howdy everyone and welcome to module two where we will explore this class's probably biggest topic of plate tectonics the first video in module 2 is going to focus on earth's internal structure so earth has layers and we're going to be talking about the lithosphere um for the majority of this module and you probably heard of these layers before if you've taken an earth science class in high school or maybe another class on campus but we have what we know as the crust the mantle and the core where the core actually has two layers where we have the liquid outer core and the solid inner core now I like this analogy for Earth's interior structure um and it helps me remember it a little bit better using earth as a peach where the skin or fuzz on the peach is the crust that's how thin the crust is compared to the rest of the earth and the flesh of the peach is the mantle then you can even see the pit has a shell or an outer core and the seed in the middle of the pit would be our inner core hopefully this will help you remember Earth's internal structure as well now when we talk about Earth's internal structure and we separate it by crust mantle and core this is when we're considering earth differences by composition the crust is a rocky material um it is the least dense of all of the layers which is hence why it's on top these layers uh differentiate themselves based on density and density is being controlled by composition here so we have the less the least dense layer the crust this is rocky material but we have two different types of crust that we'll get into more as we explore this module further we have oceanic crust and continental crust and they differ in composition and thickness quite a lot now oceanic crust is thinner it's only about 8 km thick on average and it's primarily composed of basaltt um which is a denser material compared to granite which is the composition of our continental crust so oceanic crust much denser but thinner continental crust is thicker on average 45 km thick but in some mountainous areas we could have 70 km thick of continental crust and again that composition is granite so it's less dense than oceanic crust and we'll talk more about that as we get into this module further and even our next module earth materials this will come up again so so make a note we will talk about this further now the next layer the mantle this is denser than the crust um but it's also very very thick and it's still made up of solid rocky material it is not liquid i know some people think the mantle is full of liquid um magma that is not the case here the mantle is a thick shell of densy rock material it is denser than the crust but it's still less dense than the core now its composition is primarily made of peritodite which is uh made up of mostly of the mineral olivine and again these words are going to come up again in the next module so don't worry if you if you don't really pay attention to it right now but the the mantle is still solid very dense made up of olivine now takes us to the core this is the densest layer it's composed mainly of metallic iron and it has some other metals like nickel um and we split them up into the inner and outer core because the outer core is actually liquid we have liquid outer core has the same composition as the inner core so the inner and outer core still made up of mostly metallic iron but the outer core is melted iron it's so hot um that that iron is actually melted in the in the outer core however in the inner core the pressure is so high that it actually can't exist as a liquid so it's a solid despite the super high temperatures there so we have solid metallic iron in the center of the earth the inner core surrounded by outer liquid layer of that metal then we have our thick mantle and then we have our rocky crust at the top so how do we know Earth's internal internal structure anyway have we ever been to the core no we haven't so we actually use seismic waves to infer Earth's internal structure and seismic waves take time to travel through the planet and in general the denser the material is the faster the seismic wave travels seismic waves can also change directions under certain circumstances such as when the physical property of the material in which it's traveling in changes this can cause the waves to be reflected bounce back or refracted bent now Pwaves travel faster than other seismic waves and this will come up again when we get to earthquakes but these P waves are the first signal from an earthquake or other seismic source to arrive hence why it's often referred to as the primary wave it is also a compressional wave which is why it's sometimes referred to as pressure wave so P primary P pressure now S waves generally arrive second which is why it's often referred to as secondary wave but it is also known as a shear wave and because we can't actually shear liquids S waves are unable to propagate through liquids so when humans began recording earthquakes on seismoggrams throughout the globe we noticed some puzzling observations some of our seismic stations across the globe did not register any P or S waves after an earthquake and other seismic stations only recorded Pwaves this was one of the first evidences we have that the Earth must be composed of different physical layers waves were being reflected and refracted at each layer and no S waves at all traveled through the core proving it must have a liquid shell no need to memorize the details on seismic waves now they will come up again when we talk about earthquakes um I think in next week okay we can also split up earth's internal structure by physical properties and if we do that we have different names for their layers we have the lithosphere the athenosphere and the meosphere the lithosphere is that rigid layer that includes the crust and the uppermost mantle so it's not just the crust it's also including the uppermost mantle because this is the rigid layer we're splitting up Earth as by its physical properties not composition and the rocks are cooler there and they're stronger and more rigid than the atheenosphere which is below which behaves more like a plastic now it's important to remember that strength within the Earth's layers is a function of temperature and pressure now the lithosphere is thicker under the continents than under the oceans because remember our continental crust is pretty thick and our oceanic crust is thin but our tectonic plates which we're going to talk a whole lot about in this module are formed by sections of the lithosphere so our tectonic plates are not just the crust they also consist of the uppermost mantle so that's why it's important that we talk about Earth's internal structure by physical properties because it's this lithosphere that rigid layer containing the crust and uppermost mantle that are our tectonic plates now what are tectonic plates moving on they're moving on the atheenosphere which is the region of the mantle where rocks start to become ductile instead of rigid so they behave more like a plastic think like silly putty in a way so it's still a solid but it's a movable solid so the rocks there have little strength and they're very easily deformed again like silly putty and the athenosphere exists from about 350 km to 200 km in depth so it's right it's below that lithosphere right this is our tectonic plates are moving on this silly putty of the atheenosphere below that we have the meosphere that's the lower part of the mantle it's solid with relatively high strength um because we have a lot of pressures there um but you we're not going to talk about the messosphere at all in this class i just wanted to to introduce it for completeness we're really only going to focus on the lithosphere and the athenosphere because our lithospheres are tectonic plates and they're moving on the silly putty of the athenosphere okay so here is a um diagram graphic that shows Earth's internal structure based on those physical properties but you can also see the words with the um compositional internal structure as well so here's our continental crust i told you it's much thicker than our oceanic crust this is that top layer and then we have our uppermost mantle that's also very rigid very strong very brittle so together with this crust and the uppermost mantle this is what's making our lithosphere so you see it's thicker under the continents it's thinner under the oceans below that is our atheenosphere which is still part of the mantle and this is behaving like silly putty it's ductile it's no longer rigid or brittle so it's able to flow but it's still a solid the atheenosphere is not liquid our tectonic plates are not floating on liquid magma okay so our our lithosphere consists of the crust and mantle our atheenosphere is just that ductile silly putty part of the mantle now a little bit more information about our mantle we know that the upper mantle is not as dense as the lower mantle um because the S and P waves travel more slowly in the upper mantle also it just makes more sense the further down you go in the Earth the pressure is going to increase so things should get denser now the upper mantle extends from the base of the crust to a depth of about 415 miles or so it consists of mostly olivine and silica um it itself has two distinctly different layers where the lower part of the mantle is quite plastic it's ductile remember that athenosphere this means it's a solid behaves like a plastic it's a solid however it can readily change shape and flow under stress think silly putty the lower mantle is more dense and contains mostly iron magnesium and silica so the composition changes a little bit as you go further down now you don't need to memorize these um exact details of the mantle but I just wanted you to understand why the mantle is so different than the rest of the layers of the of the Earth so that brings us to our crust and I'll start talking about continental crust first a lot of these terms you're going to see come up in multiple modules especially later on with this module 2 module three when we talk more about earth materials it's going to come up again when we talk about earthquakes volcanoes you're going to see these terms come up again so don't worry if they don't click in your head right away i'm going to try and beat it into your brain at some point later on in the semester but our continental crust it is felic felic means it is rich in silica and that makes it grrenitic in composition granite so here's a picture of our continental crust you'll see that lighter color it's kind of pink and tan this is granite so it's lighter in color and that's because silica those silicut minerals that we'll talk about in module 3 are also lighter in color so that makes sense and these silicut minerals are lower in density relatively speaking compared to our oceanic crust so continental crust is felic rich in silica making it granite in composition and that makes it lighter in color and lower in density now for module two the density is key continental crust is lower in density remember that now let's compare that to oceanic crust looks way different right that's because our oceanic crust is mafic in composition mafic means it's rich in magnesium and iron this makes its composition basaltic you know the rock basalt so magnesium and iron are very dark colored minerals so that makes our oceanic crust darker in color and you can obviously see that right away with the with the image displayed here it's usually pretty dark black and magnesium and iron are much more denser than silica alone so our oceanic crust is higher in density again for plate tectonics it's that density difference that's going to be very important continental crust lower in density oceanic crust higher in density now our crust versus lithosphere remember when we talk about our crust we're talking about something different than our lithosphere because the crust is only composed of that oceanic and continental crust but when we say the lithosphere we're talking about both types of crust as well as the uppermost mantle remember that the crust is just crust oceanic and continental the lithosphere is both of those crusts as well as the uppermost mantle our lithospheres are plates so here showing just our crustal materials again hoping to dig it into your brain that continental crust that's average density is 2.7 g per cubic cm compared to our thinner oceanic crust that has a much denser average density of 3.0 g per cubic centimeters right so even though our oceanic crust is thinner it is denser it's going to become very important and you'll see that here very soon and that brings us to our geologic cycle we're going to start talking about the first part of our geological cycle you'll see there are many different parts of the geological cycle we'll get to the hydraologic cycle at some point next module module three is all about the rock cycle but module two is going to be about this part of the cycle the tectonic cycle we'll talk about seafloor spreading subduction crust formation and this is going to be so important when we're talking about almost every single hazard we deal with on Earth and with that I'll see you in our next video