Plate Tectonics

hi everyone I figured I'd throw together a quick video just to review some of the content you've been working on in class let's get started obviously you've been working on this theory called plate tectonics which is basically an extension of Alfred Wegener work this theory that we're going to look at was put together in the mid 1900s and built upon what Vagner began working early in the century so let's get started the first thing we need to know is this the the outer shell of the earth which I gave an example of being the magic shell on a scoop of ice cream the solid outer crust of the earth we call the lithosphere it's actually broken into pieces that we call plates you see em here you can call them lithospheric plates you can call them tectonic plates or simply plates but this is the solid outer part of the earth that we live on some of the plates are all oceanic crust some of them are all continental and some are a combination of the two that's going to become important keep in mind that we do have these two distinct types of crust if you look at the gray here this is our continental crust it's thick it's made of the rock granite it's not quite that dense only about 2.7 grams per cubic centimeter but if you look at the dark gray area under the oceans that's the oceanic crust it's very different it's much thinner more dense because it's made of the rock basalt the density of the basalt is about 3 grams per cubic centimeter keep in mind that information can be found on your Earth's interior chart in your reference tables now that crust whether it's oceanic or continental along with this kind of orangie section called the rigid mantle those two together are what make up our lithosphere this solid outer shell of the earth keep in mind that beneath that is kind of a gooey plastic partially melted bubble gum layer called the asthenosphere and so the plates that we live on are actually floating on the asthenosphere beneath now down in the asthenosphere that melty gooey bubblegum like rock is actually able to move and it moves because of a process known as convection essentially convection says that hot material becomes less dense and Rises and then when it rises it cools down becomes more dense and sinks and what we end up with are these rising and sinking currents called convection cells in the mantle those end up being the Machine the force that is driving the motion of the plates so in this diagram we have a type of boundary called a divergent boundary where the plates are literally being ripped apart by the movement of the convection beneath keep in mind this convection takes place in the asthenosphere right below the lithosphere so let's take a look at a map of the plates okay it's very similar map at your reference table which we'll see shortly but for the moment take a look you notice that the plates some are very large some are a little bit smaller but they're all moving and what that means is that there's lots of action at the places where plates meet for example that's where we find all our earthquakes shown here with pink dots that's where we find all our volcanoes shown here with red triangles and this is also where we find a lot of the earth active mountain ranges so keep in mind that all seismic activity earthquakes volcanoes and mountains tends to happen at or near these boundaries between plates which brings us to those boundaries themselves now all of plate tectonics comes down to the fact that things happen when two plates interact with one another so we need to know what those interactions look like and it really boils down to three types of interactions three types of plate boundaries the first is known as a transform boundary and that occurs when plates slide past one another as seen by these arrows we also have convergent boundaries where plates collide and divergent boundaries where they drift apart you should be familiar with each of these and the kinds of things we see at each of them so let's go into some examples here okay I'm going to be alluding back to this chart this is a map and your reference tables that shows plate boundaries so keep that handy as you're continuing to learn about this let's start with transform boundaries so these are the boundaries where plates slide past one another - take a look at this animation you'll notice the lithosphere floating on top of the asthenosphere is being driven by convection currents which is causing them to slide laterally past each other right past each other and so right along that boundary in the middle you get a lot of pressure built up a lot of grinding over the plates which is often released in the form of earthquakes transform boundaries very commonly of earthquakes however they don't tend to have volcanoes they don't tend to have major mountain ranges and certainly not trenches the only real feature we see at transform boundaries are earthquakes this is an example of a place on earth where you can actually see a transform boundary at the surface this is known as the San Andreas Fault and it's found in California it's responsible for the big earthquake risk of cities like San Francisco and LA let's go back to our reference table for a minute here and see that the transform boundaries are shown down on the bottom with this symbol shown in green here there's a couple of them not too too many but the most famous and important one would be the San Andreas Fault shown right there again if you forget how transform boundaries actually work the arrows kind of reveal the motion so it's a good tool to use your reference tables next type of plate boundary would be convergent boundaries now these are a little more tricky because we have to consider not only the fact that two plates are coming together and colliding but what type of crust is involved for example we see different things happening when it's oceanic crust colliding with oceanic crust as opposed to when it's continental colliding with continental and so forth and so we have a few different types of convergent boundaries the first type we call a subduction zone and as you can tell by this little animation here this happens when continental crust on the Left collides with oceanic crust on the right now notice that the oceanic crust is sinking underneath the continental crust and that's because oceanic crust is more dense than continental crust it's heavier and so when the two collide the heavier oceanic crust sinks underneath the lighter continental crust when it sinks it melts because it's hot down in the asthenosphere and that melting rock finds little cracks and crevices to rise through and ultimately erupts at the surface forming active volcanoes so some of the features we look for its subduction zones are a chain of tall mountains and action of active volcanoes a deep ocean trench seen by this dark line just off the coast as well as lots and lots of earthquakes which tend to happen along that subducting plate we also have island arcs now very very similar to subduction zone the only difference is that this is oceanic to oceanic so no continents involved here the one thing it's tricky is it's a little hard to tell which plate is going to subduct being that they both have the same density but eventually one plate usually the older rock will sink under the younger rock and again we will get this melting which causes rising magma to create active volcanoes which in this case form islands volcanic islands because we're talking about areas out in the middle of the ocean here we also get a trench which you can see clearly in the water there and of course we get lots of active earthquakes along the subducting plate before I go back to the chart here we also have a third type of convergent boundary called a collision zone and that would be continental continental the best example of that would be India crashing into the Eurasian Plate where the Himalayas are forming but again convergent boundaries can be seen with this symbol down here and probably the most famous one that we will discuss is the Peru Chile Trench on the west coast of South America so that would be an example of a subduction zone because you notice the Nazca plate is in the ocean so that's oceanic crust the South American plate is a continent that's continental crust and so that gives us a subduction zone that's where the Andes Mountains are lots of volcanoes lots of earthquakes finally we have divergent boundaries where three plates drift different plates rather drift away from each other again we want to consider the type of crust involved so if it happens on land we have what's called a Rift zone where where the land actually rips apart into two we see this happening in eastern Africa right now right now at an area known as the East African Rift zone as you can imagine when the rock rips apart it allows magma to escape so we get active volcanoes we do get some minor earthquakes here though they're not as significant as the convergent boundaries would be often we will see this out in the middle of the ocean we call that a mid-ocean ridge shown here so the convection currents are actually pulling the plates apart the magma is coming up in the middle forming mountains and volcanoes and again some earthquakes now there's one tricky thing of this are actually a couple tricky kind of points here one is not too bad and that's the idea that the younger rock is always going to be found right along the ridge because that's where all this magma is cooling to form new rock as you get further away from the ridge the rock age will increase it gets older and older and older as you get further from the ridge now here's the trickier thing this rock when it forms it actually records the Earth's magnetism if you recall we talked about how the earth is like a giant magnet but that magnetism changes every so often and so those changes are recorded in the rock and so what we see in the rock on either side of a mid-ocean ridge are these alternating bands of magnetism which match up on either side that's an important observation because it proves that the rock is actually spreading apart and it proves that the the divergence is actually taking place so if we look at a reference table here's the symbol for divergent boundaries and again the most common one is probably the one that spans the Atlantic Ocean the mid-atlantic ridge right through here it actually extends all the way down south but that's probably the best example although you see them all throughout the oceans of the world now we do have one problem here because plate tectonics says that all volcanoes and earthquakes should happen at these plate boundaries but if you really look at the earth carefully there's a handful of places where we get volcanoes that are not at a plate boundary and the best example I could give would be Hawaii smack in the middle of the Pacific plate and so geologists had to consider this and come up for an X with an explanation as to why we had some volcanoes that weren't at plate boundaries and the answer is that these locations are hotspots so hotspot is simply an area where we have a single area of magma that's able to punch through in the middle of a plate so we're not talking about a boundary here we're talking about one simple individual solitary point where you have active magma rising up and it rises from something called a plume we can't predict where these will happen we notice there's maybe a dozen of them on earth right now sometimes they're very active sometimes they're not as active here's how they work if you look at this diagram you're going to notice shortly a mantle plume there it is coming up from the asthenosphere it's just a hot spot and that magma is strong enough to actually punch through the lithosphere to create an active volcano but the problem is the lithosphere is moving because remember all the plates are constantly moving around and so you'll get this active volcano which is then dragged to the side making way for a new active volcano to form and then that is dragged to the side and a new one will form and so forth and so what you end up with is this chain of islands if it's happening in the ocean with one active volcano over the hotspot and then a chain of extinct or dormant not active volcanoes extending away from it and what's cool about this is if you look at where the extinct volcanoes are they tell you the direction that the plate is moving so that's pretty much in a nutshell the content that we need to know about for this chapter you guys will be learning a lot more about it in class coming up but I just want to do a quick overview for you thanks a lot

Channel: Mike Sammartano

Views: 628,012

Rating: 4.8041716 out of 5

Keywords: Plate Tectonics, Convergent, Divergent, Transform, Earth Science, Geology, Tectonics, ESRT, Earth Science Reference Tables, Regents, Volcanoes, Earthquakes, Plates, Convection

Id: ZTRu620bIsE

Length: 12min 43sec (763 seconds)

Published: Thu Dec 12 2013