 Today we get to really go into deep time. If you remember, we built mountains, we turned 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 Miller Peak. There's nobody goes there. If you ever get a chance, 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 layman caves, and layman caves is the most beautifully decorated cave. It has all these helectites. If you look in the middle picture here, you'll find that it's what they call helectites. Helectite come from the Greek for helix. And so you get these things that 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 the layman, or very nearly so. The ones on the right, you'll see in a lot of caves, but they're prettier in layman. 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 now. It's a huge glacier, but up on top there are bristle cone pine trees. Now, bristle cones, if you give them lots of water, lots of food, they live fast and they die young. If you treat them like crap, they will 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. It was 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 winners had taken the bark off all the way around and set this little strand in the leaf. And that was keeping this thing alive. It was an amazing, amazing place. Now, if you get a 5,000-year-old tree, you can count the annual layers in 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. And so you can play a game of cross-dating. A tree that is happy grows a big, wide ring some year. And 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. Now, 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. And so if you look at your tree and then you look at the wood next door, you can start mashing 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 house, they're old wood. And 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 caught 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. Now, the oldest tree in 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 wakes also get annual layers. Most lakes do not have annual layers. If you've got worms growing around in the mud, they stir it up and there's no annual layer. But there are a few places where there isn't much oxygen. The lake may freeze in the winter and little bits of clay sift down. And then it falls in the summer and sand washes in and it makes a layer and so you'll get a layer of sand and clay and sand and clay in one per year. And so there are a few lakes that do have annual layers, not all, and a few lakes that are annual and the longest one so far is about 40,000 years. It's a lake in Japan that is... 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 that work that I was involved with counting ice core layers. And 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 and so we finally got to clean off. And so I've actually been there behind me is snow and ice. There's actually an Antarctic picture but I've drilled at both poles. And so this is a picture from Greenland. It's a wonderful day in Greenland as you might imagine. They take us up in the plains 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. Now, 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 up there trying to do science, as you can see here. Actually, this is a Fourth of July festival and Eric Soltzman is in the snow trap over here and this is a world famous scientist, Tater Grittis, who's hitting the volleyball there and this is pretty stride laundry. And 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 one will shine into it. And one hole over here that we're going to put boards on top of and make a lift. 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's a picture of that. It's just a little bit taller than I am here. And you'll notice that there's layers. One 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 more previous summer. So right in 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 the 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. And so you get a unique signal the sun did this. And below that is the next summer down and below that right at the bottom it's getting pretty dark and 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 build an ice core. Big drill you paid for it. 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. And there's Catherine from Alaska sitting on top of the drill. This is a big science. In the under snow laboratory we had this giant lab cut in the snow. You've got an astroturf on the floor so that you don't wear out the snow and the walls are snow and 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 Timble of the University of New Hampshire and there's an ice core. And there they sit. These are from 1547 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 Toppe who 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 you look on its side so you'll see the layering in the core is still there and the blue bands are the summers so this is a summer and this is a summer and so we have come down and we can still see these and we can still count them. Now 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 we have to do is we have to check if I tell you that that's an annual layer I better do more than just telling you that I got to show you something and so we did several things I counted and Kurt counted and Demi's counted and Tony Gao counted we recounted I went to a Fraser and Denver the National Ice Core Lab and I repeated a half a mile just to see that if I got the right answer we used different techniques in the chemistry there are layers in the electrical properties as well in the appearance and so we do 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 okay this is a big undertaking because the thing is two miles long there's a whole ice core and there's a lot of layers 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 year 1783 there's a big volcano called Waki 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's 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 tiny pieces of volcanic glass you can analyze those with a scanning electron micro probe and you can ask is this the same composition as Waki if we find pieces of the Waki eruption and the layer that we date to the age of Waki we know what we're talking about and if we don't we don't know what we're talking about and 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 when 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 sort of start comparing to the lake people and the trearing people to see if we match them we counted about 110,000 years in the ice core now clearly once we get older than recorded history we can't directly check this but like I say older than history 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 the other records all of this works and so far the ice core that we did in central Greenland is the oldest one that's been done and it's the ice gave 110,000 years now there's more ice below that but it got messed up you remember that mountain buildings sometimes bend things well the glaciers flowing and the bottom is bumpy it's like pancake batter flowing over a waffle iron and so the bottom is messed up but there's still more ice down there so we didn't actually clean out all the ice we got to 110,000 years to that 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 Cooney Holm as a professor at Cornell works all these archeological sites around the Middle East and up through the classical Greece and they actually help in the archeology because they see in the tree rings that they can date these sites and it works and so what we find is 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 haven't gotten down to the rocks yet this is all in the sediment discuss 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 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 technically they count it as being 277 miles long the width is a few miles it's not the longest canyon it's not the deepest it's not the widest there's lots of other, Zion is deep the black canyon that doesn't sit as steeper and you can just go right over the edge of that one and 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 that's a huge amount that nobody ever gets to we had the good fortune to take our costs there the cost students got us safely to the bottom and back when we went down we finished the hike by moonlight headlamp and you can see all our cause 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 to have 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 there's 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 for a long time the native peoples they have reintroduced the California condor you can watch those 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 introduce this flock of condors in the canyon and they're going so this is good if 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 it started that much a year it 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 now we're going to talk about history and we're going to walk out of the canyon here and so I've named all the things you're not going to have to memorize the names of every one of these Leo 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 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 what we've got to do bottom metamorphic rocks think Rocky Mountain, 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 the rocks, the pink actually were melted, they're granites the black around them are just the nicest these rocks are from the heart of a mountain range they have been hot they have been toothpasteed 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 toothpasteed rocks they've been very way deep they started life as sediments sedimentary composition but they've been very 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 and so these hot rocks 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 and 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 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 sediments are broken by 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 those layers are out there and you follow that fault and up here some place 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 start to get complicated already the sediments on top of the old mountain range are thick in fact there's two miles of those sediments and how do you get two miles well they've sort of been dropped in death valley type fault blocks and there's been some push together but mostly pull apart fault things 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 pretty 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 follow a layer along and after you get tired of following that layer along 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 there's an old mountain range the top is eroded and then you sort of put more sediment on top and you drop it down in bases 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 little 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 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 those that mild 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 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 so it's in the mud and so everywhere you go in the marine settings you find things crawling through mud and they leave trackways and you can see the trackways and there's 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'll find mud cracks this is a block that is from the Kyvab limestone that has turned upside down and fell off the cliff 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 angels shale and you can see the cracking that happened and the mud cracks that have happened again, if you make mud cracks you sort of have to give it time to dry out the crack and it pulls down in that two miles of sediment and you don't find many fossils but once you find there's just these algal map 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 map 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 and that's built up and so you see these giant algal map 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 mass 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 the wall of the canyon that was sort of like the modern Nile there's flood plain 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 tracts of blizzards and other things and a place where the mud dried out and then the sand fell down in the crack heading way down here like this sand dune over here sand dune marched across another one came over the top up and then 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 over here something has certainly walked through and you'll find a layer with tracts and then another layer of tracts and another layer of tracts and another layer of tracts and there has to be time to cross 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 that 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 flood plain deposit like the Nile and you find these beautiful ferns that are sitting there and the ferns have to grow fall down deep and then another layer will have more and another layer will have more and another layer will have more and on and on and on and so what I hope that you're starting to see is that there's a lot of time there a lot of time there and so what I want 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 and so this is the old mountain range and then the old mountain range was eroded and so we'll try some other color in here like this they sort of an erosion surface that cut across the old mountain range 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 paths there's little ripples there's mud cracks 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 above that these erosion surfaces by the way have a name they're called unconformities and so there is an unconformity there's a time gap an erosion surface technically an unconformity can also include non-deposition and 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 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 come 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 cracks you have mud cracks you have all sorts of things on the way up you go from being sand dunes to being lakes to be 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 and 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 okay how long people can count annual layers they get a few tens of thousands of years that's nothing the river is carving how long to carve this canyon millions of years and then 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 and so what we can say is that when the early geologists looked at this they said whoa this is old they said we just can't we see this it makes sense 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 a sand dune and then there's another layer and another critter and then the river came in 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 they can 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 state 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 that mess at the bottom is that's been toothpaste in the heart of a mountain rage there's more time there but we don't know how much so they needed plus the time for the oldest rocks they're all messed up technical term okay and that is starting to get to be deep time the little item of written history the much longer scam of annual layers remember still up on top we find archaeology in the mud we don't find archaeology in the rocks and 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 and so what you find is that today most of dating is done in a third way and 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's additional enrichment material that may prove to be helpful that goes into 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 aren't types of atoms that break down all the atoms are sitting there wobbling and sometimes they fall apart and so that's called radioactivity and 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 and so the rate at which one breaks down really is a fundamental of physics it's how stable or unstable that particular thing is and so the rate is basically controlled by fundamental physics now what do I mean by control by fundamental physics what that means is that you cannot glibly suggest oh, didn't they decay at a different rate in the past but just to make that happen you have to change the time since the control of 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 to leave a world that we would recognize and so the decay rate is not something there's lots of people 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 and the sun blew up because the fundamental physics do not allow the decay rate to be changing so the decay rate can't change and it can't change because first of all this business of fundamental physics it is just 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 and look at ice core records if you go and look at light statimates we can date events using radioactivity and we can compare to the annual lives and we agree and so radioactive dating agrees with the annual lives wherever we can check it it works very well it agrees with the uniformitarian calculations it says yeah it is pretty pretty old there in addition uniform calculations and there are many different radioactive clocks we can use potassium argon we can use parenbiodivium and the different clocks agree with each other so various radioactive clocks agree so it gets really really hard now you gotta be a little bit smart you gotta be a little bit smart if you search the web long enough you'll find the 5,000 year old living claim 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 seats and the oil has been down there a long time and if you go ask in that seafloor community it looks oiled because it's eating nothing but old stuff so you gotta 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 basically all volcanic and metamorphic and all other 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's 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 it 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 and 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 and 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're worried about that the enrichment goes into great detail on how this works and so on so you can look at that it is perhaps just a little step aside most of the radioactive stuff in the planet was included in the planet when the planet formed and so most of the earth's radioactivity was included when the planet formed it came from outer space it came from explosion of stars and so it's included when the planet formed a little bit of the radioactivity is made by breakdown of that so it's just a daughter thing so you break down from one radioactivity to another one before you get something stable so a little bit comes from breakdown of these a little more is made by cosmic rays you get zap 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 planet 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 as 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 sort of 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 the fastest estimate is that the planet is about 4.6 billion years old and so that's time that there's been little changes and things probably put a little faster way back in the pre-Cambrian 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 and 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 who was bringing our error 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 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 and so we're going to look a little bit more at the drama that has been brought on this stage over the millennia next time