GEOL 101 - #15 - Plate Boundaries

i mean you guys can stretch out let's come up here just for fun just kidding emily won good morning everybody nice to see you all thank you for joining us the local time is 9 45 pacific time and we will begin our session at 10 o'clock on plate boundaries plate tectonic boundaries thanks for joining us we're starting a little early because it's lab day so we're not doing anything after class i'll do a quick goodbye to you as soon as class is done and then i gotta go over across the hall and teach some labs so thought i'd spend a couple extra minutes with you beforehand are we functional good morning ashley ashley your artwork is amazing and uh i don't know i'd love to be able to share your artwork with others uh i think you're just putting it on instagram stories aren't you i'm talking about ashley in seattle formerly ashley of portland i don't know i if you're comfortable with that i'd love to be able to share what you're doing it's it's uh it's really outstanding uh hey hey tawny katrina hold it hold it christina okay i promise to talk to you guys a little bit more here we go looks like i'm five by five everybody's noticing i have a haircut this episode of nick from the classroom brought to you by northwest barber shop jaime gutierrez does nice work down there downtown ellensburg you've got to love his hair clipping and uh just bookkeeping uh these shirts that i love so much are l.l bean not land's end my bad i just feel better correcting my actually actually these shirts are l.l bean okay uh hello uh james in michigan and uh jill copp in meridian idaho exit 44. please chat it up back there man oh todd's here geneva help me with that man you're here every time d-j-e-n-e-b oh we got a mason sighting thank god good to see you yeah yeah oh my god are you seriously are you serious right now yes okay um i was still taking notes because i was back in western washington oh you were i still had all the stuff from the live lectures okay but i was really confused about um sorry you'll get it catch your breath okay i'll share your uh your brown nosing gift today what was it okay so as we have our seafloor spreading we're doing all of that and they're moving further apart right is that what eventually drives the cycle background or reform the new super continent or why does that part of the cycle happen today's lecture will help with that okay so let's let's let's uh let's be patient that way this episode of nick nick from the classroom brought to you by l.l bean you gotta love it snickers the original brown candy bar in the wrapper pack full of peanuts emily too hello mason mason jolt of electricity huh all right no they were good too not the albino snickers few more hellos to everybody hello todd uh carol good morning joe in denver half moon bay in the san francisco bay area that's ray oscar reports that snickers are good jerome is watching from portsmouth virginia dan in pittsburgh pennsylvania uh beau's in copenhagen denmark notre dame indiana hello thurston shake down the thunder don is in los angeles he says hi uh glasgow uk hello gordon ah rainy california glendon we actually have a little bit of sun today wasn't there sun emily was there sun on the way or blue sky oh you're up from clay elm so you were all socked in with clouds probably but there's a foggy up in cleon fog as hell reports emily number two i think we're supposed to anybody see blue sky walking in here no reaction oh boy oh boy oh boy i don't know i might have to walk out right outside right now and see if there's blue sky but maybe not uh it's very rainy in oak harbor washington and toronto toronto ohio hello brent forest grove oregon not to be confused with cottage grove where my son teaches high school amarillo texas yeehaw i was corrected in the live stream yesterday this is not a scarf this is a bandana thanks actually it's a banana it's a bit diana all right feeling perky today um there's some energy in the room let's see if i can't dismantle that in the next hour you don't have to be hushed back there by the way let it rip let's fill this room with energy let it be filled with energy especially since uh jack's here i'm going to get a double dose of you today jack in the lab too right tawny oh see now tawny says get out of the way i don't blame you i don't play you're welcome yeah i can i can stand i can i can stand right here and i can not block anybody pat lol john's in chapel hill north carolina go tar heels tom from squim ghostman5826 reports it's overcast in western minnesota actually it's a neckerchief says chris thank you hey jordan i listened to jasmine all morning and yes i tried emily one i tried i tried uh desrocks i made it a couple songs in but not not my thing that's okay we can still be friends saber i thought you were in baja what are you doing is this being translated from mexico from spanish into daryl reports my globe stops occasionally oh there's there's all sorts of things you know i need another snickers bar bite here hello alice in ontario canada bill's in sonora arkansas that's an interesting place name bob is reporting from sandy spring georgia oh i'm still prop blocking people and now i'm spitting snickers all over my laptop not good today's lecture i've given uh pretty much the same way since 1987 so i'm not proud of that but i think it works we'll see what you think so i'm not really concentrating on what we're going to talk about because i can do it in my sleep another ontario nice to see you canucks here pennsylvania poconos yeah groundhog pakistani phil good morning remind me remind me of your name jay oh how could i forget uh okay may we have more than 570 about the same for the room here 20. i'm gonna go chat with the students um thanks for being with us hope you enjoyed today's lecture and another quick reminder that uh no no live q a today because i have lab it's tuesday so i'll be saying a quick abrupt goodbye and that'll be the end for today but i will see you next on thursday and thursday the first 10 minutes of thursday will be a quiz that these guys will be doing and i'll have a little slide show to keep you entertained and then there will be a lecture on geologic hot spots on thursday just 40 minutes instead of 50 minutes okay that we're like really going to go back and look at i don't know if today's all right all right my breakdown and everything your breakdown oh your note your breakdown you're not no breakdown and then i was like i'm gonna make a pun about panchi oh that's you ruined it you were go oh oh break down break up well jc you're going to get a double dose of me today right i'm going to see you in the lab as well yeah absolutely well that's i'm excited for that i mostly did zoom tours oh [Music] yeah yeah because like you're trying to retreat students i'm sorry i guess but it's like all the inside [Laughter] have you lived there no i've been like once yeah um oh damn i didn't i didn't even know this part of the story okay well yeah yeah so welcome to come to central yeah right you too can no yeah this was like one of the nicest ones oh yeah yeah so why did they even tell you the story if if they come right around and say don't don't tell it well just to be like aware of it if someone like during a tour well i can't imagine doing a tour for new students by zoom i mean uh yeah it wasn't very fun i'm sure only like people were like right but we got a lot of bling that's always fun yeah wow you do it all so you run cross country and then you do they did pretty sure i don't know i know my boss got laid off like that was the last i've heard they let me go oh yeah yeah people no that that part's grim i mean if your whole job is dorms and there's nobody here to live in the dorms it makes the whole campus yeah all right good stuff christina good stuff let's let's uh all right all right all right pedro good morning karen's here jc the emily's michaela tani's got something else on her mind connie you what'd you have for breakfast this morning yeah what a monster this episode of the classroom brought to you by monster energy drinks they're scarves just kidding somebody corrected me yeah right uh yeah i don't know i uh pretty early on somebody said uh you know those bananas are not safe and you really need it's like that gave me motivation to buy like 10 more bandanas so it's better than nothing it's better than nothing actually that bandanas and like the surgical masks yeah thank you thank you would you walk around with me from now on so that you can give them that message as they shame me for having a bandana excuse me scarf thank you so i've got a few [Laughter] all right okay we got good energy let's get rolling ah students good morning thank you for joining us i welcome you with open arms i welcome you with open arms we are uh in the heart of the matter now we are in the guts of the class we are in the whatever phrase you want this is important stuff and today we'll get a lot of mileage of for the rest of the quarter so without a whole lot of cheerleading we're going to get right into it because the energy is good uh and we're going to we're going to make it happen please remember that the next time i see you will be thursday no class tomorrow of course tomorrow's wednesday and yes i am giving you a quiz i talked about it yesterday most of you were here yesterday or if you weren't you saw it on the live stream um 14 points not the 70 point midterm but 14 points you know it's more than one point let's say so it's a a sizeable quiz and i'm again just checking in with you to make sure that you're mastering this material we don't want too much of this to get past us and the midterms coming after another week or so of this but i want you to be ready to go multiple choice questions four of them on thursday one short answer where you're going to be drawing a little bit of stuff good morning kade and so that is the story from for uh thursday and we'll do the same routine that we did the first quiz remember you just sit you just sit you do your quiz i collect them as a group and then right off we go with the next topic and it will be magmas on thursday and hot spots on thursday and that sort of thing mason is counting students okay oh i see okay so that's the plan i'm clapping for some reason this morning i don't understand why the good news for you today is that this lecture in particular is well organized it will be less freelance and sometimes that's a challenge when you're trying to take notes or trying to keep pay attention or whatever i don't know i i can't speak for you but this is our game plan today and let's just make sure we know what the game plan is plate boundaries page 19 let's do it oh we got late arrivals today i guess i should be grateful that you're here we'll just choose that so page 19 is an important page but it's no page 20. it's no page 20. page 20 is the sorry patrick page 20 is one of the most important pages in all of the yellow book but before we get to page 20 let me just set the hook page 19 has a bunch of black lines on it the black lines are what they're play boundaries so the only thing we're doing today i'm saying only but there's there's a lot we're going to talk about today what we're doing today is globe trotting and going around to these black lines and realizing that plate boundaries are different at different places and we have labels for the plate boundaries some of these black lines are divergent plate boundaries some of the black lines are convergent plate boundaries and some of them are transformed you're settling in now you're settling in and bookkeeping wise there are three different types of convergent plate boundaries that we will break out today so it is a systematic day it's an organized systematic day we start here we'll spend almost all of our time with the middle convergent plate boundaries and then we'll finish with i'll toss in just a little bit about transform stuff as well so if we want to look quickly at page 20 let's go ahead since i said page 19 we've already discussed that was last time last couple of times but page 20 is a big big deal you've already heard me say it three times so i won't say it much more we're going to be marking up page 20 if you're doing well in the course you're going to be jotting down places mountain range names rock types earthquake and volcanic hazards right on or next to or somehow corresponding in your notebook to page 20 but this is a big big deal that's the last time i'll say it and page 20 corresponds perfectly with these three things right here so if you want to be active about this page 20 is going to be convergent plate boundary types c versus c o versus c o versus o you're like i don't know what we're doing we haven't gotten there yet but you can see it graphically on page 20. is it clear what we're doing today is it clear what our mission is today now i've already mentioned that earthquakes are found at all plate boundary no i haven't said that i'm saying it now i don't care what kind of plate boundary you are so there's actually five different types of plate boundaries that we're working through today three major categories but again there are three different types of convergent plate boundaries so there's five plate boundary types and earthquakes are produced at all five all five of these plate boundary types are making earthquakes they're earthquake generators the faults will be different depending on which plate boundary we are but we're making earthquakes all over the world generally almost all earthquakes are found at plate boundaries and i'm saying that the plate boundaries i don't care what kind of plate boundary it is it's making earthquakes now we do know that there are earthquakes that happen away from plate boundaries but that's not today that's not today oh another opening statement you're locked in and i appreciate it not all plate boundaries produce volcanoes so the statement i have given you in a past lecture is that almost all earthquakes and almost all volcanic eruptions happen at plate boundaries that one i've said a few times but now i'm being a little bit more careful and i'm saying that all of these plate boundary types today make earthquakes but not all of these plate boundary types make volcanic eruptions they don't make volcanoes and so again if you're really on top of your game together you'll kind of keep score i don't know make a little system for yourself which of these five plate boundary types are our volcano makers and which are not okay we can't do any better energy-wise let's go i'll give it to you verbally first of all i don't need to write it all out this is pretty simple if you are a divergent plate boundary the plates are diverging you're at one of those black lines and the plates i'm i'm pantomiming now the plates are going away from each other they are moving away in opposite directions away from that plate boundary where can i go on the planet today to find a divergent plate boundary that's an easy question i know you're writing but can somebody shout out at the same time where can i go today to find a divergent plate boundary thank you emily too i love you the mid-atlantic ridge and the east pacific rise yesterday's lecture those are divergent plate boundaries those are places where the plates are diverging from each other iceland sits at a divergent plate boundary i'm pausing giving you a chance to jot a few things down a lot of this is just organization none of this is very hard today but it's it's a challenge to keep things organized let's keep going are earthquakes generated in iceland yes or no yes i just said that all these plate boundary types make earthquakes and so there are earthquakes in iceland what kind of faults this is yesterday what kind of faults are in iceland normal faults thank you so normal faults we pull the crust apart crustal extension that's what we know about normal faults better have this ready for thursday wink wink and if we do crustal extension on a small scale we're going to make an earthquake when we have the hanging wall drop with respect to the foot wall we'll have a normal fault earthquake that's happening all the time in iceland that's happening all the time all along the mid-atlantic ridge stretching from the north pole to the south pole that's happening all along the east pacific rise so the east pacific rise also is a divergent plate boundary what kind of rock is created at divergent plate boundaries basalt thank you you know and you're you know you're showing up today for the first time in a week because there's a quiz on thursday well how do they know that well first of all you got to come to class every freaking day please but secondly i'm trying to get us to keep using stuff from past lectures and sew it in to what we're doing today that's a theme that will continue and many of you are you can see what i'm doing now you're ahead of me almost you're you're answering questions as i'm finishing the question and that's great let's keep that going so basalt is the main rock type created at the mid-atlantic ridge in the east pacific garage therefore that's true for all divergent plate boundaries do we want to see it on page 19 because page 20 doesn't have divergent does it page 20 is convergent instead i'll do it this way so divergent we're done talking about because we know all about divergent plate boundaries from yesterday seafloor spreading is happening at divergent plate boundaries but convergent plate boundaries are the opposite convergent plate boundaries the plates are colliding again this is sixth grade content or less than that probably not hard the plates are heading towards each other and again a little sneak peek where we will spend most of today page 20 are the three combinations jordan off the phone three different types of collisions depending on what type of crust is involved in the collision convergent collision in other words between two continents between an ocean floor and a continent or between two ocean floors so we're going to slow down and really spend a lot of time with those three different collision possibilities but you're hearing me convergent plate boundaries are collisions between plates earthquakes possible that's easy every type of plate boundary makes earthquakes but we will see that not all of the convergent plate boundary action makes volcanoes so let's save this one because i'm going to go a fair amount into depth with that let's backtrack where can i go visit a divergent plate boundary east pacific garage where else could i go mid-atlantic bridge what country could i go to iceland thank you now let's do the same real uh brief quick approach to transform plate boundaries because i've got a full lecture on the san andreas fault for you next week maybe at the end of this week i forget and transform plate boundaries are not divergent they're not convergent but they're plate boundaries it's a black line so what's happening at a transform plate boundary one plate is scraping past its neighbor i'm doing it with my arms here it's a horizontal motion there's no splitting apart it's not divergent it's not convergent instead the plates are moving past each other two trains going in opposite directions whatever you want two boats why do car i don't know it's not a hard concept and the san andreas fault is the most famous probably in the world famous example of a transformed plate boundary it's in southern california what's the name of the two plates that are on either side of the san andreas fault transform plate boundary the name of one plate is north american plate the name of the other plate is the pacific good have you found where we are right here that section right here of page 19. is the san andreas fault you can see that the north american plate is moving west and the pacific plate is moving northwest those are our directions by the way we're going to kind of commit to memory the north american plate moves west the pacific plate moves northwest and so if you do those two motions simultaneously you're not going to get a collision they're both kind of heading west but one has a northwesterly trend versus the other that has a a straight westerly trend and so you get this slippage this horizontal slippage so are earthquakes possible on transform play boundaries of course they are the san andreas fault is famous for earthquakes we continue to be very concerned about earthquakes in the short term coming on the san andreas fault and you're like wow thank god we don't live in california well we'll talk about it but volcanoes are not produced by the san andreas fault there's no magma created at transform plate boundaries there are no eruptions of lava in los angeles 20 years ago there were it was popular to have feature-length movies and movie theaters about lava coming down the streets of los angeles okay not possible are we feeling okay so far anybody want to say anything i'm hesitating because we're about to dive into page 20 and spend the rest of today looking at different kinds of collisions got it good let's do it then you've got page 20 kind of next door to you you've got your notebook uh also handy i'm going to just do a bunch of drawing i feel like drawing is going to be better than just kind of standing here and i also want to use a couple of maps i don't know when i'll go to that but i've got the laptop ready to go as well okay document camera ready to go so let's let's pick up the pace okay the first possibility on page 20 is a convergent continent versus continent collision now from the last couple of lectures we know that continents are not the same as plates continents are portions of plates but we're focusing on the crust type in these two tectonic plates that are colliding so i'm not saying that the entire plate is continental crust on either side i'm just saying that when we do have two tectonic plates these are all cross sections all this stuff i'm going to draw up here oh person with scoliosis all this stuff i'm going to draw on the chalkboard is side viewing now i'm not going to put a little stick person every time but you've you've got those cross sections on page 20 as well so we have a head-on collision between two continents we are going to make a rather impressive mountain range which you can see on the top of page 20. earthquakes absolutely absolutely volcanoes no so we're going to make monster mountain ranges that are non-volcanic if we have a convergent continent versus continent collision you see how we're kind of using some of these words convergent is the is the plate boundary type meaning the plates are colliding and now we're at convergent continent versus continent the two pieces of crust involved are two continents earthquakes yes no magmas that are erupting out of the surface so what do we have well here is the signature here is the telltale sign uh in other words here are the rocks that you get when you have this type of a plate boundary these are the rocks that you always find in the guts of the mountain range folded sedimentary layers metamorphic rocks a little deeper into the mountain range and granite in the core of the mountain range i'm talking about let's split that mountain range open and the only reason the mountain ranges there is because of these two colliding continents and let's look at the geology inside of that mountain range what are we going to find folded sedimentary layers what are these things i'm showing you right now they're folds what are the two names of folds that we have already anticline and syncline the anticline folds up this incline folds down do you remember the warm hug big hunk warm big hunk candy bar so geologic fold this is what i mean by saying folded sedimentary layers they are a direct result of the collision between those two continents are there going to be any faults sure we have earthquakes right if we have earthquakes we must have faults what kind of faults would we have in this kind of a mountain range see it's going to get sluggish now because we got too many words juggling in our minds but we can do it may somebody besides basically mason's on fire i want to involve other people what kind of fault with this type of a plate boundary go ahead it would be reverse emily ii why is that because we have crustal compression and hanging walls are going to go up so here's a thank you emily too you're also on fire this is a scale thing and it's it's a thing that typically gets confused i'm going to give it to you verbally ready i'm going to backtrack just a little bit divergent plate boundary that's a global scale thing but if you zero in at the divergent plate boundary there are normal faults normal faults are small little cracks that are showing up at the plate boundary so if you haven't yet please associate normal faults small little cracks with a much larger thing called a divergent plate boundary like the mid-atlantic ridge or the east pacific garage now here's i'm doing the same thing here what's the main story what's the global scale story convergent plate boundary two plates colliding head on but if we zero in and look carefully inside of this mountain range we'll find not only folded sedimentary rocks anticlines and synclines but emily too just helped us understand we must have reverse faults because why crustal compression makes reverse faults crustal extension makes normal faults are you on top of this you slipping behind i'm hoping we can all be on top of it and have these things ready to go they're just ready at the end of your tongue ready to go instead of having to call this stuff back after four steps of mental gymnastics that's not going to help us on thursday or in any lecture in the future we have to be brisk and sharp and ready to go with this stuff that's the goal that's where i'm trying to get all of us you know like relax just relax all my other classes everybody's just like going through the motions just go through the motions you cute little scarf just just mail it in sorry patrick come on well that's not me i like to sleep well at night so reverse faults in the heart i've got them from there for you right there i i did this here but i also did it for the top of page 20. you saw that right folded sedimentary metamorphic granite in the core now we got to be careful there is some magma in the heart of this continent versus continent mountain range but we don't have eruptions the the heat is so high the pressures and heat are so high in the guts of that that mountain range that we have some magma generated but we never have eruptions we never have volcanic events we never have volcanic ash so who's ahead of me where can i go what mountain range can i visit on the planet today to find one of these convergent continent versus continent collisions there's more than one possible answer let's go the himalayas should i spell that for you all have you heard of the himalayas can i spell the himalayas himalayas it's possibly the correct spelling the appellations probably not spelled correctly either i'm moving fast so are you challenged do you not know where those are let's find a map let's go around the world and find a couple of convergent continent versus continent collisions mason can i help you in uh to what kind of boundaries they are because i may or may not remember that some of that we might be quizzed or examined on i mean maybe maybe we should put it on page number okay thank you all right well what do we see here divergent plate boundary i'm reviewing just to touch with you just giving us a little break divergent plate boundary that's the mid-atlantic ridge divergent plate boundary that's the east pacific rise but these are the himalayas right here can you see them these are countries like tibet or northern india or western china or nepal the himalayas you heard of mount everest i think you have haven't you that's the tallest point in the world that's the tallest peak in the himalayas or the himalayas if you've been in higher education too long don't get me started now the appalachians you might be surprised to hear is also an example of convergent content versus continent collision you might be surprised because the appalachians are here and there's no continent today actively colliding and that is correct but we've already talked about the appalachians quite a bit haven't we when did the appalachians form how many years ago older than two can we do we have a number 300. i haven't talked about many numbers that old so far in the class let's get it down hang on mason 300 million years ago you have that date somewhere because that was when pangaea was put together remember or maybe you don't but i'm reminding you so what does that mean well we took a different global geography and specifically we took north america i'm talking about preceding 300 i don't know 320 million years ago this geology 101 this is fine there's exceptions but it's fine north america was inching closer and closer to africa and europe before 300 million we were closing the freaking atlantic ocean as we were putting pangaea together and what was my question when did the appalachian mountains form the answer is 300 million years ago as pangaea was put together and you're like i don't think i get what you're saying now well i'm saying that north america's continent collided with africa and europe's continent and that's what we're talking about right now convergent continent versus continent collisions and so what kind of geology do we have in the appalachians folded sedimentary metamorphic rock and granite in the core stone mountain georgia vermont granite that's the granite of the holy trinity within a convergent content versus continent collision but the only thing we have to add is what the appellations aren't happening anymore we know that because the continent versus continent collision that form the appalachians isn't happening anymore starting 200 million years ago i've already done it the mid-atlantic ridge forms the atlantic ocean starts to form so just to confirm that you have it how old are the reverse faults in the appalachian mountains 300 million years ago how old are the normal faults in the appalachian mountains 200 million that is correct i made a weird face there but i was just stalling for you that's new but not new if you're really on top of this i'll say it again the reverse faults in the appalachian mountains of kentucky the reverse faults are 300 million years old but the normal faults are 200 million years old why because reverse faults are from crustal compression normal faults are from crustal extension well why was the crust compressing 300 million years ago in kentucky because north america was freaking colliding with africa well why was the crush starting to extend 200 because we started to form the atlantic ocean floor and we broke up pangaea why is he yelling i'm proud of myself i came to class the guy's yelling at me for an hour why i blame it on the snickers either one brings out the hate brings out the anger in me yelling no i hope you can hear the excitement it's genuine i've given this lecture a lot of times but i still get excited when i talk about it like it's the first time i've ever thought about it part of it's acting but part of it's actual genuine excitement and i'm hoping that you can feel that mason's got a hand of rocks and you up in north america and rocks in north america we'd see igneous coast because we have the convergent convergence where we don't have the volcanoes over on the east coast our loose atmosphere forming because of that heat and pressure that's from creating some other reason which is coming right now perfectly timed question okay we're moving on one step down in page 20. what type of plate boundary convergent page 20 is nothing but convergent but it's a different type of a convergent plate boundary it's a continent colliding with a piece of ocean floor it's not a heavyweight bout this was a pretty even fight if you want to think of boxing and who doesn't so let me get on my knees and draw the next picture for you this being a convergent oceanic versus continental plate boundary uh oh what are we doing oh good lord what's happening what is happening the answer is we have a mismatch we have two tectonic plates colliding convergent but ocean crust by definition is significantly different physically than continental crust and so for the first time we're going to destroy a tectonic plate now let me give you some verbal rules or verbal statements and then we'll flesh it out a little bit but we're gonna do the same process with this tie we're gonna eventually go to places and the whole thing okay statement number one continental crust never subducts you're like well i didn't know what subduction is what do you mean subduct i mean this do i have to write it for you do i have to subduction is the process of sending a tectonic plate into the earth subduction is the process that i will casually refer to as destroying a tectonic plate destroying in the sense that it goes back into the earth yesterday we talked about how to create tectonic plate material right seafloor spreading we were actually making ocean crust and tacking half onto one plate and half onto the other that was creation well this is the opposite this is destruction subduction is the destruction of a tectonic plate and my rule is continental crust never subducts it's the ocean crust that always goes down and we should pause and try to discuss why this is why is it the ocean crust that subducts and not the continental crust and we've got people at home right now doing the actually thing actually there is continental crust 101 101 i'll give it to you verbally why not the ocean crust first of all is far thinner it's thinner crust than continental crust so just physically it's easier to send this thin ocean crust down into the earth as the collision continues to happen i mean this is a collision just like up here this is a collision too but it's not much of an ongoing heavyweight bout it's a light weight matched with a heavy weight and the the heavyweight's going to just dominate so let's just add to that just to touch so you can see your sketch in the middle of page 20 and there is our subducting oceanic crust subduction zone etc but let's look just real quick at 21 would you mind you're writing furiously and i i'm grateful that you are engaged 21 is a very simple way to look at differences between the ocean crust and the continental crust just to help us see why this subduction is happening with ocean crust only so there's the first difference between ocean and continental crust the thickness and you're like oh those those numbers look kind of familiar why were we taught five and 75 and 70. remember that was the depth to the moho what's the moho what's a simple sentence what's the moho this couple lectures ago now boundary between crest and the mantle correct not the lithosphere and the stenosphere the moho is the sharp boundary good job hope the sharp boundary between the crust and the mantle get it down get it down it will pay off so if the moho is the base of the crust and it is it's the boundary between the crust and the mantle the moho then now i'm actually giving you global depths to the moho in the oceans of the world the moho is only five kilometers down so therefore the crust of the oceans are thin whereas the depth to the mojo in continental settings the continental crust on average is 70 kilometers thick far thicker than ocean crust so if you really want to be on top of your game you could go all the way back to 17 if you wanted and there is graphically the difference between 55 kilometer moho depth and 70 kilometer moho dip and you might actually put so weird i gotta do this all backwards for the document camera i always struggle if i'm not a lefty and then i'm going just fyi so there's ocean crust only five kilometers thick and here's continental crust 70 kilometers thick substantial difference so if we have these collisions and it's two pieces of continental crust that are 70 kilometers thick well that's fine that's fine they're both 70 kilometers thick and we do not have subduction in our top picture but now if we have ocean crust this thin ocean crust involved in one of these collisions it's going down baby it's going down it's going down hard but it's more than a thickness difference that encourages ocean crust subduction and you're like i think i'm losing interest i don't think i really care about subduction well you better subduction is what's happening right here in washington if we didn't have subduction we wouldn't have these absolutely mind-blowing scary great earthquakes that are a major problem they are subduction related mount st helens mount rainier active volcanoes here in washington they wouldn't be there if it wasn't for subduction subduction is a huge concept that we'll talk about the rest of the quarter so we're slowing down to make sure that we really get why this abduction is happening here not in tibbuck 2. it's happening here a subducting ocean plate is subducting beneath your backyard literally so i'm skipping around now i promised this would be systematic but i guess i am freelancing a little bit you know that was coming the other significant difference between ocean crust and continental crust is the density of the material and this is also a factor in why ocean crust tends to subduct way more than continental crust and i've got to choose my words carefully now okay you can see it there on 21 high density low density but what do i really mean okay do you know what density is density is mass per unit volume you have a unit volume you have a unit volume and the mass of that unit volume is the density so let's have two rocks i should have brought in two rocks that were identical in size i did not you can see my hands home viewers can see my hands let's say that this is a rock that's the size that fits into my claws here that is ocean crust and this is an identical sized rock that i'm going to grab from continental crust ocean crushed continental crust mass per unit volume this is the unit volume same volume in both claws i'm a raptor the minerals in the oceanic crust have a higher specific gravity the minerals in ocean crust is different than the mineralogy or the kinds of minerals that are in continental crust we don't want to go into a discussion of why because i don't really understand why but the planet has evolved to the point where the ocean crust is a very different collection of minerals and therefore different kinds of rocks basalt than the continental crust and you know what i'm doing now i'm holding this ocean crust lower it's heavier the ocean crust has a higher density than the continental crust and i'm choosing this and then doing the claw routine weirdly because the mistake you can make is oh man i bet you that continental crust is super heavy dog i bet you that's really heavy that's so thick look at the weight of that continent versus that ocean crust that's not what i'm saying i'm saying mass per unit volume this ocean crust is dense so if you have thin crust that's also high in density of course it's going to be the plate that goes down of course it's going to subduct so think of continents as almost like styrofoam compared to the ocean crust think of continents as thick and buoyant like it's like it's a styrofoam floating in the in the lake got a little beer out there okay fine hot summer day the continent is thick and buoyant compared to this ocean crust i'm not even asked if there's questions because you'll ask about physics and i don't know about physics very much but now that we have that difference i hope that you can see that we do have subduction of only the ocean crust ocean crust this is continental crust it's thin and dense it's thick and buoyant and now i've hit that real hard why do we care about it well magnitude nine earthquakes generated worldwide in these subduction zones magnitude nine have you heard of such a thing they're a thing here in washington in japan in sumatra we'll have two lectures devoted to those great earthquakes it's scary and humbling all at the same time but also and i'm sure you've heard this at some point subduction is also a magma generator there you go mason the magma is typically forming at the margins of continents because we usually have subduction at the margins of continents when we do have a convergent setup so this is a call back to that sketch a few weeks ago where i had this these scoops of ice cream on the margins of a continent what i was drawing i didn't tell you at the time what i was drawing was a subduction zone and the destruction of ocean plate material making large batches of magma and sending that magma to the surface and yes feeding a volcano is it melting the ocean crust no it's more complicated than that we don't care we don't care it's 101 we are generating large volumes of magma as a direct result of subduction of the ocean floor before i lose steam with you with this sketch let's finish out our program today where can i visit what mountain ranges can i visit today that are a great example of convergent oceanic versus continental cascades thank you our volcanoes the cascade range are a great example of convergent oceanic versus continental earthquakes yes volcanoes yes i'll give you one rock name i'll give you not folded sedimentary metamorphic that was that was up here i'll give you one rock name that is a common rock found in mountains formed in this plate tectonic situation can you read that a rock called andesite a-n-d-e-s-i-t-e andesite mount rainier is made out of andesite mount saint helens is made out of endocyte endocyte is forming in a mountain range from this very specific plate tectonic situation where else can i go in the world to find not mason where else can i go to find a mountain range made out of andesite anywhere in the ring of fire where could i go mason you are not to be denied i said anybody but mason you put your hand down and then you put it right back up again you just you just want it you just want yes i like it i like it look at this yeah i wonder why they call it undecide have you heard of the andes mountains they're in south america it's just one volcano after another all lined up in fact let's go to the laptop mason you know i'm playing with you you know that i love you let's spend a little time with this now we've done most of our work we only have five minutes left and that's just fine okay andes are here can you see that mountain range along the west coast of south america it's not an accident that they're here or that they're there this is ocean floor coming from a spreading ridge divergent moving east and subducting beneath the west coast of south america so from last time we looked at ocean floors of the world we did the sexy transition spla dam we decided that the east pacific rise was actually spreading faster than the mid-atlantic ridge but that doesn't mean the pacific ocean basin is getting wider why because we're subducting all that pacific ocean floor crust i'll give it to you verbally where you have subduction you usually have something called an oceanic trench you've heard of it like the marianas trench maybe it's where the ocean floor just drops out of sight trenches mark where subduction is happening and we have yes trenches all around the margin of the pacific ocean so therefore we have destruction or subduction of the pacific ocean crust spread quickly by the east pacific rise and we essentially have zero trenches around the margin of the atlantic ocean and therefore even though the mid-atlantic ridge is spreading slower it's not being destroyed and you're like why is that and i'm like well where was the last supercontinent and if you think about that it will make sense don't need it don't need it there's a much more detailed view of south america i guess i need a a cute little dainty pointer i guess i just decided i need that east pacific rise divergent trench you see that little shadow there and these mountains made out of what kind of rock and decide oh animation time oh it's on a loop it's damn girl it's on a loop this thing's on a loop we'll look at this a lot trying my hardest to reach the guy in the back row i guess it's not going to work today thin dense ocean crush subducting beneath thick lightweight continental crust producing volcanoes and earthquakes generating endocyte magma so that's what we've been working through i've got two minutes and that's fine because i've got two minutes worth of the bottom picture here it is verbally our last plate boundary type is a completely submarine collision two pieces of ocean floor colliding so this is almost a fight who can subduct first they're both ocean floor they're both thin and dense so there's no basic rule about why one subducts beneath another except you know one plate might be moving faster than the other it might be an oblique collision we don't care depend the point is we have subduction we have a trench we have volcanism if i only had to pick one kind of rock i guess i would pick basalt it's kind of a lie but let's do it anyway so the mountain range in the top of page 20 folded sedimentary metamorphic granite than the core the rock in the middle picture endosite the rock in the mountains formed by this bottom which is what a convergent ocean versus ocean basalt and we'll finish with a picture of one of the u.s states do you recognize it oh thank you that makes me feel better alaska and these are the aleutian islands a-l-e-u t-i-a-n aleutians a-l-e-u t-i-a-n look at that beautiful string of pearls and that is marking especially way out here a convergent oceanic versus oceanic collision trench subduction and for years people couldn't like why do we have these islands out here in a line this is so weird like this this is like early ship people dude check it out we got more islands in a line this is so weird and we now know those island arcs or those lines of volcanoes are a direct result of colliding ocean tectonic plates and the volcanoes are marking a plate boundary which conveniently is the topic today so to be precise the outer aleutians meaning the aleutian islands that are further out would be a good example maybe the only example we use today and i'm not happy just saying basalt so let's say basalt and andesite basalt and endocyte for the rocks that we're creating in the volcanoes yes it's volcanic for sure subduction always makes volcanoes convergent ocean versus ocean outer illusions basalt and andesite i'm done you're done we are done and i thank you for coming today i won't see you tomorrow i will see you here in lecture thursday 10 a.m for our second quiz i will see a subset of you in just a few minutes across the hall in lab otherwise please go to your lab please continue to go to your lab and enjoy all the wonder fill in the blank thanks for coming i love you goodbye couple quick questions from you before i say goodbye do you have a question or two in uppercase i can just try to grab them real quick before i go nice things nice things nice things delay in the chat uh amazing indian is the marianas trench in ocean versus ocean subduction yes the marianas islands are the result that would be another example of like just like the outer aleutians why is the ocean floor thinner uh i guess it's the process of seafloor spreading and how we're creating ocean crust but i don't have a better answer than that what geologic forces made mountains in antarctica i don't really know chris the trans antarctic mountain range i'm guessing is a continent versus continent collision from long ago but that's a guess can you find data on the ancient slabs yeah but this is geology 101. let's say it together geology 101 when do plates fuse depends on the place we're talking about double subduction near anchorage okay you guys know things but um kent we're avoiding the rocky mountains on purpose the rocky mountains are good surprisingly complicated and controversial kind of a surprise huh it's just you and me mason now what do we do all right mason's telling me to wrap it of up in charge of what like are you the supervisor to zach yeah yeah zach's lab on thursday it was a nice small group we worked together really well i want to say my kudos i mean that was one of the better laps done by ta that i've been in wow nice and i didn't know who i should say that too well it was really enjoyable why don't you tell zach too but thank you for the thank you for the message that's good yeah he's he's he's a good one you're welcome mason okay i'm glad things are okay with you that's good i'm still doing everything okay very good all right we'll see you goodbye mason goodbye uh let's do three more i can be a couple minutes late is glacier peak an andesite volcano yes but remember now this is just the first message about cone volcanoes we'll have a detailed lecture about complications within the cascades but yeah if we just pick one rock type you bet do we know how tall the appalachian range may have been interesting question william i don't know if this is backed up with any sort of science but i don't do it much anymore but i used to say the appalachians when they were active like 300 million years ago were like the himalayas are today and the appalachians have been dead for at least 200 million years because of the breakup of pangaea and obviously there's been erosion of the appellation since then but i've never followed up on if that's just kind of a thing to say or if there's ways to really prove that the appellations were that high boy people want to talk about celestia they can't wait to talk about celestia well you're going to have to wait to the last week of the class would i consider a zoom meeting once in a while so we can ask questions face to face no i don't think so it's never enough i appreciate your enthusiasm but it never seems to be enough i appreciate your enthusiasm okay i gotta go uh thanks for joining us i love you you know i'll tell you a quick story before we quit uh there was this beautiful woman walking by the house i mean i've never seen her before she was just like a goddess walking down the sidewalk and she saw me in the backyard and she's walked right into the backyard and i couldn't believe what she was starting to do

Channel: Nick Zentner

Views: 12,697

Rating: 4.9695816 out of 5

Keywords: Nick Zentner, Plate Boundaries, Divergent, Convergent, Transform

Id: FeMCgn0jV_E

Length: 71min 46sec (4306 seconds)

Published: Tue Feb 02 2021