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.