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Continental Drift & Plate Tectonics
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0:00
hi welcome back to kiersti's virtual classroom today we're working on learning a little bit about continental drift and the rise of plate tectonics
0:12
so one of the things that a lot of scientists questioned early on was why we have mountains and valleys and i'm sure you've questioned this as well um why do we see vast landscape when we look outside why is fresno flat in the sierra nevadas are these high beautiful mountain ranges so take a minute to kind of think about that and through this lecture you will learn why exactly we have mountains and valleys and different topography so in the early 20th century scientists
0:45
didn't know much about how our earth interacted with itself and a lot of scientists thought the continents and the oceans didn't move that they were fixed and that for all time that earth existed this is exactly how it looked and that nothing had moved or anything like that so their reasoning for why we had mountains and valleys was from crustal fracturing and if you remember when i talked about mars in our solar system lecture that crustal fracturing can cause the rocks to
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either form high peaks or big valleys and that's from the expansion and contraction of the earth as it's heating and cooling so when something is warm it expands and when something's cold it contracts and so as you see the earth slowly cooling over time after the formation of earth where it's a hot molten body of rock as it cools down there is shrinking and cracking and so that's what a lot of scientists contributed um the mountains and valleys to early on alfred wegner on the
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other hand was an explorer that looked looked at things a little bit differently so we're going to talk about kind of what he discovered and how he came to this theory of continental drift
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so on earth at any given point there are a lot of different earthquakes occurring so that's what all these little stations that are popping up are and so this kind of will tie into continental drift then we'll move to plate tectonics we'll start talking about faults and earthquakes and they'll all kind of be interconnected okay so the continents as we know it today are not this have not always been the same so throughout earth's history we have seen a shift
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in the earth's continent now this is what we know today this is a screenshot or a snapshot of what our continents have experienced if you will we've had areas or periods of time where the continents have all come together and formed a large land mass or an all lands mass one of the most popular is called pangaea and then right now we do have seven different major continents where our continents are all in different locations throughout the world
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so as alfred wagner started developing this theory he started taking a lot of observations that he saw in what surrounded him he did a lot of voyages to greenland and iceland and he went to greenland specifically about four times to measure ice thickness and ice may sound trivial but when glaciers last for decades upon thousands of years they become really important to the rock record because they're basically their own ice record of atmosphere and how much
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snowpack you might have seen in a particular year so he did die during his fourth trip but he was able to successfully develop four different hard evidences for his continental drift theory so
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i want you to take a second to think of what you think those four pieces of evidence to suggest that the continents are not fixed and that they do move would be what kind of things would you look for if you were a geologist or just an explorer like alfred wegener happened to be what would you look for to figure out if these continents are moving or if they've moved in the past or if they've been stationary since they've existed
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so if you need to pause this video you can or you can continue on and we'll start talking about them so one of the most obvious that a lot of people recognize is that there is something called a puzzle piece fit a lot of the continents look like they may have fit together at some point in time the most notable is how the east coast of south america looks like it fits perfectly into the west coast of africa here and we can make some inferences based on continental shelves and
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those alignments to kind of line up all the other pieces or all the other continents so the first piece of evidence for alfred wegener's continental drift theory is this puzzle piece fit
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number two is fossils so we won't learn about fossils entirely for quite some time in the semester until we get towards the end but fossils are just um preserved animals or preserved species and if they are found in a particular region that tells you something about what environment they lived in i mean one of the things that's interesting is there are several fossils that are found on separate land masses so for instance um we have the mosasaur here which is
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the swimming guy it's kind of like a kind of like an alligator type of uh creature and he is fresh water he's a reptile he's fresh water um which means he can't swim across the atlantic ocean and as you know today south america and africa are separated by the atlantic ocean so in you know millions of years ago when the mosasaur or existed did he swim across the atlantic ocean or was there a land bridge between those two continents and so this was one of the real important theories
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because um fossils and prehistoric animals did not have the capability of crossing large seas and land masses that we do today so using some of those different fossils that are found on separate land masses helps indicate where we might have found
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where the continents might have sit previously so this is evidence number two evidence number three would be rocks so in a particular mountain building event which we call an orogeny an orogeny is just a mountain building event when mountains are built generally the chemistry the age and the composition of those rocks are fairly similar and we found rocks along the appalachian mountains
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on the east coast of the united states and in some of the mountains in africa and up into europe correlate with each other they look like they're about the same chemistry they have the same rock composition and they're approximately the same age which means they formed around the same time so that being said that means that those land masses might have been connected at one point in time so rocks will tell you a big deal of um what the continents looked like previously
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and then the fourth is climate or glacial evidence so what's really interesting about glaciers is they do leave behind particular features or deposits that indicate that the glacier existed at some point in time there so there are things called like glacial moraines or glacier glacial erratics so if you go to the canvas page the first picture that you see there where there's a kind of a perched rock in yosemite that is a glacial erratic that is a basically a pebble it's
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obviously a boulder much larger than a boulder a piece of something that was stuck in a glacier as the glacier moved over the landscape and it dropped it off as the glacier receded or melted and so things like that give us an indication that there was a glacier there at some point and because of that we've seen glacial evidence in africa and south america which is really close to the equator and as we know glaciers cannot really form in areas that are hot right so you
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need something close to the poles or really high elevation in order to see glaciers form well if we see glacial evidence in africa or south america that would indicate or give a really good indication that they might have been closer to one of the polls where maybe antarctica is at some point in time so that gives us our fourth piece of evidence there
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okay so just to reiterate are four pieces of evidence from alfred wagner is the puzzle piece fit the fossil evidence the rock evidence and the glacial or climate evidence
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okay so he had all these pieces of evidence and he was really sure that this is what was happening but he didn't know how to explain it he he had all of these things that he observed happening but he didn't know how they were happening so he was missing the mechanism behind the actual theory um so he has his theory of continental drift and to get to the theory of plate tectonics he needed the how okay so we're gonna kind of discuss how the how came about but first we need
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to know something about the interior of the earth so inside the earth we're not just one solid rock there are different layers to our earth okay so we have three main layers and then we'll kind of break it up from there so our three main layers are the core the mantle and the crust okay the crust is what you stand on the mantle is below that and then the core is the center of the earth
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okay so let's start with the crust the crust is where we stand so we have continental crust and we have oceanic crust i'll talk a little bit more about that in a second and then below that we get into something called the lithosphere so the lithosphere is the crust and the uppermost solid mantle so the mantle is this very vast thick layer and it is segmented into three different parts the upper the middle and the lower
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and depending on where you are in the mantle you will see a different um
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state so closer to the crust it's going to be solid and then as you get closer to the core it's going to be more molten okay so the lithosphere is cross and upper mantle which is all solid material okay so that's a term that includes the crust and the upper mantle then below the lithosphere you get into the asthenosphere and the asthenosphere here is molten so it's a little bit gooier it's not solid rock anymore it's more like a laffy taffy
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okay and this is what allows the crust or the lithosphere to actually move and we'll talk more about that in a little bit and then below that you just have regular mantle and then we have the outer core and then the inner core the outer core is liquid and the inner core is solid
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all right so let's talk about the crust a little bit so i did mention that there is oceanic crust and continental crust and they vary slightly so the continental crust is thicker which is why we don't have oceans covering much of the continental crust and the oceanic crust is thinner which is why that's where most of the water sits so it's um at the low point on the crust the continental crust is much more buoyant and it is less dense than the oceanic crust so the
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oceanic crust sinks below the constancy crust when they interact we'll talk more to that a little bit but just make sure you're aware oceanic crust is denser than continental crust okay and the oceanic crust is much younger than the continental crust okay so what is the crust made of so normally in a class i would have you look at rocks just look at the pictures for now um and try to distinguish which one you think might be oceanic crust or continental crust so you can
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write down some thoughts think about it i will tell you that this one on the top here that's all black that one is much denser than the one on the bottom here which is a you know black and white in a particular class period you would actually be able to feel this in your hand
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but use the photos and the information i've given you to kind of think about that and then
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okay so the one here at the top is the more dense rock it is basalt it's an igneous rock actually both of these are igneous rocks but the basalt would be your denser crust so this would be your oceanic crust and this one here on the bottom would be your continental crust is much lighter so most of the oceanic crust is made up of basalts and most of the continental crust is made up of granite so basalt is spelled b-a-s-a-l-t and granite g-r-a-n-i-t-e
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so granite is continental crust the salt is oceanic crust so this is in general
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okay so how do these solid rocks actually move in real life so to understand kind of how the continents might move we need to understand what would maybe drive them so a good example would be a lava lamp normally i would have a lava lamp set up for you to look at but i think most of us have seen a lava lamp perform um and what they kind of look like so here's a picture for you um what do you how do you think the lava lamp actually works if you're not sure you can take a
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second to try to figure out what you think might be happening if you've never seen a lava lamp before here's an example it's the blobs of orange who start here at the bottom and then eventually they'll rise up and once they get towards the top they'll start to fall again okay and that's just kind of the cycle that you'll see a lava lamp take on okay so take a minute and see what you think
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okay so the way that the lava lamp works is there is a heat source at the base so let's go back and look at this for a second so we have a heat source here at the base and it heats up the oil which is this orange stuff and then there's resin as the purple stuff so it heats up the oil and when something heats it will rise as it comes to the top it cools down and becomes more dense because it becomes more dense it ends up falling so this is a heat transfer method that we can see
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even in the basic form of a boiling pot of water this is the reason why your your pot of water will boil so we have three different types of heat transfer radiation conduction and convection radiation would be like receiving heat rays from the sun or rays of heat from a fire a campfire or something like that and then conduction is when you touch something and it's hot you feel that heat transfer into your hand or whatever body part actually touched the hot pan
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and then convection is what's happening in the water or in the boiling pot here so we have a heat source obviously the burner is our heat source the water that's closest to the burner heats up first and when it heats up remember it becomes less dense so it's going to rise as it comes towards the top of the pot it cools down again when it cools down again it becomes more dense and sinks and so you see the cycling and when you have a bunch of little convective cells in your boiling
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pot of water it creates bubbles and so you see boiling water okay so how does this translate into the earth well as i've said before as you get further down into the mantle it gets a little more molten and things can actually move around in that case you have this center core that is really hot in the center of the earth is acting as your heat source so there's something called mantle convection that is where the mantle material is being heated by the core down here
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cooled by the crust and forms these convective cells so the mantle is hot iron-rich rocks and the texture is behaving like an elastic solid and it flows in these convective currents and this convection mantle convection was founded by a fella named arthur holmes in 1928
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okay so then we get into the core which is liquid iron and nickel at the outer core and the inner core is solid iron and nickel and the reason that the inner core is solid and not liquid is because it's under so much pressure liquid cannot flow at those pressures and temperatures so you see a solid basically rock of iron and nickel the outer core however flows at several kilometers per year and the flowing motion creates the magnetic field which we'll talk about in a little bit
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okay so one of the next things that happened after mantle convection was sonar mapping by this guy named harry hess and so around world war ii we were using a lot of sonar technology and we used sonar mapping to map the sea floor so what they would do is they would take a boat out into sea they would send down a signal or basically a sound and it would hit the bottom of the ocean floor and bounce back and so this was really important in furthering
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the development of plate tectonics but also understanding what the seafloor looked like
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for a long time a lot of people thought seafloor was flat right when you you know put water on top of some soil outside most of the time what's below it the topography is just flat
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so doing the sonar mapping they found some really interesting things
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one of those interesting things was the mid ocean ridge so in the middle of the atlantic ocean there is a ridge and in the middle of the pacific there's another ridge which we'll show in a second so as they came off the continental margin of north america they saw a definite dip which is what they perceived the entire ocean would look like but then once they got towards the center they noticed that the sound waves were coming back
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faster which means that it's a higher topography and you're no longer at a low point so then that's where the moon ocean ridge was found and then there's another deep ocean basin before you get over to the coast of africa okay so it revealed that right in the center of the ocean there's this very large ridge still submerged beneath water but there's this ridge down below the ocean
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okay so between the 1940s and 60s harry has started kind of connecting all of these dots connecting things that he saw from arthur holmes and from alfred wegener and he developed this
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theory called seafloor spreading so he echo sounded the sea floor which showed him the sea mounts and the topography at the mid-ocean ridges he they did note that at the ridges the temperature was a lot higher and they did notice volcanic activity and this appeared to be the younger material so the sickness the thickness of the sediment that's actually accumulated on the ridges was very thin compared to the basins so he also suspected that
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the continents did not actually plow through the oceans so after continental drift a lot of people were surmising that maybe the continents moved but the oceans didn't so that the continents sort of floated on the water and moved around but that's not what harry hess thought after doing some research so when we combined harry hess holmes and what alfred wegener developed we see where the
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continent and ocean meet at the trenches the ocean is being returned to the mantle at the same rate it's being generated at the ridges or henceforth developing this seafloor spreading theory
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so let's take a look at the diagram this will kind of show you all encompassing what's happening at the sea floor and what's happening with these convective currents so here we see two convective mantle convective cells and they're going in opposite directions so this one here on the right is moving in a clockwise direction and the one here on the left is moving counterclockwise when convective cells move in different directions they start to pull whatever is above
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apart and that's what we're seeing in the middle of the ocean is this pull apart and then material starts to fill in which would be the development of a ridge so material is being created at these mid-ocean ridges and then where this crust or this lithosphere is hitting the continental lithosphere you see it return back into the mantle as it's drug down by the convective cell okay so these are just cycles so we're pulling apart at the top and it's being taken back down
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into the mantle at the continents okay so to further this theory of plate tectonics as we start to get further and further away from just continental drift now we're talking about plate tectonics where we have plates all interacting with each other so this is the snowball after
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the seafloor spreading storm essentially so before we start really talking about how we um got ages and things from the seafloor we need to know something about paleomagnetism so paleomagnetism is the study of iron-rich minerals in rocks that have been magnetized by the magnetic field when they were crystallized so when a rock forms out of a lava there are magnetic minerals or iron-rich minerals that will magnetize themselves and align
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to magnetic north at the time of formation and we do see flips every 300 000 years of our poles so right now magnetic north is where geographic or true north is right but over time the magnet flips and we see magnetic north become true south and magnetic south becomes true north so the way that the magnet works around earth is the flow of the liquid outer core and the interaction of the inner core that's solid allows for the magnetism to occur and it basically turns
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earth into a giant magnet with a north and a south pole just like any magnet would have and the magnetic field around that magnet flows from the south into the north just with any magnet right and in a reversal it would flow in the opposite direction so your rocks would point in a different direction depending on where magnetic north was at the time of their formation okay so take a look
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at this diagram this is going to kind of go over how we relate seafloor spreading to paleomagnetism so look at these three diagrams and then kind of think about what you think is happening and then we'll discuss so you can pause the video now and think about it write down some notes
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and then we'll talk about it so what do you think is happening so here in period a or time a excuse me um we have a normal reversal and you see magma coming up and filling in this void as two plates are moving apart okay and then in time b we see a reversal and we see more material coming up and you see the arrows changed direction right so that was indicating that the true north is or sorry magnetic north is no longer where true north is we have a reversal here and so you see
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them shaded a little bit differently and the arrows are pointing in a different direction okay and then time c we see normal magnetism again so we have magnetic north is where true north is and you see the flip of the arrows once again and you kind of see these stripes being formed
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so the stripes if we lay that across all of the seafloor give us a really good indication as to how the sea floor is actually spreading and how things are actually moving so in the center we have a ridge that we talked about right and that's where all the youngest rocks are so if you look at this map that's created using paleomagnetism we see all of the youngest rocks the ridges and all
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of the oldest stuff closest to the continents and they are symmetrical as you move from the ridge so whether you're going east or west from the ridge you see a symmetry so you see the same age rocks with the same orientation to magnetic north as on the other side of the ridge and so that just further indicates that material is being created at the ridge and then being spread apart created at the ridge spread apart so this really didn't come up until the 60s
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which is really interesting because a lot of people think the geology is old because the earth is 4.6 billion years old but a lot of the theories that surround geology are not that old i mean you probably have parents or grandparents born in the 60s and so you kind of you even know somebody that's still alive since then so these are very new ideas um relatively speaking
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okay so i already said the youngest rocks are at the ridge oldest are up against the continents so if we look at a map across all of the oceans on earth you look at something like this where we see all of the ridges at these very dark reds and that's where all of our youngest new rocks are being formed and the oldest rocks are all up against the continents
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okay so with that we'll look at the different tectonic plates so there are a lot of different tectonic plates some are very small and some are very large so we have the north american plate which is where we lie if you go over to the coast of california you'll also be able to stand on the pacific plate so you can stand on another if you go up into northern california you can go up and stand on three different plates at one time
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which is pretty cool um and uh the plates of all these arrows here are in are showing you the interaction are they coming together are they pulling apart from each other are they sliding past so the next lecture you'll see is the interaction between the tectonic plates and how all that works and what that forms this was just the rise and development of the theory okay but before we go let's look at the seven major plates so north america south america
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eurasia africa india pacific and antarctica so the pacific is the largest although on that map it doesn't necessarily look like it if you look at an actual globe the pacific is stretched much larger than the north american plate or even the antarctic and plate so remember these seven major lithospheric plates we'll talk about a lot and we'll talk about some of the smaller ones as well
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so in summary we talked about the continental drift which was founded by alfred wegner and we talked about his four pieces of evidence um seafloor spreading and massive convection led to the rise of plate tectonics we also talked a little bit about the earth's composition and that the continental crust is much less dense than the oceanic crest which will be important for interactions in the next lecture we looked at the map of the sea floor which
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used sonar mapping to determine that there was young material at the ridges and the oldest on the continents and then we reviewed the seven major tectonic plates on earth so i hope that makes sense and i will see you guys in the next lecture bye
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