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  • Hi. It’s Mr. Andersen and this environmental science video 3. It is on geology, which is

  • the study of rocks and how they change over time. And in this class more appropriately

  • how that affects human society. Now the problem with rocks is that they change slowly. They

  • change over geologic time and we simply to do not live long enough to see all of these

  • changes occur. You could imagine how this sandstone is being shaped by the wind but

  • you cannot see it. And it is not until we see lava that we really start to understand

  • the dynamic earth. And to understand these systems on the earth we should really understand

  • how the earth is put together. And so if we look at the layers, on the inside we have

  • the inner and outer core. We then have the mantle and finally we have the crust. Now

  • we live on the crust. And we have only been able to dig just a little bit into the crust.

  • And so we have been able to figure out everything else by looking at how earthquake waves move

  • through the earth. And we know this, that the crust is made up of rocks and minerals.

  • Rocks are made of minerals, which in turn are made of molecules and atoms. And they

  • are constantly being reshaped. And we can measure that through the rock cycle. Now the

  • crust itself is made up of these large continental and oceanic plates. And they float on the

  • mantle itself. And so this is rock under here, but it is rock that is moving. As we generate

  • heat, as it moves up, it is pushing those plates around. They move very slowly, about

  • the same rate that your fingernails grow. But they have huge force and therefore huge

  • impacts. And so when those plates run into each other we have boundaries. An example

  • of that would be the ring of fire. So if you look around the Pacific Ocean you have this

  • area where almost all of the earthquakes and volcanoes take place. We also have what are

  • called hot spots. And those are going to be areas where a plate will move over a hot area

  • in the mantle and we can form islands like Hawaii. Now if we look at where those boundaries

  • occur we will have structures like volcanoes and mountain chains. But we will also therefore

  • have hazards around those areas where it can impact human society and we will discuss a

  • lot of those. And so to understand what is going on on the earth we should really understand

  • what it looks like on the inside. So we have a solid inner core, a liquid outer core and

  • then we have what is called the mantle. Now around 85 percent of the volume of the earth

  • is going to be in this mantle. So it is rock, but it is rock that is moving. We are generating

  • a huge amount of heat down here. And then we finally get up to what is called the crust.

  • That is where we live. And so on that crust we have rocks. And those rocks are constantly

  • being shaped from one into another. And so if we take a look at this rock cycle, since

  • it is a cycle you could start anywhere. Let’s start with an igneous rock. So an igneous

  • rock, like granite, is going to be formed when we have crystallization of magma. So

  • as it crystallizes you can see those crystals right here. So this granite is made up of

  • minerals. So I can see for example this quartz and this pink felspar and we have this mica.

  • So these are the minerals that make up the rock itself. But, this could experience erosion.

  • So erosion on the planet, water, wind, can cause it to break down into sediments. And

  • therefore after it has done that it could get squeezed for a long period of time and

  • we could have a sedimentary rock, which is going to be compressed sediments. Now that

  • could be squeezed, we could squeeze it under the surface of the earth using heat and pressure.

  • It could something like quartzite. Or we could take that igneous and we could squeeze and

  • make something like gneiss. And so g-n-e-i-s-s, it is a type of metamorphic rock that is formed

  • by the squeezing of granite. Now you can see those minerals are still there but it has

  • a different shape. And so the rocks on our planet are continually recycled over and over

  • and over again. But if we get back to the structure of the earth, what is driving all

  • of this are going to be convection currents within the mantle itself. And so if you look

  • at this and say this is some boiling water, so if we generate a little bit of heat underneath

  • it, that heat is going to be transferred through the water, so if you held your hand up here

  • you would feel some of that heat above it. But if we look inside the water itself we

  • are going to have these convection currents, areas where we are heating it up. And so we

  • are decreasing the density and then it cools down and then it is going to sink again. And

  • so we are going to find the same thing in the mantle itself. It does not occur as quickly

  • as it does in the boiling water, but it has huge implications on the crust above. And

  • so if we look at that crust it is actually made up of plates. And so if I trace out a

  • plate like this, this would be a continental plate. And so it is being pushed to the left

  • and the reason why it is being pushed is because this convection current is moving like that.

  • It is forcing the plate in that direction. And it is running into another plate. So we

  • have an oceanic plate. The oceanic plates are going to be more dense and they will be

  • pushed underneath a continental plate. And what we are going to get, right along this

  • margin is going to be a convergent plate boundary. They are running into each other. But we could

  • look over here and maybe there is another oceanic plate that is moving in the other

  • direction. Why is it moving in the other direction? Because the convection current is pushing

  • it here. Or pushing it there. And so we could have this mid-Atlantic ridge or this mid-oceanic

  • ridge being formed right there. So we have known this for a long time. If you look at

  • the ring of fire, we find around the Pacific Ocean we have an aggregation of volcanoes

  • and earthquakes, something like 75 percent of all volcanoes, 90% of earthquakes are found

  • in this area. If we start plotting where those earthquakes are we can start to see where

  • those plates exist. And if we look at the plates on our planet it is hard to wrap your

  • head around this picture for a second. So this is North America right here. And then

  • it sits on this giant north american plate. And then we would have the pacific plate right

  • here. And so those plates are constantly moving around. You are probably familiar with Pangea

  • which was a time when all of the continental plates had come together. So what happens

  • when plates meet is that they can do one of three things. They can slide past one another

  • in this transform boundary. They could move apart. We call that a diverging boundary or

  • they could run into each other. That is converging. And so if we look at an example of that, right

  • here we have a convergent plate boundary. So what is going on? We have this oceanic

  • plate, which is being pushed of subducted underneath a continental plate. I described

  • that just a second ago. What is happening is we are melting that rock and that is forming

  • this volcano chain that goes all the way back here. An example could be the Cascades in

  • Washington state. We could also have a convergent boundary right here where you have an oceanic

  • plate going underneath another oceanic plate. And we get this island arc like the Aleutian

  • Islands in Alaska. We could have divergent boundaries. An example could be right here.

  • So we have this oceanic plate moving away from this oceanic plate so we get this mid-oceanic

  • ridge. We could have a rift valley where this continental plate is being pulled apart. We

  • could have transformed boundaries. Here would be an example where they are sliding past

  • one another. San Andreas fault is a good example of that. But we could also have structures

  • where this no boundary. So if we look right here we have what is called a hot spot. Remember

  • that is going to be an area where the mantle is close to the surface. And so for example

  • Hawaii was formed as the plate slid over a hot spot. And let me show you what that looks

  • like. So if we imagine that is the hot spot, it is going to be underneath the plate and

  • what has happened to Hawaii over time, is it slowly slid over the hot spot. The hot

  • spot stays in the same location. And so we have had volcano after volcano after volcano.

  • And so this the most recent volcano and we will have another island right here. And the

  • reason they are smaller islands out here is that there is more erosion. Yellowstone National

  • Park is another example of a hot spot and a plate simply sliding over the surface. Now

  • we get to humans and human society and how we start to interact with the structures of

  • the earth. And so living around these boundaries can be dangerous. It is just a matter of time.

  • And so an example could be volcanoes. Some volcanoes, like the ones we would find in

  • Hawaii can ooze out. And as long as we are able to move out of there quickly we are going

  • to be fine. But some are highly explosive. And it depends on what minerals make up that

  • rock that determines the explosiveness of the volcano. A very explosive one I remember

  • is Mount St. Helens. And so this is a picture of Mount St. Helens in Washington. This is

  • the day before it exploded. And so just take a second to imagine that is the structure

  • of the volcano and now it is gone and rebuilding again. And so it literally blew apart. And

  • if you were on or near that you died. Earthquakes are another example of a natural hazard. What

  • we have our faults and this would be a fault right here. And we have one plate, in this

  • case transformed fault, where it is sliding past one. We could also have a divergent boundary,

  • so these two are moving away from each other. And so we have what is called a normal fault.

  • It is slipping down. Or we could have a reverse or thrust fault, when we have an convergent

  • between these two areas on either side of the fault. Now the names are not as important.

  • Really understanding what is going on in an earthquake is. If you think about it, if we

  • have two plates that are pushing on each other eventually they are going to build up pressure

  • and it is going to slip. And as it does that we have an earthquake. So if we watch this

  • right here, let’s say there is pressure in this direction, in this direction and eventually

  • it builds up and we have a slide along that fault line. Now it is not like it stopped.

  • There is still pressure there. We could have another earthquake in the future. And another

  • earthquake in the future. As these move past each other it is just going to move in small

  • slips. And every time we do we have an earthquake. Now if you are standing on the surface and

  • there is an earthquake you are going to be fine. The problem is if we build structures

  • on that and it is not earthquake ready then those fall in and humans are going to be impacted.

  • We could also look at tsunamis which are caused by earthquakes. What we have here is a subducting

  • oceanic plate. This would be a continental plate. And what is happening is this is being

  • pushed underneath, but it will just stick. And so it is not going to release. And as

  • it sticks then we build up pressure. And eventually when it slips what we get is a huge push up

  • on the water above it. So you get this vertical motion in the ocean and that leads to these

  • giant tidal waves. And so if you are near the ocean you would also suddenly notice the

  • ocean is going way out and then it is going to come way in and there are really bad consequences

  • from that. We could also have mass wasting. So an example could be a landslide moving

  • across this road. This is a picture taken before and then watch it, after a landslide.

  • Now it does not occur really quickly. It could be triggered by an earthquake, but generally

  • if we get a lot of water in an area, it cannot support that weight. And so did you learn

  • about earth systems? Can you stop the video and try to fill in the blanks right now? I

  • would pause the video. But if not, I would say we have a center core. We have a mantle

  • and a crust. The crust is made up of rocks which in turn are made up of minerals, which

  • is reshaped using the rock cycle. These plates move on the surface. We call that plate tectonics.

  • An example would be the ring of fire. Where we have boundaries we can build up structures

  • like volcanoes, mid-oceanic ridges. But we could also have natural hazards. And then

  • remember, not along boundaries, but just within the plate we can have hot spots.

  • So I hopeyou learned all of that. And I hope that was helpful.

Hi. It’s Mr. Andersen and this environmental science video 3. It is on geology, which is

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B1 中級

地質学 (Geology)

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    Amy.Lin に公開 2021 年 01 月 14 日
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