Geology 21 (Geologic Time)

welcome back guys we’re going to be talking about geologic time today this is a short little lecture but it deals with some important concepts it answers the question of how we know what we know one of the major questions is how old is say a dinosaur bone how old is a rock how old is a geologic event the way that we figure that out is gonna be told right here in this little lecture so hang in there it’s about 30 slides long and I’m sure you’re gonna enjoy it by the way we’re just looking at kind of the penultimate in how we figure out geologic time the Grand Canyon right this is the inner core to the Grand Canyon and we can see a whole lot of rock layers in here the layers of course you can even see here in the rock we see layers within the layers within the layers and so on and so forth so there’s a lot of time being reflected here and of course the time not only to pile the sediment up and to turn it in to rock through the process of diagenesis but also of creating the canyon that then exposes that rock for us takes a tremendous amount of time to be able to do this how do we know you’re gonna see how we know that now alright so we need to establish some basic principles as we go forward right one of things that we need to come up with us how we measure this time and usually we think of in terms of minutes and seconds and hours that’s in our day-to-day human life scales but in the geologic time scale we’re dealing with a lot longer periods of time than minutes we’re dealing with periods of times that could be millions of years right billions of years in some cases so if that’s the case we need something a little different so we need a time scale well rocks the isola talk about the importance of a time scale rocks record geologic and evolutionary changes throughout Earth’s history without a time perspective these events have very little meaning right so if it happened 6,000 years ago if it happened 6 million years ago we need to know the difference otherwise we could really get messed up in our conclusions so there’s two ways that we could do this right we can state things numerical and relative dates so numerical dates specify the number of years that are passed since that event happens an actual number right the limestone is 250 million years off that’s an actual number that we can apply to it by contrast we have relative dates right relative dates place rocks and a sequence of formation so the Hermit shale is older than the Coconino sandstone we do this all the time right it’s one thing to say that if you’re say 20 years old and your little brother is 15 years old those are absolute dates right there’s a five-year difference between the two of you but if you just say he’s my little brother or my younger brother then we don’t really know what the age difference is between you we can glean that if we look at you mid we can get some idea but we really don’t know how old or that age gap is right unless we have that information all right so you get the idea right the relative dates versus numerical dates or absolute dates of course one of the most important principles that we need to start off with is the principle of superposition it’s almost too common sensical to really go over but it is in fact important to talk about because everything else kind of builds off of this so in an undeformed sequence you know a series of layers of sedimentary rocks each bed each rock layer is older than the one above and younger than the one below so provided that there’s been no problems going on during the deposition of rocks the oldest ones gonna be the oldest or the bottom ones going to the oldest and the top ones going to be the X that’s the principle and this principle also applies to surface features like lava flows and beds of ash so you’re not gonna get a lava flow that’s gonna creep underneath other lava flows generally speaking they’re gonna pile on top of one another slope the youngest ones on top and they’ll just want us on the bottom same thing with ash when it falls out of the sky and so when we see this kind of illustrated in this diagram here you see this scientist looking at Lake deposits the obvious thing that we get out of this is the length deposits do not appear to be deformed in any way nice and flat and so we can apply the you know principle of superposition and surmise that the older is going to be on the bottom and the younger is going to be on the top that’s the idea okay now there’s some other principles we need to bring out the principle of original horizontality so layers of sediment are generally deposited in a horizontal position that means flat line okay rock layers that have our observed rock layers that are that are flat have not been disturbed right so here we see on the Colorado Plateau we see these rock layers nice and flat flat flat everywhere throughout here here we see

in these Ordovician rocks the same thing nice flat layers right not performed not disturbed the principle of lateral continuity right beds originated as continuous layers that it’s done in all directions until they eventually thin or grayed out any different sediment type we can see that here right rocks that are on one side of this little gully are probably the same rocks are on the side of the gully it just extends across right there probably there were continuous at one point there wasn’t originally this gap here and we can see this demonstrated here this lateral continuity is that originally there was a rock layer that extended across the valley here and when this valley building event whether it be glaciers or rivers carved this valley out it left it on one side but at one point it was continuous that’s the principle that you need to get in mind right in the matter of fact down here it’s still relatively continuous except for this part up here at the top all right another really cool principle and these are really really neat in geology to find our cross-cutting relationships this is where something cuts across an older feature something that is young so here we see a bunch of layers you see layers here and cutting across it is a dike you’ll notice that the dike is not going in the rock layers it’s cutting across and so the question you have to ask yourself then is what is only is the dike older or is the rock layers older and of course the answer is obvious it’s it’s the rock layers are clearly older the dike has been intruded and it’s right it’s you you you are intruding something that has crossed cross-cutting something that is flat this is undeformed and probably came in this way this came in much later okay inclusions inclusions are fragments of one Rock unit that are enclosed within another rock unit so here we see it this in this case this is an igneous rock we see a rock right but the intrusion or the rock that’s stuck inside of it is actually younger and this is pretty common that we find bits of rock stuck in other on so it’s a matter of fact that’s with sandstone that’s right sand stones and conglomerates and branches they’re little pieces of rock that are older that are now in a much younger Rock say a sandstone and in this case we see a bigger class a big piece of rock that’s stuck in a in another one and this happens where say a volcano is erupting and during the process of erupting it grabs or the lavas moving up is grabs off piece of the country rock or the sidewall rock and takes it up with it into the volcanic eruption and then we find it up here on the surface okay that’s kind of a normal way you get an intrusion so in this case the rock containing the inclusion is younger so here’s younger this of course has to be older right this has to exist before it could be a part of this rock that’s the idea all right another these are really really neat here in Hawaii I’m recording right now we see a lot of these types of things its features okay these are called angular unconformity penalties here in a moment so layers of rock that have been deposited without interruption are called conformable layers in other words where things just continuously happen and change it’s all conformable right there’s it’s a continuous recording of time but an unconformity is a break in the rock record produced by non deposition and erosion or brockett’s in other words a period of time where or location where for whatever reason that deposition stops right the layers just stop forming they might take a break for a while and form again but it’s basically a period in geologic history of lost time right where we don’t see that record anymore and sometimes maybe you might have layers being the positive you get some type of tectonic event like a fold that it’s always wrote it off so it destroys everything off the top and then it starts to pile more stuff on top of it again that that’s one way you get conforming so there are made three basic types so angular unconformity is just tilted rocks that are overlaying by flat lines and that’s what does this here so here we see some nice angular or rocks letter at an angle right here some erosion has happened along its edge and since then these this new layers have been or these new layers have been deposited across the top and those have been tilted even there in that drop okay so other than an angular unconformity can go to a disk Conformity a dis Conformity is simply sedimentary strata on either side of an unconformity you should say in an unconformity and they’re parallel right they’re gonna be parallel in other words you have a layer that’s put down there’s a period of missing time and then all of a sudden more comes in again right so there’s just a period of missing time and everything’s nice and flat nonconformity is sedimentary strata

overlaying metamorphic or igneous rocks right so in other words when the igneous rock is made it’s not recording fossils or anything like that a matter of fact it gets distorted fossils and then for whatever reason am i getting doubled off to a nice flat surface and you get sedimentary layers put on top of it that’s when you get the real recording of real time almost like that tap the tape recorder the difference between that igneous rock and that sedimentary rock is called the nonconformity so here’s an example of how you would create an angular unconformity right you get deposition deformation you getting rosin where you basically chomp off the top of this these rock layers and then you renew the depositions with these angles over here so you get an angular relationship right in here and there’s different ways that you can work out a hypothetical region knowing this information right so here we have maybe you would go out and map it or we get seismic data and we see the land that looks like this and what we could do is we can work back the way this work works here is actually working towards creating this we can work back also right and figure out what happened by using the cross-cutting relationships and we can figure out something for example we know that K has to be much much younger than a and we know that the Dyke had to be younger than both a than ABCD and E okay there’s a lot of different things that we can put together and so there’s an interpretation right we got some layers that were put in maybe in an oceanic environment so this would be the law of superposition from there we can intrude a sill so that this would be the sill right here the cool thing about Asil is that we could date that we’ll talk about how we can do that later on we know that Sylvie is younger than Betsy and E because of the inclusion of the sill frat of I’m sorry the sill of fragments from beds C and even so in other words we have a bit of E in D that’s what we see that that’s the law of inclusions right so next is the intrusion of dike F so we see another dike come in here so this is what they believe the landscape would have looked at some time and we know that because it cuts across everything else right and then at some point this had to be eroded and tilted right so probably that well of course tteyuu the tilting had to probably happen at the same time as the erosion so this gets tilted and eroded this on its bevel off it’s nice and flat and then all of a sudden we get new layers being deposited again maybe the ocean comes back that’s what the interpretation I have here we had g h i j k all being deposited let me wind up for this kind of landscape eventually we are trying now interpret right so this would be a hypothetical region this is a cover of real location so anyway you get the idea take some time maybe pause this slide and see if you can work through it yourself these things are nice little puzzles that you can work through and they’re pretty normal homework assignments and a lot of geology classes to work out the geologic history using these laws that I’ve been cover and another really neat thing to know about the geologic past is the evidence of past life the evidence of past life is usually within the rocks and if we know something about the evidence of the past life we can use it to date rocks so fossils are traces or remains of prehistoric life preserved in rocks so here’s up for example of fossils this is a fossil frog it’s 52 million years old and you can tell pretty quickly it’s a frog it looks very similar to what we see today there are differences well we’re not going to get into the differences here but you know this is a frog it’s 52 million years old and this is the kind of a science some people would say the art but science I’m going through and studying these fossils and interpreting what these creatures look like and how they lived is a study of paleontology so knowing the nature of life that existed in particular time helps researchers understand pass environmental conditions obviously wherever this creature lived it’s a frog it’s an amphibian it had to live near water that gives us clues and insights as to what was going on in that environment that created this fossil there’s different ways that we can fossilized material there’s we can basically go and make a petrified material here this is a petrified wood it’s called permineralization mineral rich ground water flow through porous tissue and precipitates minerals so you get a petrified wood for example it might get covered by an ash there’s a bunch of minerals that gets deposited water usually it’s quarks or something similar to this and it winds up being precipitated into the tissue and it replaces it this is actually even though

it looks like wood there’s almost no wood here this is actually a fossil that has taken the place of the original tree but you can see that we got the structures in some cases we can even get the cells a little bit of information about the cells by looking at these these things really really neat molten caps mold is created when a shell is buried and then dissolved by underground water so you bury it into the ground and basically the thing disappears but the hole that is left is the mold right and you can have other things precipitate inside there you can have course you can have calcium carbonate all kinds of different things form and replace what was inside that hole and that’s called a mold so a cast is created when the hollow spaces of a mold are filled right so here we see a mold right here here we see a cast where it’s filling it in this is really really cool this is actually busted off but the original fossil is gone this is the imprint that it left this is the mold that it left into the rock and then the other side basically took on that shape and we can study this creatures features even though the creature itself is not present there’s carbonization and impressions this is probably the best way that we know about ancient plants in certain smaller life so carbonization happens when an organism is buried followed by compression which squeezes out gases and liquids leaving a thin film of carbon basically the same processes that make coal except for the fact that instead of making coal ore fills – a process where it preserves nicely the leaves and we can see that in fact in coal we frequently do find carved carbonization and impressions of leaves in the coal deposits so it’s effective at preserving the leaves and delicate animals so we also get some information of the on the chemistry of the carbon that tells us information about those things and there’s a lot of interesting technology that is looking at the carbon molecules and their arrangements to figure out information about these fossils especially very very ancient fossils at the beginning of kind of life’s history on earth and of course the impressions remain in the rock when the fifth carbon film is lost and so here we see a clear reptile impression of a lizard of some sort this is I believe dates back to the time of the Jurassic but you get the idea right this is just carbon the actual animal is gone just the carbon molecules forming okay probably the most famous of all fossils in the last 20 years are the amber fossils made famous by the film Jurassic Park and so I decided to go ahead and put a piece of amber in here so amber is the hardened resin of a ancient trees rites this tree sap it’s effective at preserving insects really really good in this case here we see a Jurassic insect and amber really really neat you can get pieces of amber that have dozens of insects in it and you can study them there’s also trace fossils which is indirect right so here we actually see the creature this is a Jurassic insect this is a you know maybe 100 million years old or maybe older sitting here and preserved this insect no longer exists on planet Earth but here it is in the amber so anyways back to trace fossils indirect evidence of indirect evidence of prehistoric life includes tracks burrows coprolites and gastroliths so tracks it here we see paleontologist he’s believes that this is actually a t-rex track it gives the pods at better than 5050 that this is a juvenile t-rex F was walking through here and left this track this would have been at some points before 65 million years ago copper light it is exactly what you think it is just by looking at it it is this Dino do the best we can put it the best coprolites we find actually are you are probably small reptiles that liked them and we could use them to figure out information about their diets so it’s a copper light Burroughs right with they left holes everywhere they went we can actually look at these burrows and sometimes we can find interesting things in them and gastro lists are rocks that a lot of dinosaurs and reptilian cousins of theirs would ingest into their digestive tract and to help with the breaking down of different materials especially those plant materials that were eating would help them grind it up and kind of in their gizzard if you want so there’s certain conditions that favor preservation right most organisms are not preserved there’s only a few that really are for softening rapid burial and the possession of hard parts to increase the chances of present another thing that was really helpful is if you wind up getting deposit or you

die in an area where your body falls into a zone where there’s no oxygen you don’t have any oxygen there’s nothing there that can live to eat you and so your body gets preserved in the sand in the sediment so here we see two creatures that have died these are both dinosaurs relative with small ones but dinosaurs nonetheless and they’ve fallen into this into the sedimentary deposit right next to each other really good details right you can see that ribs the spinal cord the whole thing long necks you can even see the small intricate details of the their hands and feet that hard parts so it’s kind of like everything you need to make sure that you get a good preservation and this is of course it’s some kind of silt or sandstone or probably a silk stone and silts tend to deposit readily and all the time and so yeah you got everything you need to be able to posit these two guys all right so that allows us to do some interesting things right if we’re finding rocks in one place that have certain fossils and we can correlate it with rock soil and then we find another place so that allows us to date rocks even further so for example here we can go in the Grand Canyon we know what the fossils look like and we can go over to Zion Zion is right here then we go to Bryce Canyon and we notice that these things stack up nicely we can look at Bryce and this is what it looks like turns out these two formations down here of a Navajo Sandstone in particular correlate very nicely over into the area that we see under Zion we can go through his eye on through the chi anta all the way down into the Kaibab and the monaco formation these to correlate from Zion National Park into the Grand Canyon so as a consequence we get a very large section of sedimentary rocks that we can correlate across this entire region what’s really cool is we actually have dates on these right so we know from the Grand Canyon what these ages are all the way up to the paleo gene which is the youngest period of time that we see expressed in these rocks really cool so correlation provides a more comprehensive view of the rock record something really big by the way going from the bottom here all the way to the top that is something that geologists referred to as the grand staircase because you’re basically walking up these rock layers as you go across from Grand Canyon through Zion to Bryce Canyon National Park alright so there’s also the court so this would be broad correlations right from all the way across them you know northern Arizona all the way up into Utah we can correlate other things right and the best way of doing this well there’s several ways we could do this right especially in limited areas sometimes they don’t extend the cross right you get a lake in one place you get a lake in another place so you don’t really know if they’re correlated both through environment or whatever the best ways of doing this is to note the position of fossils and to look at the rocks crash let’s read this here often accomplished by noting the position of the bed and a sequence of strata right so we get an idea of whether they’re all about the same age involves matching of rocks of similar ages from different regions and the correlate over larger areas fossils are used for correlation this is the key and so if we’re looking at different say ocean basins worldwide it turns out there are certain fossils that are present at different times that allow us to correlate different areas for example if we’re finding oh I don’t know this fossil here during the Permian period in this fossil here during the Permian period and we find it all over the place we know that there at least time correlative that they correlate in terms of time and so that allows us to be able to know that these were both happening at about the same time so fossils are really big so the principle of fossil succession is really important things that are really really simple begin evolving right into the Cambrian we find really nice things called trilobite switch no longer exists but they were worldwide along the coastlines back then and we find things although kotor narrated period such as human beings or hank they tell us that these things are a quaternary age so for example we get these things called the age of trilobite the age of fishes the age of reptiles the age of mammals the age of mammals at the time we live in and so we’re finding large mammals we know we’re in the age of mammals we’re finding dinosaurs we’re in the age of reptiles if we’re finding the large fishes and the trilobite we know where we are in that succession so index fossils and fossil assemblages so in index fossils are widespread geographically and limited to a short period of geologic time and that’s what this is okay these are basically the main index fossils if you find something for example this fossil here and we find this fossil here we know that their core they’re correlated timelines and they probably lived at the same time or very very close to the same time during the Cambrian period all right so that’s I

know I’m kind of thrown a lot of jargon but this let’s just show how this works right it’s kind of better to just show how it works and you’ll get it so index fossils and fossil assemblages so we have a rock right here’s a rock unit B here’s a rock unit a and we want to know its age right well we know that the one on top is younger the one on below is older but we have some evidence we have some fossils in here and we can use it to look at the different indexes that are in there right so fossil assemblage is a group of fossils used to determine the rocks age so here we got some plants some leaves here’s a dinosaur scallop starfish and we can go in there and Orsi star I’m sorry that’s what the oceanographers like to call it age of rock unit a has all of these in here so we know when these things lived we know that there’s here we see the scallop here we see this what is probably firm the dinosaur here here’s a leaf and we find the star and we find in this case do we find this here I’m looking for it it’s not present up here but that doesn’t mean that it has to be right because these other ones exist and what we do is we find out where they all lived at the same time so this lived during this period of time this lived during this period of time this lived during this pink period of time you’ll notice that it can’t be any older than that because if it was any older we wouldn’t find this leaf anymore that leaf right here it only exists during this period of time so this is one bracket the dinosaur here had the t-rex head had to be in this time so we know that it had to exist during this pink period of time same thing is true down here right we find the starfish but instead of finding the belief right instead we find the trailer right so we know that the trilobite lived only up to this point and this leaf only lived down to this point so this is the only thing that matches all of the data that we see there and that allows us to get an age relative on the honest so this might be Cambrian whereas this up here might be belonging to the age of mammoths or I’m sorry to the age of reptiles because there’s a dinosaur so fossils can be used to infer information about past environments all right so this is kind of almost so obvious right shells can be shells of organisms can be used in four positions of ancient shorelines and seawater temperatures even just on the first order right if you’re finding a t-rex head you’re not in the deep ocean if you’re finding scallops you’re probably somewhere near the coastline if you’re finding firms you’re probably in some type of marshy environment or coastal environment things like this right you’re not going to expect to find a bunch of vulture bones and fossils in the middle of the ocean right that’s the environmental indicators all right so we kind of got the idea that there’s these animals that live at different times and they kind of evolved in into different times at different times I should say so how do we put dates do with them the answer is is that we’ve figured out how to do this using radioactivity that’s the main way that we do it I wish there was a more sophisticated way but this is probably 98% of the ways that we do our absolute dating we get a real number and say how old something is is using radioactivity so radioactivity is the spontaneous decay in the structure of an atom’s nucleus we actually talked about the structure of the atom at a much earlier lecture on the on the principles of matter and how it operates and if you have any need to go back you should go back find lecture and review that before we go ahead if you don’t remember the structure of yeah so I’m gonna assume that you still have that freshly you know fresh in your mind or that you have some fresh experience with it we’re gonna continue so the types of radioactive decay there’s several okay the alpha emission so in other words the decay in that structure of the nucleus we can see an alpha emission which is an alpha particle so two protons and two neutrons are ejected from the atoms come out right and so happens the mass number is reduced by four and the atomic number is lowered by two right because we have two protons that are lost in the process but we add the number of neutrons protons that tells us the atomic mass so think it’s lighter by four and the Ute and the element changes by two down on the periodic table so beta emission is a little different so this is where an electron is simply either ejected from the nucleus and this usually happens at the expense of a neutron so we lose an electron by shooting it out of the nucleus usually from a neutron so the mass number remains unchanged right it doesn’t really change but the atomic number is increased by one why because that Neutron by virtue of shooting out

the electron becomes a proton right remember you know if you this is confusing to you please go back and review that lecture and lastly electron capture an electron is captured into the nucleus so this is where it’s absorbs the exact opposite of the beta emission is the electron capture so in this case a proton takes on an electron it fuses and it becomes a neutron so the mass number will still remain the same is not going to change but the atomic number will be decreased by one why because the proton is now a neutron and anyway so we have the mineral you’re a night here this is a primary uranium or in uranium there’s one of the lead elements that we use to date ancient rocks so here we see this kind of exhibited directly this is a nuclear reactor we see the kind of glowing and the light is due to the emissions going through the water this water here it’s making the water below blue it’s kind of cool so anyways here we see this unstable parent basically we have a proton Neutron situation here and we just take part of it we just shoot it out and that’s gonna change the nucleus it’s gonna become one atom to a different kind of atom this is actually a helium atom that’s emitted in the beta emission the neutron here shoots out an electron it becomes a proton so that changed the nucleus again to something different so it becomes a different element and the same thing here right the electron comes in it combines with the proton becomes a neutron and we wind up with a new nucleus again so this is a different kinds of radioactive decay that we can experience okay now we have the really cool thing about this process in radioactive decay is that it happens at a statistically predictable rate right the amount of decay that will happen so radioactive radioactive dating is completely dependent upon this idea so it uses the decay of isotopes and rocks to calculate the age of that rocks or of that rock and so we wind up with something called a half-life half-life is the amount of time required for half of the radioactive isotope to decay so we might have a hundred atoms in this case and over some period of time we’re gonna wind up with 50 so we can put a dot right there and that’s our first half-life now the second half-life is kind of cool because that 50 is a second half-life away so it’s the same amount of time but we go from 50 to 25 so it doesn’t go from a hundred to 50 to zero that 50 becomes 25 and then it goes half again that 25 becomes 13 that 13 becomes 6 that 6 is going to become 3 that 3 is going to become 1 in half and so on and so forth and of course the number of daughter atoms that we’re going to be accumulating it while we do this also goes up by the same amount right so if one is forming at the expense of the other so the daughter atoms are accumulating while the parent atoms are being destroyed so radio I could parent isotopes decay to stable daughter isotopes that’s the idea when the ratio of parent to daughter is 1:1 one half-life is passed so if we’re looking at something that originally had 100 of say uranium and not only has 25 atoms of uranium to me 75 Adams daughter material maybe LED is the main thing then we know that two half-lives have gone by and so we can use have to figure out how with the Rockets that’s the idea so with each passing half-life 50% of the remaining parent decays to another app that’s what we just discussed as the parent atoms decrease the daughter atoms increase so that one goes up the other one goes down several natural naturally occurring radioactive isotopes are useful for dating rocks so we’re gonna talk about those one of the main ones that we’re gonna be talking about or kind of briefly bring up is potassium to argon this is a really useful one it’s common potassium decays in the argon in case bar and potassium feldspar this is one of most common rocks in Commons it’s found in a lot of Sam stones and it’s a really hard rock and it stands up to weathering very well or mineral I should say so this is really kind of cool it has a half-life of 1.3 billion years so that’s kind of not that you know in terms of Earth history it’s about 25% of our history is the half-life so that means that it has some good resolution to go beyond that as long as we have several half-lives in we go three or four half-lives and we could figure out how old these minerals are it can also be it rocks as young as a hundred thousand years so I can go the other way right you look very very young as well this really cool process it’s it’s there’s some other ones right potassium-40 a decays to argon-40 and calcium for you’ve seen both of these an argon is a gas and only President rocks as the daughter product of the decay of potassium so essentially what we see here is is that we can go through I can say hit this with a laser at some

special place right on the crystal where I’m gonna have a good crystal hit with a laser blast it and I measured the amount of argon that comes out of it and I know that that argon had to have come from potassium because potassium feldspar doesn’t use argon to form potassium feldspar makes he uses potassium it’s through the radioactive processes through the decay process that argon is produced from long the minerals form so it’s trapped in that mineral and so we’re able to go through and measure the amount of argon that we find in this rock and that allows us to get that nice date this is a really cool technique so anyways here we see a fantastic feldspar up here here’s some other ones on uranium-238 this is a common uranium it’s the most common one to lead-206 is the common decay stable daughter product it takes about 4.5 billion years so it’s actually pretty stable radioactivity in uranium 238 is fairly low when we compare that to other things that’s it’s not too bad at uranium 235 which is really famous because this is the variety that was used in nuclear weapons it decays to lead 207 and that takes a shorter period of time 7 704 million years thorium 232 to lead 208 14 point 1 billion years this is almost as the age of the of the universe right as we know the Big Bang was about 13.7 rubidium strontium look at this 147 billion years is the half-life that’s longer than the excepted age of the salt of the universe by far even this is so we have the ability to look at information way back in geologic time this is not always a straightforward process right I’m talking about you know uranium 238 going to lead 206 it turns out that uranium 238 to lead 206 it’s actually 14 steps right it goes uranium the thorium back to uranium you know thorium radium radon polonium lead right polonium again lead polonium and then back to another isotope of lead so it goes from lead to 14 to 210 to 206 it’s a 14 step process supposed to go through all of these things so it can be a little complicated right you’re gonna find these other things in your rock – and this could actually be helpful it’s a lot of geologists are now looking at the K ratios in rocks between 238 and say radon-222 or radium 226 but the by far the best friend dating the Rockettes office 206 this is you some of these isotopes up here are useful for figuring out other bits of information such as uplift rates or how fast rocks pull off things like that all right I’m kind of digressing on that but you get the idea this is not always a real simple thing okay there are other isotopes that we can use I’m not going to get into them we’ll talk about carbon either this lecture next carbon-14 which is used for dating things in the modern era but there’s sort there’s all kinds of sources of error right the system must be closed right you can’t have a bunch of fresh argon flushing through your potassium argon crystal or you get all kinds of mistakes right no external addition or loss of parent or daughter isotopes so freshwater is a I’m sorry groundwater is a real problem if it’s flowing through rocks it can cause things to change fresh unweathered rocks are ideal for use to use for radiometric dating for that purpose so Earth’s oldest rocks are found on the comments write all comments have rocks that exceed 3.5 billion years whether it be Greenland Canada South America whatever you go to any of the major continents you’re gonna find at least a portion of it somewhere that’s three and a half billion years old so the comments are quite old at least it starts at the very hearts and it confirms that geologic time zoom is very very hard in the earth we as we’ve discussed is over four billion years old for now we have 4.6 4.5 five billion years old and individual minerals have been dated to 4.4 billion years we have never found anything older than that we as as we’ve said before and I’ve said many times we assume the earth to be about 4.5 5 so this is about a hundred million years after the creation of the earth maybe for less than 200 million years for sure we have found individual minerals and this is where we find the place called the jack hills in Australia or we have sand stones that contains arapaima crystals that date to about 4.4 billion years old that’s right after the formation of the earth we find cellphones really really neat oh here it is actually I wasn’t sure if it was in this slide or the next one but anyways gonna talk about carbon 14 so

carbon 14 is really cool that’s used in archeology it’s used in very young geologic studies to save young rock or young processes geologic studies so radiocarbon dating uses the radioactive isotope carbon-14 half-life is fifty seven hundred and thirty years they can be used to date events as oldest seventeen thousand years right beyond 70 thousand years there’s so little carbon-14 left that we don’t see that anymore so essentially this is the this is the chemistry c14 is produced in the upper atmosphere from cosmic ray bombardment so we have basically here’s nitrogen 14 which is pretty common there’s some neutron capture that happens so basically a neutron comes in it pushes our proton so a proton emission and we’ve got wound up with a carbon 14 it’s radioactive this is radioactive carbon-14 so the carbon-14 is absorbed by plants through photosynthesis why because carbon-14 just like carbon-12 and carbon-13 has the same chemistry the plant doesn’t know the difference and it just dates it in so c14 is only useful in dating organic matter as a consequence right we have to have plants or wood or something like that in there we you know rocks don’t absorb carbon 14 in all organisms contain a small amount of c14 including you you know you’re you’re watching this little lecture right now and yeah you’re breathing in c14 into the carbon or you know or you’re consuming plants that have absorbed c14 and so as long as you’re alive you’re consuming new c14 you’re keeping that reservoir c14 up to date it’s when you die that everything starts to break down and you’re not consuming any meat anymore c14 so your radiometric clock is started at that point when you’re dying so that’s the cool thing about c14 is it it doesn’t necessarily measure when you were born but it does tell you when you died alright so we can now put together what we think of or what we see is the geologic time scale so geologic time scale encompasses all of Earth history everything there’s a lot of stuff in it I wish I could go through each each part of it this is actually a bigger there’s a larger class that deals with the geologic time scale called historical geology we talked about that and let you’re number one that there were two branches of geology the physical which is the course that you’ve been taking and the historical geology which is the course that is usually taken by people who have a deeper desire to learn more geology and they learn the geologic time scale and what happened throughout earth history they go throughout this let’s go through this really quickly so it’s subdivides geologic history in two units then originally created using relative dates we didn’t have the radiometric dates until much much later so this is the generalized one is there’s a period of time called the phanerozoic which is the period of time in which we live and this is the youngest eon and below it was a period of time called the Precambrian pre-cambrian is subdivided into the Proterozoic and the Archaean and we’ll be talking about that in a future actually in the next lecture in detail what we know most about the history of Earth is in the phanerozoic and the reason why is because this is where we have the fossils right visible fossils are present in rocks from during the period of the phanerozoic and so we can use fossils succession to figure out everything we’ve got the Paleozoic the Mesozoic and Cenozoic and these can all be subdivided into different groups the Permian most famous of which is the Jurassic or the Triassic and the Cretaceous all right in here in the middle of the Mesozoic putting all this stuff together has taken several lifetimes and a lot of hard work okay to be able to put dates to the actual timescale so the structure of the timescale we’ve kind of been going over this really quickly and eon represents the greatest expanse of time this is an eon here the phanerozoic eon is the most recent eon which began about v 5 hundred forty-two million years ago right the phanerozoic eon has the Paleozoic which stands for ancient life the mesozoic which means middle life this is also known as the age of the dinosaurs and the Cenozoic era which is recent life also known as the age of mammals so this would be the basically ancient life middle life or dinosaurs and Cenozoic is the period of time of the mammals when you look at all of our history that’s not very long it’s a very very small period of time right up here at the top Cenozoic now if we even break up the Cenozoic we realize that human beings which have dominated the earth during the period of time the Holocene is really just a very small sliver of time in the general view of what’s going on Holocene being only the last 10,000 years Pleistocene going all the way back to point six million years

ago we find evidence of hominids populating the earth all the way into the Pliocene in Africa so humans in their origins really are our only major players in Earth history in the last five million years and really agents of major worldwide change in the last 10,000 it’s extraordinary how much effect we’ve had during that period of time during the rest of this we’ve seen dinosaurs and meteor impacts and all kinds of different things happen major extinctions right six extinctions that have happened you know almost wiped life out and we’re just kind of sitting there right at the right the very edge up here during the Holocene so anyways just some quick notes here the each fan each fan or zoic era is divided into periods so here we see the phanerozoic and here are the arrows here and here the periods right this is what it’s talking about so here we’ve got the Turner and the tertiary and the tertiary in Europe is usually split up into the Neogene into the paleo G but you get the idea now the Paleozoic era has seven periods right here we see the seven periods here that came during the Ordovician Silurian and Devonian Carboniferous if you live in europe in the united states excluded into a Mississippian and Pennsylvanian and the Permian most epochs are termed early middle and late so we can even say early cambrian late Cambrian middle Catering late Pennsylvanian that kind of thing we don’t normally do that up here at the Cenozoic and even with the mesozoic we usually resist doing that someone but we do that frequently with the Paleozoic the reason why we do it is because the periods of time that are here are very very large right the Permian is lasting a period of roughly 50 million years but 50 million years is as big as the entire Cenozoic almost right Cenozoic being 65 billion years so it’s 15 million years longer the Devonian of course is well over the period of time the Cenozoic so this is this is just a huge period of time so most detailed geological time scales in the phone centers don’t care right that’s where we have all the fossils so that’s what allows us to break it all open the four billion years prior to the Cambrian period are divided the too young so this is actually where we know the least history of the earth this is a real problem for us as geologists and kind of keepers and interpreters of Earth history there’s a couple of reasons why we’ll go into this here momentarily so it’s divided with two yawns and often collectively referred to as the Precambrian everything before the Cambrian right the Cambrian is the beginning of the Paleozoic here so everything below the Cambrian is the Precambrian that’s what this all is okay and during these prettier times during the Proterozoic which means before life it was called that because all of a sudden he wouldn’t get into the Paleozoic he starts seeing fossils we started finding shells and teeth but before that we find nothing right almost nothing and so people believed that there was nothing here now they know that in fact there have been life all throughout this period of time it just took us a long time to find those fossils because they didn’t have the hard part I didn’t have the fossil earlier the the bones and the teeth and whatever it is that we now find in most of our phanerozoic fossils up here these things were all soft bodied and of course the Archaean is the period of ancient time the Neo archaea and the majority and the Paley are key and the eor km all part of the Archaean period of time very very ancient and less is known about Earth further back geologic time why cuz plate tectonic processes the rocks actor destroy old evidence if there was any fossils there most of them are destroyed we get lucky from time to time we find some evidence that for the most part you know you have a plate to run into each other you get metamorphism of the metamorphism destroys the the evidence of any life and so it’s very difficult to date these things and put together these stories so it was during the Precambrian simple life forms that lacked a hard part algae bacteria worms funds you dominated you know the first abundant fossil evidence is not a period at the beginning of the camp we just talked about that and many Precambrian rocks are highly deformed metamorphic rocks there’s just nothing to find him so the evidence is very scant but that doesn’t mean that we can’t put some of these principles to work to be able to figure out some of these problems so for example here we see a canyon the top of which is the Wasatch formation down below is the Somerville formation we have a bunch of rocks in sequence in between and we can put together a really cool kind of sequence of events that have happened here because we know that we have minerals of volcanic ash bed that’s

gonna have potassium argon in it and we can go through we can take that and in this case if so it’s 160 million years old and we know that it’s that means everything above it is younger than 160 million everything below it is older than 160 million years old and then we find an igneous dike that is intruded here but it doesn’t intrude the Wasatch formation right it’s clearly younger than Wasatch because it doesn’t cut across it and that’s at 66 million years old so the Wasatch formation has to be of the Paleogene age because the Paleo Jean started 66 million years old ok the Mesa Verde formation the Mancos the Dakota all of these things we know what these where these should fit in and these should be a Cretaceous age the Jurassic age rocks the Morrison the Somerville we know where those are as well ok you get the idea so let Mesa Verde formation we can assume that these also exist over here why because we can assume original horizontality and that these things were continual right there continuity between our across these units here we see continuity here we can assume continuity here then we can assume continuity here here and then as well as that pink over to the Wasatch as well so we’re able to put together a whole process that since the uplift of the Wasatch we’ve had the erosion and deep incision by some type of river system into the valley all right so I know a lot of information dating sedimentary strata and being able to figure these things out because obviously we have fossils in here that helps but that doesn’t allow us to get a real date right we can bracket it between a couple of known areas and be able to get some good dates we know that for example all of these had to have formed based on fossil evidence during the Cretaceous and the staring of the Jurassic but the exact dates might be somewhat difficult to put down other than to say it’s between 66 on 160 million years old all right okay so a lot of information a lot of chemistry basically calling upon you to remember some of our earlier lectures and I hope that you have if you have any questions as always send me an email or find me on a message board so hope to hear from you have any questions feel free okay until next time