Geology


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.

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