we can assign numerical ages to rocks using absolute dating but absolute or radiometric dating only works on certain types of rocks for rocks that absolute dating does not work on we have to use what's called relative dating and that's what I'll be talking about in this video along with geologic time and the time scale in general I'll start by introducing the geologic time scale so we're all familiar with the major periods and eras and eons of the time scale then I'll go into relative versus absolute dating exactly what the differences are there then I'll talk
about the principles that we use to relatively date rocks as well as how they were used to actually construct the geologic time scale before absolute dating was a thing and of course to talk about relative dating we have to talk a little bit about stratigraphy specifically biostatigraphy using fossils to relatively date rocks magnetostratigraphy using magnetic properties of rocks and litho stratigraphy using the rock types so just as an overview of the major eons eras periods and epics or epochs of the geologic time scale here we have the Haitian archaean proterozoic and phenerozoic eons the phenerozoic
is spread out in this time scale figure because that is the current Eon that we are living in and that is also the Eon that we know most about because it has the most complete fossil record and that is broken down into the Paleozoic Mesozoic and cenozoic eras which in total have gone from about 540 million years ago to today and these eras are broken out into major periods the periods in the Paleozoic include the Cambrian ordovician solarian devonian Carboniferous and Permian and the Carboniferous is sometimes broken up into sub-periods the Mississippian and Pennsylvanian and
then the Mesozoic is the Triassic Jurassic and Cretaceous and the cenozoic is the paleogene neogene and quaternary the paleogene and neogene used to be confined into what was called the tertiary that's why it was always the KT boundary for the Cretaceous to cenozoic boundary and Extinction event but now we call it the kpg so paleogene PG boundary and who knows why we use K for gradations I think it has to do with the roots of the word I don't know but anyway the e-box that you typically have to know for a typical you know geology
major or class or whatever are the ones that are associated with the cenozoic era because again that's the most recent and we know the most about those and have the most complete record of those uh epochs so these include the paleocene eocene oligosine myosin pliocene pleistocene and Holocene which is the one we are currently in if you have a professor that makes you memorize these for God Knows Why then you can check out my geologic time scale song where I put all of these in a song so it's easier to remember and I'll link that
to the top right for you what I love so much about the geologic time scale though is that most people put it on this not to scale figure where the phanerozoic the current Eon is spread out to immense proportions in order to show all the detail that we know about it but when in reality when put to the correct scale the Haitian Arcane and preterozoic the pre-cambrian or pre-phenirozoic time is it's it's dominant it takes up the major part of the time scale because it was literally four billion years of Earth's history so most of
Earth's history and so when you're looking at time scale figures just remember that they're generally not at all to scale so how was the geologic time scale constructed what are the breaks between time periods and epochs and all of that based on is it absolute or relative dating well before I will answer that I want to talk a little bit about the difference between relative and absolute dating relative dating is qualitative and absolute dating is quantitative you can get numerical ages of course with certain error bars with absolute dating with just one rock but relative
dating is qualitative you can only get dates of the rocks that are relative to other Rock layers or rock formations for example in this figure here you can see the lower layers are typically older than the upper layers but of course there's intrusions and exceptions to this that we'll go over later in the video so relative dating is exactly what it sounds like it's stating the Rocks relative to each other whereas absolute dating is dating the Rocks numerically based on their chemistry I talk about that in a lot of other videos that are shown down
here on the screen I will link one to the top right view I'll link the absolute dating one to the top right it's a very old one but it goes over the gist of it the stable versus radioactive is a good one too and then I also have one about how we dated Earth so you can check those out so now let's talk about relative dating principles the principles we use to date rocks relative relatively because it's not just that you know the older layers are the lower ones there's a lot of strategic principles that
come into play first things first some major people that had a hand in creating these principles are Nicholas steno and William Smith as well as James Hutton and George cabier who all came together with their you know independent lines of work to come up with these principles of relative dating over centuries of work as you can see the first principle is just what we were talking about on the previous slide that older layers are typically on the bottom and younger layers are on the top this is called the principle of superposition basically the younger layers
deposit on top of older previously deposited layers of rock we also have the principle of original horizontality this is the principle that states basically that the layers are originally deposited horizontally so that if you come to an outcrop where there is a bunch of angles and folds that was probably post depositional after deposition and in the process of deformation there's also the principle of lateral continuity basically stating that the layers of rock are laterally continuous such that if you have erosion that occurred in between them you can still correlate between the rock layers that should
be from the same event rather than separate depositional events up to this point these all make good sense but then we get to some of the ones that kind of complicate things a little bit such as the principle of intrusive relations so this is basically when it comes to igneous intrusions these can come from below and so anytime you have igneous intrusions that come up from the mantle and disrupt the previously deposited layers of rock those are younger than the layers of rock that they're intruding rather than older even though they're below Technically when it
comes to like the layers of rock like superposition principle there's also the principle of cross-cutting relations that can also apply to intrusions but not always for example when you have intrusions of like igneous Dykes for example that cut across previously deposited layers as they shoot up from the mantle or magma chamber or whatever they shoot up into the previously deposited layers and when they do so they're cutting across the previous layers because they're cutting across these layers you know it's younger than those layers because it's cutting across them I'll show an example that's more clearly
imaged later I promise this not only applies to intrusions but also faults and fractures these are younger when they cut across the layers and lastly we have the principle of inclusions which is again pretty simple basically when you have something like this for example this conglomerate with clasps you know the clasts are older than the Matrix or cement material holding them together so this is where I promised that I was going to make it all make sense those intrusion and cross-cut relation principles so let's just go through it to order these geologic events or depositional
layers chronologically first things first we know principle of superposition states that the oldest layer should be at the bottom this would be a well a is not cutting across any other layers and it's actually being cut across so that makes sense a is the oldest layer the second layer was deposited on top of a and that's B the third layer was deposited on top of B and that's C all of this makes a lot of sense so far so the fourth layer would probably be D right well no if we look at the label we
see this is an igneous sill this is what I was talking about igneous intrusions can come up and fill the in-between layer space of older sedimentary depositional layers of rocks and that forms a layer but if you know the composition is igneous and you can see that there's clasts in here that were ripped up from these other formations and incorporated into the sill then you know that this has to be younger than the blue and the yellow layers because it actually disrupted them it Incorporated their class into its its layer that means that they had
to be there before it came along and formed that sill so now we know that e is number four not D then we have the question of whether the igneous Dyke came first or the igneous sill came first the igneous Dyke is cross-cutting all of these layers including the igneous sill meaning that the dike has to be younger than the sill itself or else the sill would be cutting across the dike so the sill is next it was deposited after number four then we have the question of whether the Dyke is older or younger than
the rest of these layers up here and because G comes across and cuts across all all of the older layers it has to be younger than all of those including F so the dike layer f is number six then we have a pretty easy time with G H I J and K but remember here that we're ordering geologic events not just rock layers so always remember when you're doing a test or something to order all the events including the erosion number 12 here we have erosion of layers 10 and 11 here and that would be
the last geologic event that occurred um based on this picture at least so now that we know what relative dating is and how we do it how we use the principles to relatively date rocks and we know what absolute dating is we know we use the chemistry of the rock to date The Rock numerically we can come back to the question of what the geologic time scale is based on absolute or relative dating well the numbers associated with the geologic time scale are a little misleading because actually it's based almost completely on relative dating specifically
fossils or bio stratigraphy we use fossils to kind of indicate where breaks should be in the geologic record or at least we used to now we heavily use absolute dating and we've gone and assigned numbers to all of these breaks or boundaries in between time periods and we've adjusted them many many times and we've gone more specific with epochs and ages and all of that but originally when we first built the time scale it was based largely on the fossil record this is because early geologists found distinct fossils in different Rock units they also observed
biological overturn events in other words Extinction events where the biota distinctive of one period would kind of have this rapid ending for a lot of it or at least diminishing of their numbers for a lot of that biota of that previous period and turn over and do this new diversification thing in the next period with a whole different Suite of biota that was often very distinct from the previous period so because of all these Extinction events observed in the Rock record they were able to make these boundaries between periods based on extinction events that's why
you always hear the end such and such mass extinction or the late whatever mass extinction it's because they're always the end of periods right before a boundary because we built it with that in mind we built the time Scale based on basically Extinction events and this is easily seen if we look at extinctions throughout the rock record we can see the Cambrian to ordivision period here this boundary was marked by an Extinction it's not one of the big five mass extinction events that I talk about in a lot of my videos but remember those are
not the only mass extinctions in Earth history there's been a lot then we have the ordovician to solarian Mouse Extinction I have talked about this one this is one of the big five the solarine to devonian mass extinction the late devonian or devonian to carbon their first boundary mass extinction this is also one of the big five the Mississippian and Pennsylvania one again the carbonifers can be broken up into the Mississippian and Pennsylvanian sub-periods and there was a big glaciation event and mass extinction during this time the Carboniferous to Permian or Pennsylvania to Permian boundary
and the end Permian Extinction of course that's the largest Extinction event of all time you can see the huge dip in number of Genera or diversity in that time then we have the late Triassic mass extinction another one of the big five then we have the KT or kpg boundary mass extinction at the end of the Cretaceous when the dinosaurs became largely extinct but not all periods were separated based on distinctive fossils and major Extinction events some such as the Cretaceous Period the last period in the Mesozoic era were separated based on mythology the Cretaceous
is marked by intensive chalk deposition and this is how geologists initially separated it from the Jurassic and then its upper boundary with the cenozoic era is separated of course with the KT or kpg Extinction event since the construction of the time scale fossils and thethology and of course geochemistry and absolute dating have allowed more distinctions of periods epochs ages Etc now stratigraphy is largely what drives these distinctions and discoveries in the Rock record and stratigraphy is just the study of stratified or layered rocks a big part of stratigraphy is correlation among rocks from basically all
around the world so that we can try and recognize when two separate or multiple separate sections of rocks or outcrops are actually representative of one stratigraphic unit or one geologic time period based on fossil content this allows us to fill in the erosional gaps in the Rock record at least to some extent and we can correlate Rock units based on a few different things we can correlate them based on rock type or lithology we can correlate them based on fossil content we can use age if absolute dating is possible on those layers and sometimes we
can even use magnetic properties when we use fossils to relatively date or correlate Rock units this is called bio stratigraphy when using biostatigraphy index or guide fossils are crucial in understanding where in the geologic time scale you're looking at when you're looking at rocks what are index fossils well index fossils are basically just these perfect little time stamps on rocks they're fossils that represent such a specific time range that you know when you're looking at that rock and it has that fossil in it that you're in the Pennsylvania period for example or whatever because the
fossil was only in that period And so they're really useful when for example you have sedimentary rocks that can't be dated using radiometric or absolute dating there are five major criteria that fossils must meet in order to count as index fossils and be used this way one they must have been abundant two they must have been easily distinguishable from other fossils an example of great index fossils are basically all ammonites ammonites are big cephalopods that are now extinct but we're really dominant in the Mesozoic Era and although the Mesozoic Era is a big big time
block that isn't very specific we can get way more specific by looking at these suture patterns or these squiggly lines on the ammonite fossils because that tells us what specific type of ammonite it was and that can tell us the more specific time range that that ammonite should have been around for and that can help us narrow down the time period that we're looking at in the Rock record index fossils also have to be geographically widespread if they're not they're not much use in correlating rocks and different Geographic locations they also had to have been
short-lived like I mentioned earlier which makes sense because this allows the dating of rocks to a specific time period rather than a really broad range and lastly they had to occur in many different sedimentary rock types in other words they had to be of relatively High preservation potential for example soft bodied organisms or land animals or things like mosquitoes for example that only get preserved in Amber typically these would not be good index fossils and so things like ammonites with shells hard parts that can become preserved in a lot of different sedimentary depositional environments and
stay preserved well those are great index fossils in terms of examples of great index fossils we've got ammonites as I've just been talking about this whole time fusa Linens or other forams oraminifera these are microorganism protists that lived in very specific time ranges well fuselinids foraminifera are still around they've been around for basically the entire geologic time scale since the Cambrian and for manifera of pretty much any kind are really great index fossils but I mentioned fuse Linens specifically because I know that their time range was the Mississippian Pennsylvanian impermeon or just the Carboniferous and
Permian and they had very specific types of fuselanas around in each one of those time blocks so it is pretty easy for me to tell when I see a type of fusillanet in a rock I know if I'm in the Permian the Pennsylvanian or the Mississippian trilobites are great index fossils for the Paleozoic Era not just trilobites in general but specific types of trilobites that lived throughout the Paleozoic I have a trilobite video that I'll link up to the top right for you which goes over which types or orders of trilobites lived the distinctive periods
of the Paleozoic and they're all very physically distinct from one another so you can basically tell if you look at a certain type of trilobite that you're in a certain time period of the Paleozoic which is really helpful but pretty much guys all other invertebrates there are certain invertebrates they're not great for index fossils like lingula brachiopods for example those have been around like since the Cambrian and have not changed physically for a long time they look kind of like this um but there are so many other great invertebrate index fossils that are they fit
all the criteria of what a good index fossil will be so you might be wondering is bio stratigraphy still relevant today even though we have absolute or numerical aging we have the ability to go in look at the geochemistry of that rock and say the age why would we ever need biostatigraphy well it's absolutely still relevant no pun intended absolute dating as I mentioned previously only works on very specific rock types typically well igneous is the best type when you have rocks that crystallize directly from magma or lava like igneous rocks you're dating The Rock
when you look at its geochemistry but when you have sedimentary rocks for example that are Amalgamated grains from all different types of sources that come together to become compacted and lithified together as a sedimentary rock all of those grains from all of those sources will give different ages they're not crystallizing all at the same time and I talk way more about how absolute a radiometric dating Works in other videos so I'm not going to go into detail here the point is it doesn't really work well on most sedimentary rocks there are ways to date the
depositional age of sedimentary rocks or to absolutely date or radiometrically date layers above or below sedimentary rocks for example a lot of sedimentary outcrops have some sort of like volcanic ash layer or lava flow above or below them that you can use to then say well this is in between this absolutely dated layer at blank million years old and this absolutely dated layer at blank million years old so this sedimentary layer that we can't absolutely date has to be in between blanket blank million years old so you can use a lot of different methods to
do this but fossils are still very helpful in correlating sedimentary rocks around the world and are just it's one of the best ways to do so magnetic stratigraphy or magnetostratigraphy is used basically by looking at the magnetic properties of rocks Motion in the iron rich liquid outer core of Earth has generated Earth's magnetic field and Earth because of this has a magnetic north and south pole that occasionally switch and because iron minerals that crystallize in igneous rocks for example align with the current magnetic field of Earth at that time when it's crystallizing we can look
at this you know magnetic alignment throughout the rock record and see when the poles were where iron mineral grains can even realign when settling in water for example to become incorporated in and sedimentary rocks so sometimes sedimentary rocks can have magnetic signatures that represent the Earth's magnetic field at the time they formed as well once in the Rock the iron minerals preserve the magnetic field signature from when that rock formed this is useful for multiple reasons not only can we correlate rocks with the same magnetic signature you know from different Geographic locations but we can
also study Earth's magnetic field and win and why it switches especially when we use absolute dating combined with Magnetic signature correlations magnetic signatures are most often found and used in igneous rocks and absolute dating is also mostly used in igneous rocks and so we can look at both the absolute ages and the Magnetic signature of these rocks to kind of basically track first Magnetic North Pole movement throughout Earth's history and try and understand why it's migrated and all of that now moving to Litho stratigraphy distinguishing Rock units based on lithology or the physical and chemical
characteristics of the rock is called the Tho stratigraphy litho just means Rock lithostatographic unit types include super groups which are broken down into groups which are broken down into formations which are broken down into members formations or rock formations are probably the most common term actually used uh in you know just speaking with somebody you typically are like oh that's the such and such formation when you're like a geologist and you just like talk about these things using this categorization of lithostatigraphic units we can actually construct stratigraphic columns or sections that can be made basically
by mapping an outcrop and vertical succession so you have like the oldest rocks at the bottom the youngest ones at top and then you you know describe things about each one of those rock formations or groups this is an example from the Franklin Mountains in El Paso where I currently go to school and so you can see all the formation names here and the group names here the El Paso group includes formations like mckelligan Canyon Etc so this is kind of how you break out things into a Strack column there's a lot of different formats
of Stroud columns though and I'm hopefully going to make a future video about how to make Strack columns for those of you out there who are currently taking set strap using stratigraphic columns and correlating them across large areas we can actually create entire cross sections which is just like a cake slice looking at the in innards of a region based on what the stratigraphic columns are telling us but one thing you might be wondering about stratigraphy in general and the vertical succession of rocks is how and why rocks from different depositional environments deposit on top
of one another rather than side to side you know when we go to the beach the beach depositional environment and the deeper epistle plane depositional environment are a Js decent not on top of each other so why in The Rock record do they become on top of each other well this is because depositional environments are always migrating an example of where we see this is in the Cambrian during a transgression or basically a sea level rise when there was Inland migration of the sea when you have the migration of the sea Inland or basically sea
level rise you get a vertical succession of shallow sand deposits over Lane by deeper muddy and carbonate deposits so you've got the yellow sand here the gray mud and the blue carbonates here and eventually if you took a strap column of this middle section you can see they're all laying on top of each other because over time this is a migration of the depositional environments a migration of the sea a migration of that margin regression is the exact opposite of transgression it's the Basin word migration of the C margin of the base and margin and
this can be driven by sea level fall and when you have a lowering of the sea level you're going to get the opposite direction of depositional environment migration and the opposite succession so instead of shallower deposits Overland by deeper ones you're going to get the deeper deposits over Lane by shallower deposits and that's what's shown in this bottom left figure you had transgression and the top part and then you had another regression in the bottom part and now the carbonates the deeper carbonates and the deeper months are overlane by the shallower sand deposits and this
entire succession if we took a strat column in the middle here that would show both the transgression and a regression sequence and this is cool because it allows us to understand for example sea level rise and fall through time and when it has occurred and where it has occurred and all of that and of course I cannot put this video out without discussing unconformities unconformities are surfaces of non-deposition or erosion the most recognizable or obvious unconformities in my mind are angular unconformities where you have angled layers layers that were folded or Tilted in some way
and then they were cut off by erosion and then layers deposited horizontally again the second type of unconformity is non-conformities these are also somewhat distinctive because this separates either metamorphic or igneous rocks from sedimentary strata so if you have an igneous intrusion come into contact with sedimentary strata then you have an unconformity there you've got the younger sedimentary strata or layers on top and the lower igneous intrusion at the bottom and the last type of unconforming is a disc Conformity this is just basically an erosional surface in between sedimentary strata so the strata above and
below are horizontal there's no angles like an angular unconformities but you can tell that there had been erosion in between these depositional events so I hope you guys enjoyed learning about geologic time and relative dating I know that I have old video on my Channel about that but I just felt like I could have covered it a little bit better so that's why I'm making this one and if you want to check out more about absolute dating now I will link a couple videos up here that I think will be really helpful for you anyway
thank you guys so much for watching and I'll see you guys next time bye
