howdy everyone and welcome to the next video of module two where we're going to talk about convergent plate boundaries which are probably the most complex type of plate boundary because they behave very differently depending on the composition of the plates that are combining and convergent plate boundaries are the most hazardous place on Earth when we're talking about geologic disasters So these are the most dangerous places on Earth and we're going to talk about why that is here in a minute All right So convergent plate boundaries This is going to be the opposite of our divergent plate boundaries Instead of plates splitting apart we have plates moving together converging and they're colliding Collisions sounds violent right that's because it is a violent process Lot of hazards here guys And we see a lot of features at convergent boundaries This includes subduction zones which we're going to talk about more in detail um folded mountain ranges All of our mountain ranges on Earth are pretty much formed by convergent processes We have lots of volcanoes some of the most explosive volcanoes on Earth And we have earthquake zones The largest earthquakes in Earth history occur at convergent boundaries So you can see why this is such a hazardous dangerous deadly place But like I said this is complex So we're going to have to talk about three different scenarios of convergent boundaries because they're going to behave differently depending on what type of lithosphere you're dealing with You could have two continents colliding You could have a continent and an ocean colliding Or you could have two oceans colliding And all three are going to look different behave different and have slightly different hazards And let's talk about them here moving forward So this is our convergent margin where we have an oceanic plate colliding with a continent plate So oceanic versus continent This is where we're going to see subduction zones Now what the heck are subduction zones subduction zones are when we have one plate subducting or sinking under another one And this is where the density differences between the two crusts are going to become so important because what do you think is going to slide underneath the other the more dense material or the less dense material the more dense material right the denser plate is going to sink underneath the less dense plate So this is really important when we're studying convergent margin So here we have oceanic versus continent We have the oceanic plate moving towards the continental plate and they're colliding right at this zone And because they are two very different types of crusts with two different types of densities they're going to behave differently So oceanic colliding with continent the oceanic crust is thinner yes but it is denser So it's going to actually slide underneath the continent You will never have continental crust subduct That's because continental crust is not dense enough to sink into our mantle Density is what's driving this process So important to remember only oceanic crust is going to subduct Never continental Only oceanic crust So when we have ocean versus continent which one's going to subduct the ocean every time And as it subducts it's creating this trench This is what we call our subduction zone It's where we have that plates actually being pulled being sucked underneath the continents creating this trench So we get these deep ocean trenches at these subduction zones But we also have the continental plate is kind of being crumpled up a little bit We get these these crumpling these folded mountains these mountain chains on the continent side but you'll probably notice on this diagram that the there's something different about these mountains They're volcanoes So as that oceanic crust is being pulled underneath we also have a little bit of water that's in these minerals that are also being subducted in the sediments of the ocean And when when those sediments in that crust reach a certain pressure and temperature they can begin to melt and we start to see partial melting of the sediments in the crust And this this forms magma And magma is a lot less dense than the hard rocky material around it So what happens to things that are less dense they rise So that magma starts to rise through the continental crust When it reaches the surface we form our volcanoes So these convergent margins oceanic versus continent we see ocean trenches and we see volcanoes And these volcanoes tend to be pretty explosive because we are adding a lot of silicone material to it with the ocean sediments and the continental crust and they're going to explode a lot more than our volcanoes that we see at divergent boundaries So that's just one hazard But these plates sliding underneath the other it's not a smooth process like it looks here I mean these plates are very rigid very irregular They're very strong they get stuck right these plates get stuck but they're still trying to move So when they get stuck and they're still trying to move they're building up pressure and eventually it's going to exceed its strength and it's going to slip That's going to create our earthquakes So we get some pretty gnarly earthquakes at these convergent margins Now we are talking about oceanic versus continent So when those plates get stuck you'll see that that oceanic trench is underwater And those plates get stuck when they get unstuck and slip that's going to push up a lot of water during that movement and that's going to create a tsunami So here we're dealing with explosive volcanism large subduction style earthquakes and tsunami So you can see why these are the dangerous places on Earth Now what are some examples of convergent margins where we have ocean subducting underneath the continent well we have them in our very own country This is Mount Reneer in Washington It is an active volcano and some would argue this is probably the most dangerous volcano on the mainland US in terms of population nearby and its potential for eruption This Mount Reneer is a volcano and it is a direct result of the convergent ocean continental boundary that is occurring off the Pacific Northwest where we have that Wandafuka plate subducting underneath the North American plate Here's another volcano the more famous one because it has had a more more recent eruption This is Mount St Helens that last erupted in 1980 This is in our own country We do have these sleeping volcanoes in our country with the potential to create these disasters This occurs on the same convergent ocean continent boundary that Mount Per does So the Cascade Mountains are a direct result of that Wandafuka plate oceanic plate subducting underneath the continental North American plate And here we have it's called the Cascadia subduction zone Now the Cascadia subduction zone is just one part of the larger ring of fire that you may have heard about And the ring of fire are those subduction zones all along the Pacific Ocean So in the middle of the Pacific Ocean we have the mid ocean ridges that is spreading and it's pushing the plates in either direction but they're colliding with the continents here So we're creating this ring of fire These this is a ring of subduction zones So this is a ring of volcanoes volcanic mountain chains and large earthquakes tsunami Remember Japan we talked about Japan last u module So we have a lot of dangerous places here in red the ring of fire Some of the most dangerous places on earth when we talk about geological hazards Okay moving on to the next scenario Now we're going to talk about two oceans colliding Oceanic versus oceanic Now this is going to behave very similarly to the ocean versus continent because we are still going to have subduction here But it becomes a little trickier when when you try to predict which one is going to subduct because they're both oceanic They're both dense So when we have two oceans colliding which one do you think is going to subduct density is still the key here So whichever one is more dense is going to subduct And this is usually the older crust because the older crust is colder more dense So usually the older oceanic crust is going to subduct underneath the younger one And then we have the same processes occurring here as we had in the last scenario We're going to have partial melting When those ocean sediments and ocean crust reaches a certain temperature and depth we create magma Magma is less dense It rises to the surface and it's forming volcanoes But instead of forming volcanoes on a continent and forming a volcanic mountain chain this is going to form volcanoes on the seafloor And if those volcanoes get big enough they'll start to form islands And we call them island arcs So when we have two oceans colliding you kind of see a chain of volcanic islands Japan for example is a beautiful example The Philippines So that's how those islands are formed They're volcanoes that are sitting on the sea floor that are just big enough to rise to the surface to form land but they're the direct result of the oceanic versus oceanic boundaries We also have the same thing here This is not a smooth process These plates are getting stuck Pressures building up They're going to get unstuck and slip forming large earthquakes and tsunamis because that water is going to get pushed up as well So still very dangerous places on Earth Explosive volcanoes large earthquakes and tsunami So my favorite example is Japan And this is a photo of Mount Fuji Mount Fuji is a very dangerous explosive volcano that is a direct result of the oceanic versus oceanic boundary that exists here that's actually forming the entire island of Japan Japan is a large volcanic island and that that Tohoku earthquake that we talked about in our last module that is a direct result of those subducting processes So we have the Oshtosh plate and the Pacific plate These are two oceanic plates that are colliding It might be confusing because you might think this is a continental plate but this island this is a volcanic island that is created by volcanoes sitting on the seafloor So it's made up of oceanic lithosphere But that was the direct result that earthquake was a direct result of that Pacific plate subducting underneath getting stuck and then getting unstuck Large earthquake huge tsunami big bad day So this is where we have these kind of processes happening Okay So our last scenario we talked about ocean versus continent ocean versus ocean Well what about continent versus continent that's going to behave very differently Remember I said continental crust does not subduct because it's not dense enough to sink into the mantle But the Earth isn't getting bigger We're not expanding in volume So what what happens when two continents collide if we're not destroying anything if nothing's sinking down underneath the other well it's going to crumple upwards It's going to form crazy mountain chains The Himalayas is a perfect example We have India colliding with the Asian plate and that collision is causing the continental crust to move upwards because it's not dense enough to subduct So we don't get subduction It's going to move upwards So it's forming very very high mountain ranges full of folded rocks very violent place and they're going to continue to grow as long as that plate motion continues to be in that direction So the Himalayas are actually still growing and getting taller today which is crazy to think about Now you'll notice in this diagram we don't have anything melting because we don't have any subduction So we actually don't get volcanoes associated with this type of convergent boundary But this collision is still violent We still have things getting stuck and unstuck So we still getting pretty large earthquakes associated with this boundary as well But of course we're not underwater so we're not getting our tsunami So no volcanoes no tsunami but we are still getting moderate to large earthquakes And then we also are dealing with large mountains And when things shake in mountainous terrains you also have to think about the added hazards of avalanches because the Himalayas are are peaked with snow and ice and landslides as well And there was a terrible tragedy in Nepal several years ago when that actually happened And we'll talk about that when we get to earthquakes But here's an example of convergent versus con uh continental versus continental convergent boundaries The Himalayas the perfect example on Earth towering mountains because we have plates that are just colliding and squishing and pushing all that rock upwards All right So just some summary for all three types of convergent boundaries Again I told you it was a little complicated So when we have oceanic versus continental convergent boundaries What are the hazards we have volcanoes These volcanoes occur on the continental side They produce volcanic mountain chains Remember examples are the Cascades The Andes are also another example in South America They also form mega thrust earthquakes which can form tsunami because we're dealing with underwater here And we can also get landslides because in the volcanic um terrain we can we can see landslides when we get shaking We can also get underwater landslides as well Ocean versus ocean Very similar hazards but they're going to look a little bit different We're still dealing with volcanoes but those volcanoes are forming islands They're not on continents So we get island arcs like Japan and the Philippines We're still dealing with mega- thrust earthquake and tsunami And we could still have landslides whether that be on the slopes of the volcanoes themselves or underwater also And then we have our continental versus continental convergent boundaries No volcanoes no tsunamis but we do have earthquakes landslides and avalanches Again please remember no volcanic activity is associated with this type of boundary And that's it for convergent boundaries They are the most dangerous deadliest places on Earth We're going to spend several more modules talking about them um when we get to earthquakes and tsunamis and volcanoes So these topics will come up again So make sure you keep these notes somewhere handy so they make a little bit more sense when we get to the details of those other hazards