Geology of the National Parks

Video Lecture


The Unit 10 lecture features Dr. Richard Alley and is 48:30 minutes long.

Unit 10 Lecture
Click for a transcript of the Unit 10 lecture.

Today we get to really go into deep time. If you remember, we've built mountains, we've torn them down, we're trying to read history, and we're trying to put events in order. And the last time, we got as far as being able to say, this one is younger, and this one's older. And now we want to put numbers on things. How old is this? What can we do with this? And so to do that, we're going to go through a series of things to try to get ages-- the age of the Earth.

To do that, we get to go someplace really fun. Way out in eastern Nevada in Great Basin is Wheeler Peak. Nobody goes there. It's in the middle of nowhere. But if you ever get a chance to go to Great Basin, you've got to do it.

The real draw at Great Basin is actually a cave. It's Lehman Caves. And Lehman Caves is the most beautifully-decorated cave. It has all these helictites. If you look in the middle picture here, you'll find what they call helictites. Helictites come from the Greek for "helix." So you get these things. They just grow in all sorts of crazy directions like that, and you get just beautiful, beautiful decorations in this cave.

Things like this-- the shields are apparently only known in this cave. It's not quite clear why, but these sort of structures that you see in the left-hand picture are unique to Lehman, or very nearly so. The ones on the right, you'll see lots of caves, but they're prettier in Lehman. So if you ever get the chance to go out to Great Basin, you've got to do it. You'll have stories to tell everyone.

One of the things in Great Basin, though, is up on top of Wheeler Peak-- the glacier is gone new. There was an ice-age glacier. But up on top, there are bristlecone pine trees. Now, bristlecones, if you give them lots of water, lots of food, they live fast, and they die young. If you treat them like crap, they live just about forever.

And in fact, the oldest tree that's ever been known so far was cut on top of Wheeler Peak before it became a national park. It would be 5,000 years old now. The people that did that just walked out and cut a tree. They did not say, let's look at every tree that's out here and find the oldest one. So there's almost certainly older trees up there.

The thing's 5,000 years old. It was living with this little strand of bark that was running up the side of the tree. The sand blasting and the ice blasting and the winds and the hideous winters had taken the bark off all the way around except this little strand in the lee. And that was keeping this thing alive. It's just an amazing, amazing, place.

Now, if you get a 5,000-year-old tree, you can count the annual layers of the tree, and you know it's 5,000 years old. But next to this tree, there's a lot of dead wood. Trees die, and they don't immediately rot up there. It's sort of cold and high and miserable. So you can play a game of cross-dating. A tree that is happy grows a big, wide ring some year.

So a happy tree is making big, wide rings, as I'm indicating on the slide there. And an unhappy tree is making a skinny ring. In a place like the Great Basin on top of Wheeler Peak there, an unhappy tree is probably a cold one. Big volcano erupts, it blocks the sun, it gets cold, the tree doesn't grow very well, and so you have a bad year or two or three.

You get this pattern of good years and bad years. So if you look at your tree, and then you look at the wood next door, you can start matching up the pattern between good years and bad years. And by cross-dating, you can get a longer record than is possible from just one tree. You can find old wood in a variety of places. You find it just dead sitting next to the tree.

You find it in Native American sites. When they use lumber in their construction-- logs in the construction-- those are old wood. So you can work from that. If you go to Mesa Verde, in the upper right there, there's a little yellow arrow pointing to a tree that was used in construction at Mesa Verde. Tremendous park as well. Completely worth your while to visit.

You go to Mesa Verde, and you look-- you can see in the museum where they've cut a tree so you can look at it. You can see in the ruins themselves where you can see the tree rings, and you can look at the good years and bad years. You'll also find in many of the logs where little samples have been taken out for tree ring dating. And this is a little plug put into one of the beams in one of the houses at Mesa Verde. And that sample has been taken out, and it's used for tree ring reconstructions.

The oldest tree is 5,000 years. There is wood next to it that goes back over 8,000 years. The longest tree ring record now is about 12,000 years. And it's actually oak trees and pine trees that are buried up in river deposits in Germany. And an immense, immense amount of effort goes into getting those things. But one can actually count annual layers.

And if you are counting annual layers in trees, the oldest tree is about 5,000 years. But the oldest wood, using cross-dating-- matching overlaps, getting overlap from various trees-- you can now go back to about 12,000 years. That's just in trees. We're not down into rocks yet. We're not into anything old. We're just in trees.

You may know that there are a few places where lakes also get annual layers. Most lakes do not have annual layers. If you've got worms crawling around in the mud, they stir it up, and there's no annual layer. But there are few places that there isn't much oxygen. The lake may freeze in the winter, and the little bits of clay sift down. And then it thaws in the summer, and sand washes in, and it makes a layer.

So you'll get a layer of sand and clay and sand and clay, and one per year. So there are a few lakes that do have annual layers. Not all. And the longest one so far is about 40,000 years. It's a lake in Japan. And you can just imagine how difficult it is to count 40,000 years. They do a lot of checking to make sure it's right.

I'm going to show you some pictures of work that I was involved with counting ice core layers. So I'd like to take you to Greenland. There are some ice cores that are annually layered as well. This is a picture of the edge of the Greenland ice sheet. This is a picture of me slightly younger. We had been working for a while and we hadn't had a shower for a few weeks. So we finally got to clean off.

So I've actually been there. Behind me is snow and ice. It's actually an Antarctic picture, but I've drilled at both poles. This is a picture from Greenland. A wonderful day in Greenland, as you might imagine. They take us up in the planes, and they leave us up there and come back in six weeks to get us. So in fact, these newspapers, the USA Today over here is a month old at this point. It is, actually, perfectly functional. It does the job.

You may know that this is a good day in Greenland. This is a bad day in Greenland. The wind does blow, the snow does go, and it's a little bit different up there sometimes. Where we are, we're two miles up. The ice is two miles thick. It is 200 miles to the nearest rock. And we're sitting out there trying to do science, as you can see here.

Actually, this is a 4th of July festival, and Eric Saltzman is in the snow trap over here. This is a world-famous scientist, [? Peter Grutis, ?] who's hitting the volleyball there. And this is freeze-dried laundry. So all the comforts of home.

Now, what we were doing up there was studying snow and ice. And here's a picture-- I'm the gent down here with no shirt on. And right next to me there is John Fitzpatrick of the United States Geological Survey-- a wonderful scientist. And you'll notice what we've done is we've dug two holes in the snow-- one hole over here that we're going to leave open and the sun will shine into it, and one hole over here that we're going to put boards on top of and make a lid. Now, if we put boards on top of the one over here, and the sun shines in the other one, it will shine through this little wall in between, and you're going to be able to see the layers in the snow.

So let's do that. And here is a picture of that. It's just a little bit taller than I am here. And you'll notice that there's layers. The storm comes in, and it puts down snow, and it comes in, and it puts down more snow. So we see layers. And you'll also notice down a little bit there, about so far down, is marked "previous summer." So right here are layers that were cooked by the sun. We have watched this happen. We have monitored it from space. We've monitored it on the surface. We've done all sorts of things.

When the sun shines, it doesn't get up to freezing. It's still colder than that. But it does change the snow, and it gives a different look. So this light layer, some storm came through, and it sculpted the surface down here. And then it gets a deposit, and then it gets cooked by the sun, and then another storm and another one. And then winter comes, And there isn't any cooking by the sun in the winter, because there's no sun in the winter. So you get a unique signal, the sun did this.

And below that is the next summer down, and below that-- right the bottom, it's getting pretty dark, because there isn't much sun getting to the bottom of the pit. But below that, there's another summer, and below that, there's another one, and another one, and another one, and another one. So we do this. Now, let's drill an ice core. Big drill-- you paid for it, or your parents paid for it.

So we get this big drill, and you take the drill, and you go down, and you pull up ice. And you go down, and you pull up some more ice, and you go down, and you pull up some more ice. There's Katherine from Alaska sitting on top of the drill. This is big science. This is really cool stuff.

And you get the ice cores, and you take them down in your secret under-snow laboratory. We had this giant lab cut in the snow. You've got AstroTurf on the floor so that you don't wear out the snow. And the walls are snow. The ceiling, actually, was boards. But if you need a bigger lab, you get a chainsaw, and you whack out a bunch of the wall and take it out, and you get a bigger lab.

And you take the ice core down there, and you slice the top off the ice, which is going on right here, from Bill Kimball of the University of New Hampshire. And there's an ice core. And there they sit. These are from 1,547 meters down. So that's about a mile down. And you take it, and you slice it, and you look at it. And here's Kurt Cuffey. He was from Penn State. Now he's a professor at Berkeley. And Kurt is looking at it.

And if you look at the core, lo and behold, those sun-cooked layers made big bubbles, and the big bubbles make it look dark in this. And it's lying on its side, so you'll see the layering in the core is still there. And the blue bands are the summer. So this is a summer, and this is a summer. So we have come down, and we can still see these, and we can still count them.

Let's go back over here for a minute. I just told you that we know what we're doing, but why should you believe me? What is it that we could do to do this? So what we have to do is we have to check. If I tell you that that's an annual layer, I'd better do more than just telling you that. I've got to show you something.

So we did several things. I counted, and Kurt counted, and [UNINTELLIGIBLE] counted, and Tony [UNINTELLIGIBLE] counted. We recounted. I went to a freezer in Denver at the National Ice Core lab, and I redid a half a mile just to see that if I got the right answer. We use different techniques. There are layers in the chemistry, there are layers in the electrical properties, as well in the appearance.

So we count in various ways, and we count at various times. We don't cheat. We don't tell each other the answer until after we've done it various times by various people. This is a big undertaking, because the thing is two miles long-- the whole ice core, and there's a lot of layers. So this is a serious effort to do this.

And then we check ourselves. As far back as we can, we look for time markers. And this is probably the most important thing. The year 1783, there's a big volcano called Laki erupting in Iceland. It's a disaster in Iceland. It's blocking the sun, it's putting out poisonous gases, it's killing things. It was a very unpleasant thing. Benjamin Franklin is representing the young United States. He is over in Paris as our ambassador. And he notices the blue fogs that are blowing in, and he says, there is a volcano erupting somewhere.

If we know what we're talking about-- Greenland is right next to Iceland-- you should see the deposits from that big volcano in the snow in Greenland. Now, different volcanoes have different chemical compositions. So you can go and look for the acids from the volcano. You can cut out that piece of ice. You can filter it. You can look for little, tiny pieces of volcanic glass. You can analyze those with a scanning electron microprobe.

And you can ask, is this the same composition as Laki? If we find pieces of the Laki eruption in the layer that we date to the age of Laki, we know what we're talking about. And if we don't, we don't know what we're talking about. So we look for time markers such as the chemically-fingerprinted fallout of historically-dated volcanoes. So how about that? The chemically-fingerprinted fallout of historically-dated volcanoes.

We also can find atomic bomb fallout. We can see when the no-lead gasoline came in, and the lead falls. We can see a whole bunch of things like that. And when we do this, what we find is as far back as history goes, we're good. If I tell you it's 100 years, it's 99 or it's 100 or it's 101. I may have made an error. It could even be 98 or 102, but it basically works that way.

Once we get older than that, we start comparing to the lake people and the tree ring people to see if we match them. We counted about 110,000 years in the ice core. Clearly, once we get older than recorded history, we can't directly check this. But like I say, older than written history, we end up comparing to, do we see the same signal of climate change as they see in the tree rings as they see in the lakes? So older than written history, we compare to other records. All of this works. And so far, that ice core that we did in central Greenland is the oldest one that's been done. And the ice gave 110,000 years.

Now, there's more ice below that, but it got messed up. You remember that mountain building sometimes bend things while the glacier's flowing and the bottom is bumpy. It's like pancake batter flowing over a waffle iron. So the bottom is messed up, but there's still more ice down there. So we didn't actually clean out all the ice, but we got to 110,000 years to there.

So annual layer counting goes way past recorded history. And annual layer counting matches recorded history as far back as we can see. My friend, Kuniholm, who is a professor at Cornell, works all these archaeological sites around the Middle East and up through Classical Greece. And they actually help in the archaeology because they see in the tree rings that they can date these sites, and it works.

And so what you will find to start with is that annual layers go way past written history. And it's still ice. It's still fine. It's still trees. There's no huge things. You go back into an ice age, but other than that, there's nothing weird going on.

And there's very, very high confidence that this is correct. And we're still in the mud and the lakes. We're still in the trees. We're still in the ice. We haven't gotten down to the rocks yet. This is all in the sediment-- the scuzz on top. So this is on top of the rocks.

So now, let's look a little deeper. And to do that, let's go back to the pictures. We're going to look at Grand Canyon, and then we're going to walk out of the canyon. The canyon is incomparable. You've just got to go to the canyon someday. It's just inconceivable that the thing is as big and as beautiful and as wonderful as it is.

This is a satellite image looking down on it. Typically, they count it as being 277 miles long. The width is a few miles. It is not the longest canyon. It's not the deepest. It's not the widest. There's lots of others. Zion is deep. Black Canyon of the Gunnison is steeper. Man, you can just go right over the edge of that one and it's a half a mile down. But the canyon, just taken together, is just about as good as it gets.

The N and S in this picture are pointing to the North Rim and the South Rim, and that's where most visitors go. Most visitors drive into the South Rim, and then they walk down one place or another. They often end up going right down here. Some visitors get to the North Rim. And there's a huge amount of it that nobody ever gets to.

We had the good fortune to take our [UNINTELLIGIBLE] there. The [UNINTELLIGIBLE] students got us safely to the bottom and back. When we went down, we finished the hike by moonlight and headlamp, and you can see all our [UNINTELLIGIBLE] hikers over there on the left. And you can see various people on the way back out-- some filmers and pros and what have you. This was just a wonderful, wonderful undertaking we had.

The path down, you'll see on the left, has a few switchbacks in places. And on the right is actually my cousin cutting up. He actually did not get himself in that situation, but I have seen people in that situation. Every year, they have to get helicopters down in the canyon to get people out who really did look like Chuck does there and couldn't make it. It gets hot down there. It's a long walk.

The wildlife is surprisingly diverse in the canyon. There's a lot of stuff down there. People have lived there for a long time-- the native peoples. They have reintroduced the California Condor. You can watch the suckers. California Condors are great. They're sort of an anachronism. They belong in a world with woolly mammoths and lots of big things that can die so there's lots to eat.

We don't have woolly mammoths anymore, but they're hanging on. And they introduced this flock of condors in the canyon, and they're going. So this is good. As you're hiking, you can watch them soaring.

If you go down to the river, you see rocks, and the rocks are just beautiful. But the rocks are being eroded. They're being cut. The river is working on it. The river is active. The river carved this canyon. That much a year makes you a canyon, and that river, man, when it's flowing and it's got mud in it, it can cut. So you get things such as this. It's really beautiful.

We're going to talk about history. And we're going to walk out of the canyon here. So I've named all of the things. You're not going to have to memorize the names of every one of these. Real canyon aficionados know these. They're old friends. You go down, and the great sandstone cliff of the Coconino, and down to the beautiful hold of the red wall, and the cave sitting in the bottom of that. And down to the deep inner canyon. These are old friends. And if you've actually walked the canyon, you come back, and you really know them in a fundamental way.

So what we're going to do is we're going to stop at the bottom, and we're going to look at pictures of this as we come walking out, which we got to do. Bottom-- metamorphic rocks. Think Rocky Mountains. Think what Sridhar was showing you. Here are pictures right along the river. You can see the river right at the bottom there-- the little bit of green.

And just above it, notice these rocks. The pink actually were melted. They're granites. The black around them are schists and gneisses. These rocks are from the heart of a mountain range. They have been hot. They have been toothpasted. They have flowed and squeezed and bent.

And all along the river, you look up at the heart of an old mountain range. And it's cold now, obviously, but it was hot. It was down there. And everywhere you go along most of the river, you'll see these old toothpasted rocks that have been buried way deep. They started life as sediments-- they're sedimentary composition-- but they've been buried way deep. They've been heated, squeezed, run around, all sorts of changes.

Those rocks, in this picture, are down at the lower left. I'm sort of scribbling on them. That's the heart of the old mountain range. Above that, the blue line marks an erosion surface. So these hot rocks were brought up to the surface. They were weathered. They were eroded. You can find rust down in the rocks that is most at the top and then decreases as you go down.

So you can see that these things were at the surface the way the metamorphic rocks are in Rocky Mountain now. They were eroded on top. So you have to take sediment, you have to bury it, you heat it, cook it, then you have to bring it up, erode it.

And then on top of that, there are more sediments. Now, these are very old sediments. We'll have a look at these in a little bit. But there are more sediments on top of that. And in places there's a lot more sediments, as we'll see in a minute. And then those sentiments are broken by a fault. And this yellow line on the side is actually an earthquake fault.

This particular one is a shoved-together, pushed-together thrust fault, and you can see where it bent the rocks. So it actually was dragging the rocks with it as it went. So you can follow those layers around there. And you follow that fault, and you follow that fault, and up here someplace, there's an erosion surface-- an unconformity. And the fault doesn't go through that.

So you're starting to put together a picture here of an old mountain range brought to the surface, eroded, more sediment put on the top, a fault breaks it, and then it's eroded again up there to make another unconformity.

And that's the story that the rocks are telling. And this starts to get complicated already. The sediments on top of the old mountain range are thick. In fact, there's two miles of those sediments. How do you get two miles? Well, they've sort of been dropped in in Death Valley-type fault blocks, and there's been some pushed together, but mostly pull-apart faulting.

What I want you to do for a moment is go over to the magenta arrow on the far right here. And I'm going to circle the starting point. And these are the sediments we're talking about. Now, go down and measure the thickness of the sediment. So you go down, and you go down, and you go down. There's a big pile of sediment there. That's the river way at the bottom. That would take you some hours to walk.

When you get down here, almost to the river, just follow a layer. So I want you to just follow a layer along. And after you get tired of following that layer along, now you're back above the river. So go down to the river again. You're going through more sediment. And then follow another layer along, and then you're going to go down to the river. And just keep doing that out of the picture. And if you do all of that and add all the pink layers together for the thickness of the sediment, it's two miles.

So this is an old mountain range. The top is eroded. And then you sort of put more sediment on top, and you drop it down in basins, you get two miles of this sediment on top. And then on top of that, up where these yellow things are, it's eroded. It's cut off. There's an unconformity-- an erosional surface there. And then on top of that, we're going to find more stuff.

Now, in that mile of sediment on top, if you go in, in places you will find little ripple marks. The current came along, and it made little ripples. And you're looking in a cliff, and you can see the little ripples. And you'll find little sand dunes where the sediment has been brought along. And then it's been cut off in the top. And then something else has worked on it. So in that mile of rocks on top, you'll find all these interesting things that speak of time.

You will find burrows. Over here on the left above my finger, you'll see where something has crawled through the mud. And as it crawled through the mud, it's changed the conditions just a little bit, and you see a burrow. Lots of things crawl through the mud. They're either hiding so no one eats them, or else they're eating the poop and other stuff that's in the mud.

So everywhere you go in the marine settings, you find thing crawling through mud. And they leave trackways. And you can see the trackways. You can see the trackways. And there's lots and lots and lots and lots and lots and lots and lots of these in the mud. And it takes time to let something crawl through the mud.

You will find mud cracks. This is a block that is from the Kaibab Limestone that is turned upside down. It fell off the cliff. And and you can see where the fill had gone down into the mud cracks. This one over here on the right is down in the Bright Angel Shale, and you can see the cracking that happened and the mud cracks that have happened. Again, if you make mud cracks, you have to give it time to dry out and crack.

These are fossils. Down in that two miles of sediment, you don't find many fossils. The ones you find are just these algal mat deposits. You go down to the creek and you'll find there's scuzz on a rock. And a flood, a little bit of mud will settle on the scuzz-- the algal mat-- and then the algae grow up through it. And down at the creek, the snails eat it. But in a world without snails-- we don't find any snails down at the bottom. In a world without snails, those algal mats built up. So you see these giant algal mat deposits down there, but not much else.

As you go farther, to younger rocks, then you start finding trilobites, like this trilobite tail that I'm scribbling over. And you find beautiful snails and other sorts of things that ate the algal mats. So as you come up through the canyon, you start seeing these changes in who lived where. You might remember, last time we were looking at this picture of the cliff in the wall of the canyon that was sort of like the modern Nile. There's floodplain deposits on the bottom.

And in those, you'll find footprints. You'll find fern fossils and other things. There's a giant sand dune deposit on top. In that, you'll find tracks of lizards and other things in a place where the mud dried out, and then the sand fell down in the crack heading way down here like this. This is a close up of the sand dune over here. The sand dune marched across, another one came over the top of it, and it marched down and so on.

If we go and look in that big sandstone on top, what do we find? We find trackways where reptiles have gone walking along. These are believed to be millipedes here. Something has certainly walked through. And you'll find a layer with tracks, and then another layer with tracks, and another layer with tracks, and another layer with tracks, and there has to be time for the critters to walk across those layers to make the cracks.

You go a little farther up and you find these beautiful trackways. This one is in a little museum at the top of the canyon. And this is an old historical photograph was taken along the Hermit Trail. But you can see that somebody's been here. And many times somebody has been there.

This is in the Hermit Shale-- that floodplain deposit like the Nile. And you find these beautiful ferns that are sitting there. And the ferns have to grow, fall down, be pressed. And then another layer will have more, and another layer will have more, and another layer will have more, and another layer will have more. And on and on and on.

So what I hope that you're starting to see is that there's a lot of time there-- a lot of time there. So what I walked you out of, and I will try to very briefly summarize it here, you go down in the bottom, and what do you find? You find the heart of an old mountain range. And the thing is just beat up and broken and bent and cooked and so on.

So this is the old mountain range. And then the old mountain range was eroded. So we'll try some other color in here like this. There's an erosion surface that cut across the old mountain range like that. And sitting on top of that, there are a whole bunch of layers of rock. And those layers of rock have very few fossils in them. And they're mostly the algal mat deposits. But they're sort of sitting down there at an angle.

And if you add up all the thicknesses-- go this way and that way and this way and that way-- there are about two miles of sediment with algal mats and not much else. There are volcano fallouts. There's little salt crystal casts. There's little ripples. There's mud cracks. There's all sorts of things. There are algal mats down there. But there isn't much in the way of other fossils. And then above that, there's another erosion surface.

There erosion surfaces, by the way, have a name. They're called unconformities. So there is an unconformity-- or a time gap-- an erosion surface. Technically, an unconformity could also include non-deposition-- it just sat there and did nothing for a while-- but usually it's an erosion surface. And there's another unconformity right there.

So this is really getting complicated. And then on top of that, there's another mile of sediment to the rim. So there's one mile of sediment up to the rim. And in that mile of sediment going up to the rim, we see changes in the kinds of fossils. So the fossils change going up. There are actually many unconformities in that. There are many unconformities in the pile.

There are some really cool places where streams actually cut down into a surface, and then a flood came in, and it put little deposits in the stream channels. And you can see these things. You have tracks. You have mud cracks. You have all sorts of things on the way up. You go from being sand dunes to being lakes to being all sorts of different environments up there.

When you get up that mile, if you went up the North Rim, you will look, and the rocks will slant away from you. and they slant down to just under Zion. And then there's another mile of rocks up through Zion, and they slant down under Bryce. And you're up through Bryce, and then they go under something else. And then on top of that are the Native American sites. On top of that are old trees. So above this is Zion. And above that is Arches. And above that is Bryce. And above that are Mesa Verde's people.

The geologists worked really hard to figure this out. It is not easy. The geologists looked at this and they said, wow. This is just amazing. I can't believe everything I can see here. I can't believe how well I can tell this story. And then they said, OK, how long? People can count annual layers, they get a few tens of thousands of years.

That's nothing. They said, look at this. The river is carving. How long to carve this canyon? Whoa. Millions of years. They said, we know how fast mud piles up in the ocean. How long to make two miles of mud, and then to tip it, and then to erode it, and then put more on top? And so the early geologists tried very hard to come up with estimates of this.

So what we can say is that when the early geologists looked at this, they said, whoa, this is old. We see this. It makes sense. We recognize these rocks. We recognize the tracks. There's no magic in getting a critter to walk across a sand dune, but you have to have time for the critter to walk across the sand dune.

And then there's another layer where another critter walked. And another layer, and then the river came in, and then the lake, and then the ocean, and then sand dunes were back. And they said, whoa, this sucker is really old.

And then they did something they called uniformitarian calculations. Big, fancy word. Notice "uniform" in there. Uniformitarian calculations. They said, let's say that things in the past sort of happened at the same rate they do today. Because we recognize the mud cracks, we recognize the fossils, we recognize the tracks. We see what's going on. There's nothing catastrophic in this. It's normal. And they said, at the rates that things happen today, how long would it take to make what we see?

And what they ended up with is about 100 million years to make the rocks. And the "about" there is really fuzzy. Could it be 50 million? Sure. Could it be 200 million? Sure. That's a fuzzy thing. And then they also said, but we need time to erode. There's all those unconformities-- those erosion surfaces. So it's plus the time to erode.

And they said, we can't tell what that mess at the bottom is that's been toothpaste in the heart of a mountain range. There's more time there, but we don't know how much. So they needed plus the time for the oldest rocks that are all messed up. Technical term.

And that is starting to get to be deep time. The little skim of written history, the much longer skim of annual layers, remember, is still up on top. We find archaeology in the mud. We don't find archaeology in the rocks. So now we're looking at something that's really looking very old.

This doesn't quite get us where we want to be yet, because while this gives you the great age, it doesn't really put numbers on-- that 100 million plus plus is still pretty fuzzy. So what you'd find is that today, most of dating is done in a third way. So most dating today-- most ages of rocks-- come from radiometric or radioactive techniques.

So we have to briefly mention a word for this. And it's from these radioactive techniques. They are described in more detail in the textbook. And then there is additional enrichment material, that may prove to be helpful, that goes in even more painful detail of how you derive this and why nobody is pulling your leg when they say they can do this.

What you know is that there are types of atoms that break down. All the atoms are sitting there wobbling and wibbling, and sometimes they fall apart. So that's called "radioactivity," So some atoms break down. And we have a name for that. We call that "radioactivity." And it is sort of a statistical thing. You wiggle and wobble long enough, and eventually you break.

And if you're really stable-- you just can't fall apart easily. It takes you a long time before one wiggle breaks you. And if you're really unstable, you fall apart immediately. So the rate at which one breaks down really is a fundamental of physics. It's how stable or unstable that particular thing is. So the rate is basically controlled by fundamental physics.

Now, what do I mean by "controlled by fundamental physics?" What that means is that you cannot glibly suggest, oh, didn't they decay at a different rate in the past? Because to make that happen, you'd have to change the constants that control the world. And to make that happen, either the sun blew up or it never burned.

There is not much room to tweak these sorts of things and still leave a world that we would recognize. So the decay rate is not something. There's lots of people that say, oh, maybe in the past it ran at a different rate. Well, that is sort of the same as saying, maybe in the past the sun didn't work or the sun blew up. Because the fundamental physics do not allow the decay rate to be changing.

So decay rate can't change. And it can't change because, first of all, this business of fundamental physics. It is very easy, it is very glib, to say, oh, things ran differently in the past. But that has implications.

The other way that it can't change is to use common sense. Now, if you go and look at these tree ring records, if you go look at ice core records, if you go and look at lake sediments, we can date events using radioactivity, and we can compare to the annual layers, and they agree. So radioactive dating agrees with the annual layers. Wherever we can check it, it works very well. It agrees with the uniformitarian calculations. It says, yeah, it's pretty old there.

In addition-- well, abbreviate that. Uniform calculations. And there are many different radioactive clocks. We can use potassium argon. We can use use sarmarium neodymium. We can use rubidium strontium. And the different clocks agree with each other. So various radioactive clocks agree.

So it gets really, really hard. You've got to be a little bit smart. You've got to be a little bit smart. If you search the web long enough, you'll find the 5,000-year-old living clam. If you find something that only eats really old things-- it never eats anything that's alive today, it only eats dead stuff-- it'll look dead. And there are certain places-- there are seafloor communities that are living on oil seeps.

And the oil has been down there a long time. And if you go ask if that seafloor community, it looks old because it's eating nothing but old stuff. So you've got to be just a little smart when you use these things. And if you're really, really stupid, you might screw up.

But basically, it works. There is an example in the text that might possibly be worth looking at. The example in the text is done with potassium and argon. And that one happens to be probably the easiest one. Potassium-40 is a radioactive thing. It happens to be very common in basic all volcanic and metamorphic and a lot of sedimentary rocks. There's a lot of potassium in there. Potassium is very common in the world. It is common, and it happens to occur in minerals. It is solid.

When the potassium breaks down, it takes about 1.3 billion years for half of it to break down. And it changes to a gas. So it's changing to argon-40, which is a gas. Now, why does that matter? When a lava flow comes out on the surface, the gas escapes. So a crystal starts with some potassium and no argon. As the argon is formed by the breakdown, you can see the change in the relation of potassium to argon. So you can figure out how long it's been there.

So because argon-40 is a gas, a lava flow starts with potassium but no argon. So then you can look at the ratio of the two, and it gives you a clock. Now, you do not have to wait a half life to measure the half life. If you worry about that, the enrichment goes into great detail on how this works and so on. So you can look at that.

Just a little step aside. Most of the radioactive stuff in the planet was included in the planet when the planet formed. So most of the earth's radioactivity was included when the planet formed. It came from outer space. It came from explosion of stars. So it's included when the planet formed.

A little bit of the radioactivity is made by breakdown of that. So it just is a daughter thing. So you break down from one radioactive thing to another one to another one before you get to something stable. So a little bit comes from breakdown of these. A little more is made by cosmic rays. You get zapped, and it damages something. And it may make something radioactive. And a little bit is made by humans in giant accelerator labs. But that's about it.

So naturally, those are the sources of radioactivity. Because of this, the plant is actually very slowly running down, because we're slowly running out of the radioactivity that was included when the planet formed. So the planet is slowly losing its energy source for plate tectonics. Because remember, the radioactivity is the heat source that drives the convection cells that drives the volcanoes and the subduction and so on as the radioactivity runs out.

Now, I wouldn't lose any sleep over this. You'd have to wait a billion years before you'd even notice. This is a very slow process, so it's sort of a billion years to notice. But our best estimate is that the planet is about 4.6 billion years old.

So that's time that there have been little changes. And things probably cooked a little faster way back in the Precambrian than they do now. So the planet is estimated to be about 4.6 billion years old-- 4.55, something in that neighborhood-- based on radioactivity. And that actually is consistent with everything else.

So what do we see? Again, this is science, and I hope you've seen just a little bit of the evidence that goes into it. It's not received truth. We're not getting this from somebody whispering in our ear or passing us a particular document. This is science. What I hope you've seen is that just by counting annual layers in the muds, in the ice, in the trees, that we go older than all of recorded history.

If we once look into the rocks, we see things we recognize-- environments we recognize, trackways we recognize, erosion surfaces we recognize. It all makes sense. It's not catastrophes. It's normal processes acting over deep time. We see hundreds of millions of years or more. When you then turn on your radioactive clocks, you can put numbers on that, and those numbers really are very, very old. And they are the most glorious stage for the most glorious drama that you can imagine. So we're going to look a little a bit more at the drama that has been wrought on this stage over the millennia next time.

Credit: Dr. Richard Alley

Want another look?

Check out the Unit 10 Presentation used in the online lecture.