put a big weight on the land, and it sinks. And you take that weight off, and the land bobs up. But it's slow. It's like sitting on a waterbed that's full of molasses. It doesn't sink immediately. And so, it's slow.

So, what happens is in places where glaciers melt the land is push down, and the ocean comes in. And you get beaches. And then the land raises up. And those beaches get raised out of the ocean. And so, here way over in the corner you'll just barely see the ocean sitting way over there. And here is the top of the old beach that has now been raised 200 and some feet out of the ocean as the land popped up.

And so, you can actually see this particularly one was sort of a delta/beach combination. And you can see where the sand coming down from the rivers was deposited. And it was growing out in the ocean. And then the land popped up. And here's this beach sitting up there in the middle of nowhere on the edge of Greenland. So, that's sort of cool. I don't know what you'd do with that. But it's cool.

This may be a little more interesting. These are pictures. The state of Washington has actually put pictures of their whole coast on the web. And you can find the website here. And if you want to see a particular cool place along the coast of Washington, they've got pictures of it. And that's all right.

And so, this is a place, the Westport jetty. They built this big, big wall that's sticking out into the ocean here. And there's a number of things that we can learn from looking at the Westport jetty. One of them is this business about how waves behave. You'll remember that when a wave hits friction, when it hits shallow water, it slows down and that in deep water the waves are still going faster.

So, suppose you're a wave. And you come in here, and here comes a wave coming in. And so, there is the top of the wave I've drawn in yellow. And the wave is moving in the direction of the arrow that we're showing.

OK, so what's going to happen to this wave when it hits the jetty? Well it slows down on the shallow end by the jetty. And so, if we watch the wave, you'll see the top of it sort of there. And then you'll see that it starts sort of bending around. It's moving fast out here in deep water. And it's moving slow right there in next to the jetty.

And so, later we look at the thing, and you can see that it's bent a whole lot. And so, it's wrapping around. And you look way in there, and you can see actually wave tops that have bent all the way the other way. They're being held slow against the jetty. They're moving faster elsewhere. And they're trying to turn to come in towards the shore. And so, the waves are always trying to turn to come towards the shore. And we see that happening with the jetty.

There's some other things, though, that happened in this picture. You'll remember that the waves don't quite come straight in. They move sand along a little bit. And so, the waves have just a little bit of motion which is tending to cause the sand to go this way. And so, the net is a little bit of sand moving along this way headed that way.

Now, what happens on a river if you have a dam, and you slow down the water. The river is bringing in sand, and it dumps it right front of the dam, and it fills in the reservoir. What happens if you build a dam on a river of sand that is the beach? Well, it does the same thing. It fills in with sand here. The dam of the jetty is trapping sand. You see it in that picture. You see it in the other picture that we have here. And so, this is the sand that has been trapped as it drifts along, and it comes up to this big dam which is the jetty.

What happens then is that the sand is blocked there. And so, clean water gets around. And that water promptly starts picking up sand. And so, you end up with places like this that are eroding because the clean water passing your dam is going to pick up more sand again. And so, you end up with a deposit on one side and an erosion on the other side out in here.

And so, whenever you put a dam like this up, it makes a big difference to a whole lot of things. If you wanted to save your house, and you put up a dam like this, you'd say, wow, I'm happy. And then your neighbor's house would get eroded. And the neighbor would be very unhappy. And so, whenever you tweak something like this, it matters.

There are other ways that sand moved. It's not always in the winter. It's not always very simple. 2005, Hurricane Katrina comes in. It was not a nice hurricane. It was not nice to New Orleans. It was not nice to very big pieces of the coast. Here are a couple of pictures.

Picture in the upper left here is a September 1998 picture of the coast near Biloxi, Mississippi. And if you notice the sort of building that the red arrow is pointing to with the blue roof over there-- somewhere about there there's a nice blue roof-- if you come back and look at the picture after the hurricane, you'll notice that it's different.

And you'll notice a number of other things. There's a very, very beautiful antebellum mansion in the before picture, that white building. And you'll notice that it isn't actually there in the after picture. Terrible, terrible things that can happen.

Pieces of that are now part of the beach. Some of them get washed inland. Some of them are left there. But some of them are down on the beach. And so, if you compare the look of the beach right here before, and you compare the look of the beach afterwards, you can see that nature really is, when the waves come in, they will get things. They will make a beach. They will erode what's behind them if they can.

These are two more. This is a natural setting now rather than a human setting. These are the same places. It may not be immediately evident. The arrows are pointing to things that are matched. In the far upper left of the before picture over here there's a little lake that is outlined right there that the yellow arrow it was pointing to. And that same lake can still be identified in the after picture.

And I hope it is fairly evident that there is more stuff in the before picture, more plants, more sand, and so on, that are available than there is in the after picture. If you look, you'll see some of the sand in the after picture is down there. It's been moved around. It's been eroded. It's been transporting away. So, it's very clear that when those big waves come in, they grab pieces of the beach, and they take them away.

This is not the same hurricane. There are lots of hurricanes. There are lots of coasts. These are before and after pictures for Hurricane Fran in 1996 on the Outer Banks of North Carolina. There's a yellow arrow in the before picture pointing to somebody's really fancy orange beach house up there. Boy, I bet they liked that thing. And then after they eventually had to tear the thing down because this new stream showed up. The beach was torn apart, and it undermined the house, and it started to fall in.

You'll notice that somebody had a really nice beach house right here in the before picture. And that's it in the after picture. It went away. It was not very nice. And so, you can see that there's a lot of changes when big waves come in and they attack things.

This is evidence of longshore drift in a very much nicer place. You don't have to worry about as much. We're back in Greenland now. And what you'll notice is there's ocean that's sitting out here. The waves are moving along this shore a little bit. Boy, that's not very well written. But here's ocean.

Here, this is actually Scoresby Sund in East Greenland. And what you'll notice is that the sediment has been transported across the mouth of a little stream. And so, this sediment bar has been built up here by sediment being moved along. The sediment is headed for somewhere. It's headed for somewhere deep to fall off into the deep water. But on the way it may pause temporarily and give you interesting things like this.

These are some more pictures in Greenland. You have streams that are bringing sediment down from the glaciers and taking them out to the ocean. So here is a big stream. It's carrying lots of mud and sand. And so, it gets all these bars out there. And you can see the muddiness of the water.

And when it meets the ocean, the waves start to move the sand. And the waves make bars-- outer beaches, if you would. And so, you get these beautiful barrier beaches. All of these things have geologic names that you can learn if you get really excited by this, little tidal inlets and all sorts of other things going on.

And so, you see some very interesting things. And it's obvious that the sand is being affected by the waves where the ocean meets the land and the rivers. You get interesting things going on.

This is the same thing except very much bigger. And so, now we're looking down on the Outer Banks of North Carolina. And you have rivers that are coming out like this. And they're carrying sediment. And the sediment gets down to the sea. And the sea stars banging against it. And it builds these wonderful barrier islands that we know along the coast of North Carolina.

And then behind the coast you get the great embayments that people go sailing in, and they have a good time in, and so on. But these are places that when a giant hurricane shows up, it just goes right over the top. That's very low. And it makes a huge difference to what's behind us. And so, when you look at something like this going on in Greenland, you basically have the same thing on a bigger scale going on on the coast of the US.

OK. So, let us go back. And we're now going to try to draw a few more things for you. And then we'll see if we can get out of here without getting too bored with beaches.

So, what we see is that the beach has a balance. It's getting sediment, it's losing sediment. If it loses too much in the deep water, what's going to happen? Your beach gets skinny. If the beach is skinny, what happens? The waves go over the beach, and they take apart whatever is behind it. They take apart houses. They take apart roads. They take apart natural deposits, whatever is there.

If Acadia, if it's really hard rock, they have trouble taking it apart. And you have no beach. If it's soft stuff, they take it apart and they rebuild the beach. And so, what we have to do then is if the beach gets too little sand, if there's too little sand being supplied to the beach, what you just saw from the hurricanes and other things, if there is a little sand supplied to the beach, then the beach will be narrow. And if the beach is narrow, the waves will come right across the beach, and they will tear apart whatever is behind it.

So, the waves will cross the page, and they will erode what's behind it to get sand. And that sand may start out as your car. It may start out as your house. It may start out as a natural deposit from a glacier. It may start out as granite in Acadia, whatever. The waves don't really care. They're coming across the beach, and they're going to beat up whatever is behind it. And they're going to make enough sand so that the waves can't get across the beach anymore.

And so, we have a balance now. You're supplying sand. You're transporting it. You're losing it into deep water. And you have to ask what tweaks to balance. And we humans keep tweaking the balance. For example, we build dams on rivers.

So, suppose you dam a river. What happens? People did this in Olympic National Park on the Elwha. They make a little power out of the Elwha. It's a beautiful river. It sort of cuts off all the salmon that used to migrate up there. It also cut off the sediment that the river was taking down to the coast.

Well, the next thing that happened is the waves just took the coast away. They eroded right across the beach. And the Native Americans that used to shell fish on the beach there can't do it anymore. The Corps of Engineers is spending a lot of money to shore up the harbor next door because there isn't sediment coming in the longshore drift because the sediment is trapped up in the dams.

So, if you dam a river, you stop the sediment supply to the beach. What does that do? It means that the beach will lose sand in the deep water. It won't get as much supplied. And it will narrow. And then the waves come across, and they take apart your house.

So, one of the things that we humans do that matters is tweaking this, if you would. There are some natural things that feed into this as well. And they're probably worth pointing to as well. If we switch back, for a moment, to the pictures, we're going to try to go back and see a particular slide here if I can make this work. And so, we'll view the slide.

This is along North Carolina. And what one can notice in something like this is that many of the East Coast rivers are flooded way inland. When the sea level rose at the end of the ice age, it flooded river valleys. And so, all up and down the East Coast we have bays. The Susquehanna flows into the Chesapeake Bay. Why is a bay there? The sea level rose at the end of the ice age.

And so, if you now switch back over here for a minute, we will see if we can draw something like this. Imagine, if you would, you're looking down, and you have a river. And the river is running out to the sea. And so here comes some sort of a river, and it comes out to the sea. You're looking on this from your helicopter. So, out here is the sea. And over here is the river. And you know that if you look down on the river valleys, that the river is sort of lower. And around it you'll have hills of some sort or another. And so, let's draw in some hilltops like this. And then something like that.

Now what happens when sea level rises? And when sea level rises, the sea will flood up the river valley. And so, you'll have the sea moving inland, and it will flood up the river valley. And then you have a big bay. And so, the Chesapeake Bay, or the Delaware Bay, or all those bays along the coast of North Carolina are old flooded river valleys.

Well, now what happens? The river brings mud down to the sea. Where is the mud going? Is it getting out to the beach out here? And the answer is no. The mud can't get to the beach out there. And the mud can't get to the beach out there because it's piling up here.

And so, because of the end of the ice age, there is less sediment reaching US beaches than there otherwise would. Because the sediment is trapped way up at the head of the embayments filling them in trying to build out to the beach where it can get there.

And so, if we write this down rather slowly, the ice age ended. And when the ice age ended, sea level rose because the ice melted. Remember, right under the ice it's pushed down. And then it bobs up. But most of the rest of the world was not pushed down by the ice. And so, the water coming from the melting ice sheets fills up the ocean. And it gets higher. So, sea level rose.

And as sea level rose, it flooded the rivers, and it made bays. And so, we have all these beautiful bays up and down the East Coast of the US because the ice age ended. And the rivers are now filling the bays with sediments. Rivers filled the bay with mud, sediment, whatever. We'll write mud. It goes faster.

But because they are filling the bay with the mud, that mud is not getting out to the beach. The sand is not getting out to the beach where it can nourish it. And so, the beach gets eroded. It gets narrow. And the waves take apart what's behind it to get their mud. So, the rivers filled way with mud. It doesn't reach the far out coast. It doesn't reach the beach-- reach the beach-- OK, reach beach. And so, we then see erosion going on at the beaches because they're not being supplied the sediment that they should be. And so, both human activities in damming rivers and natural activities in flooding things at the end of the ice age have contributed to this problem.

Now there's some others floating around. Cape Cod, remember, is a pile that was made by the glaciers. It happens to be a pile that was made by the glaciers in a place that there aren't many big rivers delivering sediment to the coast. And so, you go and look at something like Cape Cod, and it is a pile from the glacier. And it is a pile from the glacier that happened to be made in a place that rivers don't supply much sediment. So, there's nothing replacing the stuff that goes into deep water. OK, so there's no big rivers to supply sand.

And you know if you have a bank account, and you take the money out, but you never put any in, eventually it gets empty. And so, as Cape Cod loses sand to deep water, it is not getting a new supply of sand from the big rivers. And so, Cape Cod eventually, over thousands of years, is a goner. It's going to go away. And so eventually you lose the Cape.

You don't have to run screaming and panicking. It will take a long time for that to happen. Naturally, then, another ice age is supposed to come. The glaciers build the thing back up. And it would be happy again. Whether that happens with human activity will matter.

Another thing that's worth noting is that, in many places, we humans are making things a little worse by pumping water, and oil, and gas out of the ground. And when you pump water, and oil, and gas out of the ground, the surface settles a little bit. And if you're pumping them out from under New Orleans, and the surface settles, now you have a problem because then the sea comes in when you have a hurricane. And then you're very unhappy about this.

And so, pumping oil, and gas, and water out from under the coasts makes them sink. From under the beaches. And we do this in some places. We're getting smarter about this. But we're not real smart yet.

And so, if you're interested in this, and so, the net of all of this, the net of our damming, the net of nature, and the net of sea level rising, and sea level is rising. We will talk about the whys a little bit later in the semester. But sea level actually is rising. Mountain glaciers are melting. The ocean is warming and expanding. And so, sea level is rising.

And the net of all of this is that probably 3/4 of the US coast is moving inland. We're losing land on 3/4 of the coast. And about 1/4 is either holding its own like at Acadia, or maybe advancing a little bit in a few places. And so, the net of all of these different things is sort of 3/4 of the coast is losing. It's coming back. The coastline is being lost at this point.

That's a huge thing because we live, a lot of us, along coasts. We vacation along coasts. We love coasts. Coasts are really expensive, and they're wonderful. And everyone likes to go down to them. And they're busily being gnawed away. And that's not a good thing.

Now people try a lot of things to keep the coast from being gnawed away. They go down, and they dredge sand out of deep water. You spend immense amounts of money. And you pull the sand, and you put it back on the beach. And the next storm takes it back to deep water. And then you go it again. And so, people are trying this.

So, what do you do? OK, one thing is beach nourishment. Go get sand out of deep water and pump it back up on land. That's expensive. And it takes a lot of energy. And it doesn't last very long. And put it on the beach.

People are talking about taking out dams in places. We haven't done much of that yet. But they are actually talking about taking out dams and letting the sediment go back down. We actually bought the Elwha dams in Olympic National Park. And we were going to take them out. But now they're paying money from the hydroelectric power into the government coffers. And we're not taking them out right now. So, people are looking at that.

Some people try to change the shoreline. Remember when we saw that big jetty out along the coast of Washington that it entraps sand. And so, you'll see many people who actually have tried somehow to change the shoreline.

Suppose we get back in our helicopter, and we look down. And over here we have land. And out over here we have ocean. And in the ocean we have some sort of a longshore drift that is running this way.

And so, suppose you are sitting down here, and you say oh, my goodness. I've got this wonderful beach house here. And I love my beach house so much. And I want to save my beach house from being eroded. So, I go, and I build a giant jetty that sticks out into the water. They're also called groynes. So you're going to build a dam, or a jetty, or a groyne. They're all the same thing basically. But you may see all those different names somewhere in your life.

All right, so what's going to happen? You're going to be fairly happy for a little while. Because what's going happen is as the sand comes drifting along the shore on the beach, and it gets to where you are, it will start to pile up. And that will make you happy because now you've got a nice beach, and you're not worried about it. OK.

But the question is suppose you have a neighbor. And your neighbor also has a beach house. And the clean water goes around. So, this goes around without any sand. And what does it do? It starts picking up sand. And so, it starts eroding down here. And your neighbor's house falls in. And then your neighbor sues you.

So, when you do this, you trap sand here. But often-- not always-- but you often end up with erosion on the other side as the clean water gets around the jetty or the dam. And so, you may get erosion here. And if it erodes on this side, then your neighbor's going to sue you. And then it's going to be a real mess.

Geologists have looked at this. And they say, you can do things. You can build things, you can pump things, you can take out dams. In the long term they've said, you know, maybe we should just move back from the beach in some places. Maybe the great outer beach at Cape Cod really is a good national park. And if we just stay off of the beach and let nature play with it, and we go down and enjoy it, if we keep some of these beaches in the national park system, we'll be happier.

Credit: Dr. Richard Alley

Want another look?

Check out the Unit 8 Presentation used in the online lecture.