hi I'm Dr Johnson hos and welcome to Earth Parts these are coastal redwoods giant sequoia trees largest trees on Earth not the oldest they're pretty old though the oldest trees on Earth are smaller they're bristle cone Pines but either way if you look at a tree and if you look at its rings you're able to actually look back in time you're looking at older and older material to the core of the tree and because of the growth rings you know exactly how old it is you know that you're looking if you're looking at a 25-year-old tree when it was cut down you can count 25 rings and the wood in the center of that tree is the wood that was laid down 25 24 23 years ago if you're looking back into the past this is the most immediate way you can do with materials that are at hand the study of history and of past events through tree rings actually is called dendrochronology it's the study of using tree rings to date events you can use them to date events because a tree will lay down thinner rings in harsher years when there's less rainfall when it's bad weather in general they will lay down thinner rings and in good times when the climate is more optimal for that species they will lay down a thicker ring of growth for that year and the nice thing is you can look at some very old trees and track back through centuries through many centuries what the record of climate was did it get better or worse in a particular area and you could trace that using dendr chronology and you can actually go back beyond the lives of trees that are alive now that we cut down because trees reach back into the past with their record of what happened you can look at other trees that died before them and line up their growth rings this form of dendr chronology work is called crossdating you can cross date across different slabs of wood that lived at different times if their lives overlapped and it doesn't have to be a tree it can be a piece of lumber used in an archaeological site but using this kind of method of crossdating dendr chronologists are able to go back about 14,000 years to get a record of climate you can read chemical data from the wood itself that tells you information about what temperature it was then and so forth so this is a very useful way we can directly interrogate the past beyond our own lives and in this case beyond the history of human civilization as a tangible record of past events we can even look Beyond tree ring data and we can start looking at things like ice cores if you go to Antarctica and Greenland especially Antarctica has the oldest masses of land ice on the planet and scientists working at Antarctica today are able to go there extract cores from the ice drill down extract the core and examine it in a laboratory setting the advantage of doing this is that in Antarctica snowfall every year is crushed down by the next year's snowfall forming layers like almost like tree rings but in this case horizontal layers of snow Crush down so that if you're looking at an ice core you're looking at a layer by layer year by-ear record of past events in this case you get information not just from the water being the oxygen Isotopes and hydrogen Isotopes for example you also get information about little bubbles trapped in the ice giving you samples of the ancient atmosphere so you can read what the CO2 level was or how much methane there was and so forth pollen grains laid down by wind ash from volcanic eruptions in fact if you look at this ice score you see that dark band toward the end in the foreground that actually represents a volcanic eruption at that year a volcanic eruption occurred and you can count back layer by layer to find out exactly what year that happened in and a great wealth of information is coming out of these ice cores it allows us to directly interrogate the past going back a long way in the case of this ice core Antarctic I scores go back about 800,000 years which gives us a record of history of climate and of large geological events like volcanic eruptions in the southern hemisphere for four times longer than our species has existed this is a wonderful wealth of information for the case of ice core records and for tree rings finding out how old they are is relatively straightforward you just count them back but you can't really do that with this stack of rocks here because we don't know just by looking at it how long each layer took to accumulate now you can look at how fast settlement accumulate in rivers or at Sea today and make an estimate of that and that's what some people during the 1800s around the time of James Hutton and William Smith these people were doing thought experiments just like that and realizing that the history of the planet they were recording trying to understand was immensely longer than they expected that the Earth would have had to be millions of years old for the observable geology of the planet to be there and in fact a scientist of the 1800s named Charles LEL really brought this to the Forefront of popularity as a science he popularized the ideas of James Hutton and of William Smith myth and of others and put it all together into a comprehensive view of geology and he published the first book popularizing geology the principles of geology which became very popular in multiple volumes and it laid down this modern science of geology he named a lot of the geological time periods which I'll talk about in a different lecture but his work really made it possible to start looking at the planet and assessing that in fact it is hugely ancient and no one knew how ancient there was no way to assign an absolute date to the stack of rocks in a sediment column the way that you can with an ice core because a layer of the ice core takes a year to lay down with sediments you could have sedimentary layers where previous sediments had eroded down and new ones stacked on top of that so time can be all strange with these things and relative age dating allows you to peer into some of this you can see what happened first and what happened next and so forth you still don't get an absolute number on this and it really isn't possible to do that with the methods that Charles lyel and people of that time period had but by the mid 1800s and later into that Century a lot of people from a lot of walks of life had become very interested in the concept of the age of the Earth because geology people like Charles LEL had popularized gey is a concept and so the other major scientists of the day basically put in their two cents worth there were many estimates ranging in the latter part of the 19th century about how old the planet was based upon a wide range of things including how fast sediment accumulates an estimate about how long it takes for the oceans to get salty if you assume all the salt is coming from River drainage which it is not we'll cover that in a different lecture there were several serious attempts to estimate the age of the planet one of the most widely known at the time was by one of the preeminent physicists of his era William Thompson also sometimes called lord Kelvin the Kelvin scale is named after him and he looked at it from the point of view of okay I'm not a geologist but I'm going to look at it as a physicist so let's say the earth started in a molten State how long would it take to cool it to the temperature that we see it at today how long would that take to happen and Kelvin estimated using thermal physics calculations sound math that the Earth would be millions of years old and his estimates varied throughout his life uh from about 400 million years as the age of the Earth to I think in the latter days of his his last estimates around 1897 I think it was uh he estimated the Earth was somewhere around 20 to 40 million years old closer to 20 he thought and he made fundamental mistakes that he did had no way of knowing about the reason his estimates were so off is because the of course the Earth did not simply cool from a hot initial State the Earth generates its own heat internally from radioactive decay something he did not know about radioactivity wasn't known yet at that time Kelvin wasn't the only one working on this stuff there were a lot of people at that time period who were also interested in trying to work out independently their own estimates for the age of the Earth which varied from a few million years all the way out to a couple of billion years depending upon what kind of approach they use to try to make the calcul ation and it actually took the discovery of radioactivity orri beel famously discovered radioactivity and subsequently people took up that science and started exploring what radioactivity was all about it was quickly established that atoms that are radioactive are atoms that are decaying to another chemical element they're transmuting into another chemical element giving off radiation when they do it and in fact that radioactive elements radioactive isotopes in nature have a dis distinct very specific reproducible halflife that they will Decay statistically during a halflife roughly half of the original Parent isotope decays to a daughter product Arthur Holmes scientist working later than beel came along and picked up the idea of trying to use radioactivity radioactive isotopes to date rocks his idea was if you started with something like uranium in a mineral and you lock that into the Minal lattice of a crystal grain and let it age over geologic time then it's going to be uranium decaying to lead the amount of uranium will go away the amount of lead will come up and you should be able to use that to estimate the age of the Earth and in fact he did in 1929 he published his estimate of the age of the Earth and it was in the billion year range which was news uh but he also was able to establish that this is a useful science in fact he was put in charge of a committee a committee established by the National Science Foundation and the US National Academy of Sciences to work on the problem of the age of the Earth and so the science of geochronology then gained the ability to do radiometric dating using radiometric dating using the decay of Isotopes and the Clockwork Precision of their Half-Life time periods to assess directly what the age of a rock is by looking at its chemistry what I want to do is demonstrate a little bit of an analogy about how radioactive decay can be used to determine how old rocks are I'm going to use pennies for the demonstration you see where I'm going with this as we go so what I'm going to do is I'm going to take 100 pennies here and I'm going to just toss them on the table and then I'm going to sort them out throw 100 pennies on the table like this get them all flat doesn't matter how they land that's sort of the point here all right so I've thrown 100 pennies down on this table and what I'm going to do is I'm going to I'm going to think about these pennies as radioactive atoms I'm going to think of them as a collection of atoms in a rock and over time radioactive atoms like uranium or potassium 40 will radioactively Decay and essentially transmute into a different element so what I'm going to do here is say let's let's let's treat these 100 pennies as 100 Radioactive atoms and over the course of one of their half- lives statistically speaking about half will Decay so the half life of uranium 238 is about 4 and a half billion years which means that since the Earth has formed about half of it is decayed and about half is still here what we're going to do here is say let's say one half life has occurred and I'm going to take all the pennies that landed tails up heads down I'm going to take the Tails and I'm going to separate them out so I'll do this and by the magic of editing it'll take a lot less time than it will take me in real life to do so I'm G to put them over there so what I'm going to do now all right so I've separated them apart from each other and now what I'm going to do is I'm going to count them up I'm going to count up from this first throw so I'm counting out throws throw zero I had 100 pennies after throw one which is that how many do I have over here 44 I have 44 sort of like half very close to half all right so what I'm going to do now is I'm going to remove these completely from the [Applause] table so these items have decayed they have decayed and so the parent isotope that's radioactive is been DEC a to the daughter isotope in here I'm going to put these aside for now so now I have 44 and I'm going to take these as you might be expecting me to do throw them again and like last time I'm going to separate them out yes see what is that it's Canadian it works still it's a tail that one heads heads heads and heads okay so again again I've separated out uh after throw two and and these are the Tails which I'm going to discard into a box of doter isotope having already decayed from the parent so now I'm going to take this set and see how many I've got 27 27 now for throw three every time I make a throw I have waited a half life length of time conceptually I've let a half life go by here's another one it's getting quicker is it daughter remaining parent after throw three of got got three 6 9 12 15 many thr four okay daughter remaining parent three six8 huh interesting at 305 I only discarded two you see it's not exactly half life is exactly half it is statistically half which means actual population samples that you measure are going to be a little bit clumpy around that but it won't be exactly that in this case or five I've got six of these left so now we're starting to see the statistics of small numbers it should have been eight half of that would have gone down to four well sort of did but not quite that case exactly half throw six gives me remaining of three throw seven I have one left still heads throw eight I have one left and there we go tails the last atom of the parent isotope has decayed took me nine throws to get to zero now if you were to plot that up what you're going to see is a general Decay curve and yep there we have it right there so what this curve shows you is the actual data from this one experiment where the curve shows the numbers declining down and they get to one to one and then to zero and the thing is you can do this over and over again you can do this many times and you will get a slightly different result every time and yep here are some some extra trials that I ran off camera and I'm just adding them to the curve so you can see they're not going to look alike but you do see that there's a very clear Trend there's a very distinct trend of the number of parent Isotopes decaying according to this curve and then bottoming out which means if you were to be able to take that curve and look at for example a point on there you would be able to tell how long how many half lives have gone by if you know how much daughter and parent are both there you could say oh it's this point on this curve therefore it's gone by this many half wies so if I were to hand you a bunch of pennies and say I've just done this experiment I'm not going to tell you how many times I did it but after doing so at this point I'm going to show you that have four no let's make it a little bit more interesting I'm going to say that I've got nine pennies left you could look on the diagram and say okay if you started with 100 let's say we know that in case the experiment would do if we have both parent and daughter present in the Rock then we do if you look at that number of parent and number of daughter then you're going to say oh yeah it looks like I've been through about four Half Lives to get to that number so that's how geochronology Works in very basic terms conceptually we know that atoms are decaying at a statistically known rate over time and what we need to do is find rocks and minerals where the parent and the daughter exist in a way that we can we can measure them later and determine what their ratios are and determine how old the rock is