Plate Tectonics (and hotpsots)

Added: 2026-05-09

[00:00:00]welcome back geographers today we'll look at plate tectonics and plate tectonics is really the driving force of what we see on the earth's surface as far as mountain building where erosion is taking place where it's not taking place different it it it brings together so many different aspects of the world we see physically today everything from earthquakes to volcanoes are related to this and uh i have a textbook from the 1930s no it's not mine but in it to explain where the mountains came from it talks about how when the earth cooled it shrank and those are like the wrinkles on a dried up grapefruit and i thought wow we really didn't know what was going on we were just guessing at things and the beautiful thing about plate tectonics it's it's observable it's measurable and it it brings together a lot of things that had been confusing in the past so we'll talk about how the pl the continents are moving around on the earth's surface but they're not the only thing that's moving around on the earth's surface and part of it is um we've all done that puzzle piece of seeing wait a minute that's not the puzzle piece well we'll talk about that later the puzzle piece of how south america fits into africa and then you kind of take that coast of africa and it kind of corresponds with that curvature of north america and alfred wegner at the turn of the last century had observed i guess would have been between the 1800s and 1900s so the previous century um had observed that very phenomenal that a lot of people had but he was also liked fossils and so he was just an amateur fossil collector and would go to fossil talks because by trade he was a climatologist and meteorologist and he would travel the world doing studies for that but while he was there he would also see what the local fossils were like and fossil clubs so his one of his observations in addition to how the continents fit together is that if you looked at fossil records the types of fossils if you put the continents back together the types of fossils would line up as well and also the types of rocks would line up wagner also observed that i mentioned this briefly before when we talked about igneous and rocks that when they cool the minerals kind of go in random directions except for the magnetic minerals some of those will align themselves with the north pole as the rock cools and current volcanic eruptions you can observe that in the rocks as well however there'd be some volcanic rocks where the magnetic minerals within them although at that location all the minerals were pointing in the same direction they weren't pointing towards the north pole and so if you were to move the continents back into this position that would be in alignment with the north pole so wegner had this theory that the continents were drifting on the earth's surface there was his problem with with it he had amazing observations about what things where things used to be but he didn't have a good mechanism for how they got there so the other observation was that the oceanic floor was denser than continental material silicates that you would find in the continents made them his thought was that made him buoyant and there was something driving them to float across the ocean floor well the poor guy got laughed at and never got the respect he deserved because he he had the theory of continental drift he just didn't have the mechanism to which it it was working so his idea was there was a massive continent called pangaea and pangaea broke into two smaller continents loracia and my all-time favorite word gondwana land it's like an area at disneyland that nobody wants to go to but gondwanaland larasia broke apart and then our continents moved into our current locations now this pangaea would explain why you can find glacial erosional evidence in australia because clearly australia isn't going to be glaciated nowadays because it's up in the tropics well roll this forward to post world war ii we start doing research on the ocean floor we've got new technology that was developed during the war sonar radioactive right sorry radiometric dating that we can do on rocks we can also look at the magnetic signature of rocks to see which alignment it is for their north-south orientation the minerals we do all these at a distance and we're measuring back and forth across the atlantic ocean just looking what the ocean floor looks like so marie tharp was one of the leading scientists on this and she discovered this kind of massive zipper right in the middle of the atlantic ocean and so this was something that obviously would should be further investigated because it's kind of midway between the continents and it kind of lines up with the puzzle pieces as well as far as where these continents move into their current position also i'd mentioned the magnetic shift north-south we've known this for a long time that the poles every so many years it's it's in the hundreds of thousands or millions of years so it's not something that keeps me up at night um so then the north pole which is positive now becomes negative and then the south pole becomes positive um so that magnetic reversal is you can be seen in the rocks so if you're at the middle of the ridge that area has the current magnetic signature on both sides of that is the reverse of that magnetic signature and that pattern of stripes leading away from it is current pattern reversed current pattern reversed you go the other direction current pattern reverse current pattern reversed the relative width of each one of those is the same on both sides and so they kind of mirror each other but it all starts with that one singular one at the center of the ridge with the current directions so the thought is that in the lithosphere sorry this graphic is kind of crummy it's solid and it's solid rock but beneath it is what is the asthenosphere let me see if it's in the next one where is that here we go so beneath it is the asthenosphere which is partially molten remember it's plastic and there's convection currents in it where the upwelling convection currents are they're pushing across the solid lithosphere and they're pushing it in two different directions so if we've got upwelling and it's pushing apart the lithosphere some of that molten material from the asthenosphere makes it onto the surface that's why we've got the current polarity at the ridge it's because that's the brand new rock coming from below and it's pushing the lithosphere in two different directions so this little hypnotic graphic down here as opposed to to demonstrate it but don't stare at it too long so that mirror image moving away is reflective of as the polarity of the positive north negative south reverses you would see that polarity reversal in a pattern moving away from the center of the ridge and it repeats itself because the ridge just isn't in the mid-atlantic it goes beneath the continents it goes up into baja california it also goes through california but that's a different story we'll get to that a little bit later and then comes out again off the pacific northwest and we can see that same pattern with the ridge there current reversed current reverse current reversed as you move away and a mirror image of each other so the third thing that tells us what's going on is the age of the rock through radiometric dating we should expect by this theory that if everything is coming up at the mid-atlantic ridge it should be the youngest rocks and that's exactly what you see the mid-atlantic ridge and the entire ridge system is the youngest rocks and in theory it should get older as you move away and in fact it does it gets older as you move away now the breakup between gondwana sorry it's going to make me laugh every single time gondwanaland and laresha happened first and sure enough that age of rocks on the coast of africa and the age of rocks off the coast of north america they're the same age and they're the oldest and then when we see the age of rocks matching on the african coast and the south american coast that corresponds with that initial breakup of those continents so what we didn't have before was the mechanism for continental drift and what we didn't realize it's not just the continents that are moving it's an entire section of the earth's surf that's moving that the continents are a part of oh this is a new one i got recently and it's kind of trippy so it's not just the continent that's moving away um so you end up with the entire plate moving with the continent on it now what you're seeing here is a continent coming in this direction and an oceanic plate coming in this direction these are plate one of the plate boundaries we'll talk about in just a second i already talked about this moving apart different directions the multiple plates so if we roll back in time to where they were in the past i have apparently the world's slowest animated gif working because it's not doing anything in fact even my space bar is not working right now well we'll just ignore this graphic and i'll move on or i won't oh there we go so one of the problems though is you know if you're on one side of the split between the two plates i told you i was going to tell jokes the rest of the semester so when i talked about the entire it's not just the continent that's moving so when south america and africa are moving away from each other what's happening is these were one massive continent together gondwanaland and when the continent split apart and the pieces of the continental material started moving both directions what was happening is what was replacing the space between them is oceanic crust so south america sits on this big chunk of the earth's surface and on that is the continental material that outer crust of the lithosphere but also on this big plate on the earth's surface is oceanic crust in this area so this plate has two things on it it's got continental crust and it's got oceanic crust and what's pushing it in this direction is we're adding more oceanic crust on this side the same thing with africa is the african plate has continental material on it and oceanic material so the african plate is being pushed to the northeast because we add more on this side south american plate we're adding more on this side so that's pushing the plates in multiple directions well you can see how there might be a problem because if we're pushing plates in different directions they're going to collide with other plates so the nazca plate here is being created on this portion of the oceanic ridge and it's pushing that oceanic crust towards the south american continental crust and that's where they collide and so we end up dissolving the nazca plate as the south american plate runs over it so we keep the size of the earth the same we just kind of move the pieces into different locations some of the pieces are getting bigger some of the pieces are getting smaller as it moves around now one of the things you'll notice with where the plates meet whether it's they're splitting apart or where they're coming together that's where you find earthquakes and volcanoes because that's where you're going to get the most energy i mean you got to imagine you've got sections of the surface of the earth pushing against each other colliding against each other sliding past each other what kind of tension that's going to create on the earth's surface now the idiot who made this map and it wasn't me decided that they were going to use white on light yellow to show where the earthquakes are so it's almost impossible to see that there's a difference in color here and here because this is where all the earthquakes are concentrated and it corresponds with where the volcanoes are now you'll notice there are some areas that you can't really see anything hey look california but this is an area that's prone to earthquakes but not volcanoes so that stretch between northern california and central mexico doesn't get volcanoes but we definitely get earthquakes in this area and we'll come back to these other random things out in the ocean a little bit later so if we put look at all of our crustal motions so we've got that divergence where the crusts are spreading apart in the middle of the ocean so the crust is moving in this direction but we're also going to have the crust coming together and they're going to see areas where the crusts are colliding but this is what keeps everything the same shape and so this is when i talked about the south american plate being made up of continental crust and oceanic crust you can see it all right here so well apparently i was bored when i made that particular powerpoint slide so we've got three main types of crustal boundaries and each one is associated with different landforms and i'll go through each one and we're going to have different combinations too within each one because we've got two types of crust we have ocean anti-crust and continental crust so we can have those two coming together in different way are spreading apart in different ways giving us different resultant land forms remember the second half of the semester we're looking at landforms so this is the underlying cause of the landforms are the plate boundaries so now we're going to look at the landforms at each one the first plate plate boundary type is the divergent plate boundary and just like it sounds the plate are diverging so depending on where those plates are moving apart you're going to get different landforms so this particular image you're seeing here is a divergent boundary under a continental crust and so that continental crust is being pulled apart you may get some volcanic activity with the magma moving to the surface but primarily you're looking at that stretching of the crust and when it stretches it makes it thinner and some of the pieces will fall down in what are called rift valleys apparently my phone decided to light up right as i'm doing this lecture and so as you pull it apart the center portions will sink a little bit and create what are called rift valleys so under a continental crust as it stretches it creates a rift valley and another name for is gravin we will come back to these types of landforms a little bit later because they're really noticeable in nevada but right now we're looking at much larger ones and that rift valley as you stretch it it kind of sinks down in now eventually the asthenosphere is going to be move up here because you're going to pull the continental material far enough away from each other that it just simply isn't connected anymore but until you do that you've stretched it and it down drops so what we where we can observe that is through the red sea and down through africa as this is stretched and this is called the great rift or the great rift valley and you can see this block right here has dropped down in comparison with the highlands around it this particular block has dropped down so much that it's actually dropped down below sea level and is flooded from the indian ocean with salt water the rift valley in africa has exposed hominid remains and so it's one of the big hotbeds for fossils in that area we went through tanzania and one of the the really exciting parts for me at least was we went through the eastern rift valley in tanzania and so we're down in this area not this particular location but we're in that down dropped block in tanzania and that's the ups blocks on the other side those are called the horse it's a little bit easier to see once you're up on top of the ridge looking back down into the valley so in the oceans though once we've pulled the continents far enough apart you're bringing up that new material from the asthenosphere and that becomes the new oceanic crust and so when you've got the material upwelling it causes a little lift in the oceanic crust and so there's a ridge line right where the upwelling is coming from from it's that new magma that's making it to the surface and cooling that material is coming from the asthenosphere and is redeposited at the surface and it gets pushed across so if we look at that ridge system that you'll notice this if we were to take away all the water it would be the world's largest landform stretching from the americas all the way under into the indian ocean and all the way up into the arctic that's a massive massive structure parts of it are actually slightly above sea level and that's where you get some of the islands like iceland so iceland this is the mid-atlantic ridge it's just that it's upwelled enough that it's above sea level and you can literally walk down and so on this side over here this is the north american plate it's heading to the west and over here is the eurasian plate and it's headed toward the east so this is the edge of north america right here heading in two different directions look how happy i am and i have no idea why i only have one glove on that was plate boundary type number one number two is the convergent boundary so if plates are moving apart it would make sense that they're going to run into another plate in the process and so at a convergent boundary with one exception there's going to have to be something that changes where you're going to end up sending one piece back down into the asthenosphere to re-melt so in what you're seeing here this is the divergent boundary over here but here's the convergent boundary in this particular case it's one of our three scenarios you can have if you've got oceanic crust and continental crust your three scenarios are ocean crust meets ocean crust continental crust meets continental crust or in this case you see here oceanic crust is colliding with continental crust well oceanic crust is much denser than continental crust this has all those silicates in it that change its density so when a more dense object hits a less dense object the denser object goes down and it goes back down into the asthenosphere and re-melts that's a convergent boundary so let me go through the three different combinations possible and also figure out why my phone's lighting up all of a sudden again even though i silenced it and my daughter's freaking out so i'll be back hold on okay um that's convergent plate boundaries so of our with our three choices of ocean incontinent ocean ocean or continental continent we're going to get different land forms so i'll start with ocean to continent this would be the equivalent of the nazca plate going under the south american plate or also what we see off the north coast of the united states sorry the northwest coast in the pacific northwest or we've got the juan de fuca plate going under the north american plate giving us the cascade range so you've got this plate the denser oceanic crust colliding with the less dense continental crust so the oceanic crust goes back down into the asthenosphere and in doing so it re-melts and it re-melts some of the surface mud and other deposits on the ocean floor some of the scraped off continental material and so that has a different chemical composition than the oceanic crust itself so it changes its melting properties so it melts a little bit sooner doesn't just go all the way back down into the steenosphere and it creates volcanoes and so we get a range of volcanoes so the andes is a volcanic mountain range and also just simply the collision of the continental crust against the oceanic crust compresses it and in that compression that creates mountains as well so the compression causes the mountains to be created and the volcanoes are also going up in that same location now sometimes you'll get a coast range of mountains that from the compression this particular one for the andes is just simply you've got the compression and the volcanoes create the the what would become the andes so you've got a volcanic mountain chain you've got when the crusts collide it pulls down this one slightly it's dragging this one downward a little bit and so that crop causes a depression or when this goes down it pulls a little bit of the surface with it and so pulling it down creates a deep ocean trench our deepest ocean locations are where we have two plates coming together and they just drag each other down just a little bit so you get a trench you get a volcanic mountain range and a ocean to continent boundary now ocean to ocean you don't have that big collision and compression of a continent so you're not going to get a massive mountain chain so you'll still get volcanoes you'll get a volcanic island arc so this would be in say the indonesia or the japanese mountains you'll get a trench but that's about it now when you have continent to continent you have two things of less dense material colliding with each other and that continental crust can't be forced down into the asthenosphere because of its density so what happens is when you have a continent to continent one it's called a suture zone and they basically seal up against each other and stop the movements of the plates and from continuing to move towards each other now obviously this is going to be a massive collision and it's going to cause tremendous amounts of uplift so guess where the highest point on earth is mount everest where two continents came together the indian subcontinent and eurasia and it basically sutured the indian subcontinent onto eurasia to become part of that plate so this gives us tremendously large mountain ranges but notice there's no melting no melting no volcanoes so just a big mountain chain all of these and in all of our plate boundaries earthquakes also happen but right now we're just looking at the landforms so here's the nazca plate off the west coast of south america colliding with the south american plate and in doing so you get that tip to tip mountain range of the andes like i said you're going to get tremendous earthquakes when you've got these plate boundaries a 7.9 in peru in 2007 ironically on august 15th of 2007 i was flying out of lima i literally was in the air an hour or so before the big earthquake hit the volcanoes that you get within the andes now we got to visit oserno a couple years later a year later excuse me the volcano immediately to the south of it erupted the aleutian islands the pacific plate colliding with the i can't remember the name of it plate creating this deep trench to the deep blue you see in through here and this chain of volcanic islands the good friday earthquake was one of the biggest earthquakes in american history it was the biggest one in the 20th century and of course the volcanoes that often on erupt up in that area no big massive eruptions but sometimes there's enough enough ash that's thrown into the air that disrupts flights to and from asia in the united states and then pavlov this this volcano drooled though and that's a joke indonesia with the indo-australian plate colliding with the eurasian plate now this one does have a lot of volcanic activity activity tambour in 1815 and krakatoa in 1890 um two of the biggest eruptions in modern history and when we talk about volcanoes i'll talk about krakatoa just how massive of a volcanic eruption that was also this plate boundary the that pulling down of the plates as they move past each other sometimes as they get pulled down they'll rebound and bounce back up a little bit and so for about a 600 mile stretch the indo-australian plate bounced upward and created the massive tsunami in 2004.

[00:26:14]on a smaller scale but the same thing we had a convergent boundary pulling down one of the plates and it bouncing back up um created the japanese tsunami in 2011.

[00:26:28]so the con the convergent boundary for two continents to coming together is evidenced by india colliding with the eurasian plate and the top of everest is made up of oceanic floor material you can see there's limestone mudstone fossils what where that material came from would have been this ocean here and that's where the sediments were deposited so as the continents collided those sediments were pushed upward in that collision because there wasn't subduction and so the continents collided and pushed those sediments upwards to create everest i've always fascinated that the top of ephras used to be the ocean floor the third boundary type is referred to as a transform boundary at a at a transform boundary the plates are sliding past each other and we're dealing with the earth's surface which is a three-dimensional surface so you're going to have some plates that are colliding directly some plates are colliding indirectly and some plates that are just simply moving in opposite directions as they scrape past each other so since there's no subduction in this third boundary type there's no melting no melting no volcanoes however you still get earthquakes and you can have massive earthquakes on a transform boundary the only thing about a transformed boundary outside of earthquakes is that landforms are really subtle because you don't have that tremendous pressure you're not going to get mountains as big as the andes you may get a little bit of pressure as they move past each other at a slight angle that's what happens in southern california that slight angle of the california portion of the or the southern california portion of the san andreas fault gives us our local mountains but nobody's going to confuse mount baldy with mount everest that's for sure so your land forms this see this little depression here the rock is ground up so much as these slide past each other it's kind of weak and so it creates depressions called sag ponds even the name is kind of depressing that it's not much of a land form it's it's not even a lake it's a pond and it's not that deep because it's just a sag so it's a sag pond and then offset streams because the streams flowing across will get pushed as this plate moves in one direction we'll talk a little bit more about some of these later but again nothing to write home about for us though it's a very real plate boundary because it gives us stress over a huge area it's not just stress along the san andreas fault that stress is parallel in both directions and gives us massive earthquakes in the area we'll talk more about earthquakes on the san andreas fault when we get to the earthquake lecture this area here this is the san andreas fault and on this side is the pacific plate the world's best plate because that's where we live and then this is the north american plate which is okay there's a couple of sag ponds in here that you can't even see from the freeway and this area there's lots of fracturing of the rock and through here rightwood is literally on the other side of this little slope here that rock's all fractured and there's been a couple of landslides in that area as well this rock type does not match this rock type because this rock type would have started much farther south and slowly migrated centimeters per year up to this location so if you're wondering where it runs through these guys out in through here big bear lake wrightwood they're all going to have oceanfront property in about 20 million years so encourage them even san bernardino they're just going to be on an island off the north coast of western north america they're going to have oceanfront property so encourage them to stay out there and not move to orange county here you can see the sag ponds this is just outside of san francisco and hey look this is a great place to put residential development our last topic on plate movements and plate tectonics also works as kind of a transition to our next lecture which will be about volcanoes in our next module the hot spots or mantle plumes when we're involving movement of material in the asthenosphere and how we can see the plates are moving over the asthenosphere so the hot spots and mental or mantle plumes it's depending on who's saying it um i tend to call them hot spots because it's a cooler name than a mantle plume but the there they can show us that the plates are moving because these odd little features within the asthenosphere they stay in one place and then the plates move over them so what is a hot spot well it's a singular plume of hot material coming up through the asthenosphere and then burning a hole literally burning a hole in the lithosphere as it moves over it it when i say singular plume this is different than a convection current where you've got massive movement over thousands of miles in length of material moving underneath the asthenosphere it's something about the outer core see i said i was going to go deeper in the earth and i did sorry something about the outer core creates a singular location that creates hot material moving up to the surface so imagine a straw bringing hot lava to the surface well if the lithosphere is moving on top of that hot location it the holes that it burns in the surface are going to move away from the mantle plume so we'll see ancient volcanoes moving away from a current volcano so this lithosphere this used to be over here but it slowly moved in this direction and so those old volcanoes get eroded away but where you are over the current hot spot is where the current volcano is this animated gif sometimes works sometimes doesn't work but it's the the general idea of the volcano moving away from the hot spot and then no longer being an after active volcano so of course hawaii is a great example of that i know there's other ones in the pacific that show it but the red dot represents where the hot spot is but if i move the plate the hot spot didn't move the plate moved so it burned a series of volcanoes into the oceanic crust and hopefully this would match with radiometric dating of the rock and sure enough it does that the oldest rock is outer in this direction actually some of it is being subducted in at the aleutian islands but on this particular one we're at 60 million years here about 38 million years here 20 million years old as you get down into the hawaiian islands oahu 2.6 million years old maui 1.3 hawaii half a million but there's even current eruptions happening right now so there's brand new rock coming up in hawaii because that's over the hot spot actually the vault the big island has moved slightly off of the hot spot so the there's even activity offshore too so when you look at the island chain in the western end of it kawaii you can see that this really it it's volcanic rock but it doesn't look like a volcano your dominant features that you're looking at are erosion on a volcano whereas if you move to the big island you've got sometimes active flowing lava now we have last big eruption we had was two years ago in 2018 there was a little bit since then but this is again when you're talking geologic time this is new rock coming onto the earth's surface and when we talk about volcanoes we'll talk about how much fun it is to go see this kind of stuff happening so the dominant landforms here are volcanic landforms with evidence of recent volcanic activity and apparently there's orgraphic lifting happening in the background too all right so we'll do volcanism not peace live long and prosper volcanism we'll do volcanism in our next module thanks for listening