Transcript for:
Understanding Plate Tectonics and Geologic Time

hello scholars it's doctor aber again and our second-to-last video of the semester in this chapter we're going to be talking about the evolving earth and in that chapter would be plate tectonics and geologic time the core concept behind the material in this chapter is that the earth has an internal structure and cycles materials between the surface and the interior now we learned a little bit about this when we talked about the layers of the earth in the last chapter particularly with the lithosphere and the asthenosphere below it and we'll talk a little bit more about processes occurring in the mantle and different types of crust on the Earth's surface and how that crust is broken up into plates so going way back into 17 late 1700s James Hutton introduced the idea of uniformity and in his idea that basically that the earth slowly changed over long periods of time and he saw that that rock is broken down and eroded away he watched rock particles keep being carried downstream and deposited and then he was on the sedimentary rock and that that would form new rock over time but the problem was is that most of the earth most of the people on the earth at that time thought that it was only a couple of thousand years old and his principle of uniformity would not coexist with that process with that timescale and so most people around that time had another explanation for all of Earth's geology and that was catastrophism and and this idea behind geology is that basically all of the things that you saw mountains and canyons and seas all of those were formed by large catastrophes and so it wasn't uncommon for example for an earthquake or a volcano to occur and you know and Italy they saw Pompeii and and with earthquakes they saw rocks shift and so catastrophism worked with the time skills of a young earth that was only a thousand or three thousand years old it wasn't until many years later when Charles Lyell decided that if we accounted for a longer or older earth that uniformity would be a logical process and it turns out it's kind of a happy medium it's a little bit of both it's catastrophism and uniformitarianism e's can cause large geologic changes but then also the slow erosion of material and the reforming of new material as we learned in the rock cycle last week is also accepted so our present-day understanding is what really is plate tectonics I'm gonna come back to this slide here but our present-day understanding was that it's a little bit of both and we'll come back this is where we're headed we're going to talk about the mechanism that's going to drive a lot of these geologic processes so as you recall layers of the earth we've got crust mantle and the core and we're going to talk about convective processes and the mantle we learned about convection early in the course back when we did physical forces in motion and we learned about heat and heat transfer conduction convection and radiation well convection is where you got a lot warmer material rising to the surface and cooler material sinking and it's going to be hypothesized that those convection currents in the mantle are going to be what drives a lot of the process that we see on on the crust and we're going to define the crust into two different kinds of crust oceanic crust and continental crust so of course the crust covers the entire earth and even where the oceans are there's crust need the ocean and so oceanic crust is going to be much thinner but it's more dense because of the basaltic rock that makes it up so it's way more dense compared to continental crust continental crust is on average much thicker than oceanic crust but it's less dense this is going to become important when we start to see crustal interactions at boundaries and we'll you know we're gonna see what happens when oceanic crust meets continental crust or when oceanic crust meets out-there oceanic crust or when continental crust meets continental crust so if you recall the asthenosphere is sort of a semi elastic kind of molten material and it's gonna be right underneath the lithosphere which is the solid rock layer above it and the lithosphere of course is the entire crust the Moho and a little bit of the upper mantle that might be more solid the asthenosphere is right below that so you can think of it as you know well just Rock kind of sitting on some you know more of a fluid like magma molten rock and it's shown in this picture here we have you know the lithosphere up here and then floating on top of the asthenosphere so Alfred Wegener proposed this idea that the continents drifted and he first started by looking at the map and saying hey it looks like the continents kind of fit together it looks like South America kind of fits in here into Africa and the United States the curvature of that fits nicely over here and when they started to examine other continents and other places they're like hey yeah Madagascar fits kind of right up in there and and in Africa and so he proposed this theory of continental drift that the continents drifted and there were several other pieces of evidence - the rock strata or the layers of rock on the western coast of Africa were identical to the rock strata you know thousands and thousands of miles away across the Atlantic Ocean on the eastern side of South America there are a couple of other puzzling things they found tropical plant fossils and Antarctica and so how would tropical plants that got a lot of direct solar radiation that can only could not survive in that you know rock be found in Antarctica and so they also found fossils of the same species and rock layers on each of these and so that was a really interesting piece of evidence the problem was is that there was no mechanism to explain that day the criticism that Vague nur faced was how is you know under what mechanism would move entire continents and so it wasn't he had actually died before he saw his ideas come to be supported with with more modern evidence but we now know that based on a lot of evidence that there was actually one supercontinent Pangea and and actually even before that continents were moving around and they will continue to move around as they are right now we've measured them directly so we can actually directly measure the movement of these plates so it's not so much theater you know as a hypothesis as it is supported with actual observations and measurements so the original idea was continental drift the mechanism that drives continents to move is called plate tectonics and we're going to talk about that in a little bit more detail so the crust on the surface of the earth is made up and broken up into different plates just large sections and usually the plates are named for the continents that sit on them so this would be the North American plate in the Eurasian Plate in the African plate in the South American plate and so on there's a few the Pacific plate and a couple of other small ones that you'll see in your lab and so we actually have quite a bit of evidence when we first started mapping the bottom of the ocean we found in our transect halfway there's an undersea mountain range that extends all the way across the middle of the Atlantic north to south and so we call that the mid-ocean ridge and there's a ridge there and what's really interesting is the coincide with earthquakes and of course volcanoes it goes right through the middle of Iceland and there's volcanoes undersea volcanoes earthquakes a large amount of heat coming from the top and so this idea was that you know what new magma is coming up and cooling and driving as it come as new material comes up and cools it pushes the old material away and so we call that seafloor spreading and that was one mechanism to describe how plate tectonics occurred was through seafloor spreading now if you're going apart in some regions then that means of course you'd be coming together and others so the seafloor spreading hypothesis has been confirmed with allotted I should say with a lot of data including direct measurements using you know steaks that have been put in there and they're measured and their movement is measured and so what happens is that hot molten rock moves up from the mantle and the asthenosphere through the lithosphere all along that edge and it's going to cool to form new rock pushing the older rock so actually this rock here in the yellow is older and this rock right here in the middle would be the youngest and I think there's a quiz question about that identifying the older rock and this is also confirmed with you know absolute dating radiometric dating looking at the age of rocks to the rocks over here much older than the rocks here drilling taking sections then sediment layers near the ridge where they haven't had time to accumulate whereas they're thicker sediments over here older fossils are near the continents and there's not any you know rarely fossils near the rifts so there's a lot of data to support seafloor spreading and we also have evidence from Earth's magnetic field and you can watch the way iron in the rock Orient's itself with the magnetic north and south pole and so that kind of tells us a little bit we also talked about the pole reversals which is also detected in the rock layers so the plates that I named earlier shown in this map by color-coding also where two plates meet each other is called a plate boundary and basically the lithosphere is broken up into all these plates the plates move along the encino sphere as material seafloor spreading causes new material to form it's gonna push others and cause it to be destroyed and so we call that a convergence and where they come together as opposed to seafloor spreading which would be a divergence so there are three overall plate motions there either spreading apart which is divergent boundary they're coming together which is a convergent boundary or they're sliding past each other and so transform would be sliding past one another sort of one going this way and one going that way and they'll get hung up on each other and as the pressure builds they'll slip and you know and then we get an earthquake and that's actually how the San Andreas is that it's San Andreas Fault in California is a transform boundary we're also gonna see a lot of earthquakes volcanoes and other geological processes where we have these plate boundaries and we'll we can even form mountains if two continental plates meet we can form mountain ranges so let's take a look at each of these in detail a divergent boundary is where you have two plates spreading apart from one another so if this is a mid-atlantic ridge we would have the South American plate if we're in the below the equator and where the Africa plate and you have convection where warmer material is going to rise and and it's going to be pushed up and so this is a weak spot in the cross the mantle is pushing out material and cooling to form new rock pushing the old rock away seafloor spreading as an example but another example of this would be a rift and in a rift get the different color here a rift can occur in continents - in fact there's one in Africa that eventually at one split Madagascar off the come off Africa and now there's an there's still another one going through Africa it's widening and eventually you know once it pushes far enough water will flood that area and divide that continent into several islands so again you're gonna get volcanic activity and it may be even a few earthquakes as you're moving those plates along a convergent boundary is where to come together and there's several different kinds we can talk about oceanic crust converging with other oceanic crust Oceanic and continental convergent boundaries and continental to continental convergent boundaries we'll talk I'll show you a picture of each so when two plates are gonna move towards each other they converge this is ultimately going to destroy some of the crust if you imagine the way I explain it to my seventh graders if you have two cars that hit head-on and what happens you basically get a lot of buckling up of the hoods and you'll see them crushing in the hoods get all wrinkled and that's exactly what happened when India slammed into the southern portion of Asia and you formed the Himalayan mountain range however if one of the crustal plates has a denser material as an oceanic crust compared to the other one then what ends up happening is they hit each other but instead of buckling because one is more dense it gets pushed under it gets abducted and so we call that a subduction zone and so that more dense crustal material is going to be pushed under it if two oceanic crust old plates hit each other and they're both very dense sometimes they actually will both crumple down forming an ocean trench so I'll show you some pictures so you can get that visual but the subduction zone is when one of the plates is more dense and is going to be pushed under the other and that's gonna create volcanoes as you'll see the other way to make a volcano is a hotspot which is not related to a plate boundary and we'll talk about that in just a second so three possibilities continental and oceanic plates converging oceanic to oceanic plates converging and two continental plates converging let's take a look at the first one so because the oceanic crust is more dense and if you pay attention to the arrows here because it's more dense it's gonna be pushed under now all this solid rock that gets pushed under is going to form at not only a trench but then this rock will actually melt again as it enters the asthenosphere and them in the mantle and so because you start to melt all this rock you get this new material you start to build up extra magma and form a magma chamber right here if when this happens enough that chamber gets pressurized and becomes large enough it can actually explode or you know have an eruption it can erupt and so live volcanoes where are going to be found along subduction zones and so all around the you know west coast through Chile all the way up through North America and all the way to Alaska you have subduction zones and that's why off the coast of California the continental shelf drops off really steep very close to the coast line is you get this big deep spot as oceanic crust is being subducted and that's how we're gonna form the volcano and so another example would be on the Nazca plate and the South American plate which causes so many of the volcanoes and earthquakes in Chile as we've seen in the news we can also have this happen underwater we can have ocean ocean plate conversions usually if one is more dense it'll gets abducted or they both get kind of pushed under resulting in a magma chamber you'll have you know undersea volcanoes that will eventually grow to form land above the waterline if it starts to grow there right now there's um in the Hawaiian island archipelago there's a couple of other islands growing underneath the ocean so everyone knows the Big Island and the lava that's been flowing there started in the 80s they said it wouldn't last long and it's been going strong for thirty forty years and in fact very intensely most recently and so what happens though is you get this hot spot in this island arc situations a little bit different from the Hawaiian Islands but you have a hot spot that where hot material on the mantle burns through the crust and as the plates move that island moves off the hot spot and so it's gone it's it's just now in islands the volcanoes are extinct but what happens is another part of the plate has moved over that hotspot and so it's going to you know burn a new hole and make another island and I think the new one there's a couple new ones and Loihi is about halfway up I believe from the Big Island of Hawaii we see this also in Alaska there's an island archipelago up there as well and then I already talked about continent continental plate convergence remember the continental crust is much thicker less dense and so when two continental crustal plates converge you get Mountain folding and and you're gonna get this buckling that's gonna cause all these mountains and of course the earth's largest mountain from sea level would be Mount Everest and Malayan mountain range transform boundaries are where two plates slide past each other one's going say uh north ORD and the others going south and ones going east the others going west neither the crust is really destroyed in the process but there can be some grinding and erosion of lithospheric plate material but when you get irregularities they get kind of stuck and the pressure builds as these plates are wanting to move and then you get a sudden jerk as it slips and material breaks and you get the earthquakes that result okay so the last thing I wanted to talk about in this video would be dating rock material and there's a couple of ways to do that there's relative dating which is an approximate it's we can tell relatively the age of rocks based on how deep it's buried and we call it that the geologic column and so the law of superposition says basically that you know rock material and the fossils there in that are in shallower levels are younger than rock material and fossils that are in deeper rock material and so that tells us the relative age so you'd have the youngest rocks up here and the oldest rock down below so we can also use fossils to help date rock material a lot of fossils that are widespread across the globe we call them index fossils we know about the time period that they were around based on absolute dating and so when we find that species in a rock then we can pretty much tell the age of the rock very quickly with absolute dating we're going to be looking at radioactive decay and so as you learned radioactive elements in the periodic table will decay at what's called a half-life and it's a disintegration and so the half-life says this is how much material will be left well after half the time and so a different radioactive isotopes have different half-lives and one of the ones that we use a lot it would be carbon-14 it has a half-life about 5,700 years I believe and so we can't use carbon-14 to date rocks or any material much older than 50000 years because it would have been gone so if you know the radioactive decay and the half-life of the material after one half-life half of the material would be gone and that the amount of time it took for that to happen was you know the half-life so if we need to date rocks much older than 50,000 years uranium 38 is a good one it's got a half-life 4.5 billion years so we can look at 238 uranium 238 to date rocks or fossils that are millions of years old and that's absolute dating your chapter 16 of your text that goes into a lot of detail after section 16 point 12 on the different eons eras periods epochs and what happens in some of the major life history events and just rock history events that took place during those eras some climatic changes some mass extinctions etc I think that's a great read it's a little bit beyond the scope of what I wanted to do for this chapter so if you're interested in that please read on but I did want to let you know that we name a lot of these periods and time based on events that have taken place and and it's evident and and the you know the geologic history and so we would have the largest time zone would be the eons and then eras after that we're currently in the Cenozoic era and quaternary period and they're starting a new epoch they're talking about and and I'm not up to date on the research on this but the Anthropocene which is one the period of time on earth where humans have dominated I think there's a quiz question about this so it's worth noting now but when you look at say fossils of dinosaurs and then the age of mammals that came after that we're talking about millions and millions of years that these organisms were on the planet as evident in the fossil record and then our own species if you look at that not going back more than a hundred thousand years really we are a very young species and we have only been on this planet for a very small amount of time the earth has been dated to be about 4.6 billion years old or 46 hundred million years old and based on radiometric dating I know that might be inconsistent with some you know religious beliefs that you have and no one's out to change that for you but that that's what science has told us so far until further evidence reveals itself and so it just really makes our species kind of unique we've not been on this planet very long we've done a lot with the time that our species has been on it but you know the earth is a lot can happen especially if you follow catastrophism and so time is precious but geologic time is very long and it's hard for an average human living 75 80 years to conceptualize geologic time so we talked about index fossils the lost superposition we've got species that are in shallow iraq are going to be younger and what's really interesting is finding fossilized marine life in the deserts I was in West Texas Big Ben and you could see shark's teeth and and other marine fossils indicating that Earth has changed a lot over the years so those would be index fossils and so the problem with fossil records is it's incomplete first of all you get more animals than plants because of the hard parts and the bones and of course most animals don't even have bones you know for example invertebrates and so they wouldn't fossilize well and you have to have a very unique set of circumstances to form a good fossil usually quick burial because if an organism is left open and not buried then scavengers would come and and then you get the weathering and decay so and then what's worse is not only do we not know where these fossils are only the hard parts may have fossilized and so there's a bias and kind of which organisms form fossils and then of course a lot of fossils are destroyed in subduction of processes where you've got oceanic crust or or other in crustal she really gets abducted and melted through the rock cycle it destroys those fossils so organisms that lived in areas that favored fossilization have left more fossils and so certainly corals and and arthropods that have hard exoskeletons like trilobite s-- and ammonites the ammonites are a mollusk and trial of mites for an arthropod trilobite similar to sort of modern-day horseshoe crabs only a little smaller and you know we can use a lot of we can tell a lot of information with those with those fossils but it is the fossil record although is replete as it is there's still a lot of missing missing information we'd love to find some more so we already talked about the ammonites and they're kind of like a chambered nautilus they're a mollusk and then the trilobite stone below sorry but there's II and so that really nice thing though as I mentioned earlier these index fossils they were so widespread and so abundant that we can actually use them to quickly relatively date rock material all right guys I hope you enjoyed this this week's chapter on geologic time plate tectonics if you have any questions go ahead and call or text and send me an email and I'll be happy to take a look at that Thanks