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Welcome back to week four wherein we are going to
talk more about the internal
processes that are happening in the earth
and in particular what we are going to talk about now is some of the evidence that exists to support
the theory of plate tectonics. So what
kind of
things exist? Well Alfred Wegener, who is the kind of the father of plate
tectonics,
what he discovered uh... and was looking at are things like continental fit,
looking at geologic structures, rocks, fossils
that exist in very different locations, but are very similar
to one another.
Ah more recent things that are looked at are the occurrence of volcanoes and
earthquakes, the age of the sea floor.
And then Harry Hess was looking at
magnetism of the sea floor as well as
sea floor features as well. So
Alfred Wegener had this idea and that
continents
are like little puzzle pieces and they all once fit together.
And here you see two pictures the picture on the left showing you what's happened
from
present day in the bottom and then moving backwards in time to the Permian
when we had all of the continents once together in this
uh... puzzle fit together
to form Pangaea at about two hundred twenty five million years ago.
And this is what Alfred Wegener
was proposing. That
all the continents were once fit
together, looking at the shapes of the edges of the continents as well as
some of the other things we will look at in the next few slides.
In this other pictures over here
kind of chepper ah..
checkerboard pattern
ah.. shows
uh... how the continents fit together using the continental shelf.
Which is actually the
uh... where the continental crust ends and the oceanic crust begins.
You can see they fit together pretty well, there's a few
of the grays and black areas on here that show places where there's either some overlap
or there's gaps,
and that's due to erosion or deposition of material.
So some of the other things that
Alfred Wegener used to
piece these continents back together
are things like geologic structures. You can see in this top picture shaded
brown areas are showing you different mountain ranges
that were formed
when all these continents collided into one another.
And once they ripped apart,
these mountain ranges, such as the Appalachian mountains in uh...
the eastern side of the country,
match up to mountains that we see over in Europe.
We see rocks
that exist in Maine
and we also see them in parts of Africa
and Europe as well. So we see things that are
identical that are now
separated by a huge ocean. We also can look at fossils as well,
and
some of the fossils that, when they're looked at,
are things like the lystrosaurus
which is an
organism that is
living on land. It's not gonna swim across the huge ocean.
You see that brown range stretching from Africa to India to Antarctica. These three
continents are very very far away from one another now. It's very unlikely that
this organism was able to
travel that distance.
And then we see things like the mesosaur
which is a
fresh water
or organism that we see in
the southern tips of Africa and South America,
now separated by a huge ocean as well. So
Alfred Wegener,
looking at some of these features like continental fit,
structures,
rocks and fossils.
And poor Alfred Wegener, he
came up with this idea that the continents were once
fitting together
but people didn't believe him and support his theory because he couldn't
come up with the driving force. What
happens to create
this motion of the plates?
He didn't have a good explanation so people said "Hey, well if he can't explain it
how do we know it's actually happening?"
It wasn't until later on
with Harry Hess and his discoveries that we were actually able to figure out what is
actually happening.
And you guys are going to watch a video this week as well that
is
by
Bill Nye, the Science Guy,
one of my favorites, and he's going to talk to you about Harry Hess
and some of the things that he actually
did to help this theory of plate tectonics.
So some of the other things that we use:
earthquakes and volcanoes. Here's a screenshot of the USGS real-time,
worldwide earthquakes.
You can see um...
this is just in the last week. We get a nice pattern around
the Pacific ocean right along the ring of fire.
Lots of earthquakes there. The yellows are within the last week.
Blues are in the last day,
reds are in the last
hour. So we can see some over in
the Caribbean there.
So this is from the USGS. You can also look at the IRIS real-time
seismic monitoring as well.
And I'll show you those websites. Here is
IRIS.
Cool website, lots of interesting information, it's the Incorporated Research
Institutions for
Seismology. Great
little videos that we're actually going to watch when we talk about the types of boundaries.
And here's this
seismic monitor page.
And this one actually goes further back. It actually goes
in the last five years in purple,
two weeks in
yellow and then orange is yesterday and reds are today.
The relative size of those circles
correspond to the magnitude of the size of those earthquakes. So you can see those
tend to form a pattern around the world.
And those are occurring along those plate boundaries. And we can
also looked at volcanoes as well.
And here's the uh... USGS earthquake map here,
real-time, you can zoom to different areas to see what happened where.
And here the volcanoes.usgs.gov website you can see um...
volcanoes that exist
in this country and around the world and then they actually tell you
if they're active or not,
or if they're being watched as you can see. In the
orange
triangles, that's where there's actually something happening
right now. Which is pretty cool. And you guys can
explore
that
as well. So that
we see
volcanoes spreading along
those boundaries as well as
earthquakes
following those plate boundaries as well. So it's another example of
things that we look at
to help us figure out
where those boundaries are,
and to help prove that they actually exist.
We also look here to see things like the magnetism of the oceanic crust.
And that
changes through time. The Earth's magnetic field is
dynamic, it
fluctuates. We have times where we have normal polarity which you can see in this
picture here where the north pole is the north pole
and then we have times where it
flip-flops
and we actually have it reversed. Where
your compass,
the south arrow would point north
um... instead.
And also uh... that magnetic field changes in strength as well.
And that, it gets recorded in the rocks that are forming at these divergent plate boundaries.
And this is what Harry Hess was looking at.
So what happens, I have a
cool little video to show you.
We go to this Ocean Explorer NOAA website. See some
descriptions of seafloor spreading activity. So this is where we have
divergent
plate boundary.
So here, can't
totally
fit it all on the screen, but
we can come pretty close. So what
you know can see here is the divergent plate boundary where two plates
are moving
away from one another.
And
in the center, this is where we have new crust being formed. We have
material erupted onto the surface.
And as that material erupt on the surface, it's liquid, the magnetic minerals
in that rock can
take on the polarity and the strength of the magnetic field of the earth at that time.
Once that rock cools,
that polarity is now frozen
in the rocks. And because this is an
active divergent boundary
those two uh... that material is now going to be
ripped apart ripped in half and pulled away from the center. So that's what we
see here, zero
age in the middle, where we have new crust formed.
And then we go out from the center: we have one million year
two million then three kind of a mirror image of one another on either side because we're
actively pulling this stuff
apart and pulling away from each other.
So this is our map view
with our divergent boundary.
You can scroll this
uh... bar over and we can see
um... the
stripes that we see, the magnetic minerals that are present.
And then our map view
of what the surface would look like. So um...
we see zero
regular polarity and then we see mirror images of this polarity pattern as you move
out from the center because these are actively being pulled
away from one another.
So make sure you watch this, the video of
Bill Nye describing Harry Hess and his discoveries. It's pretty interesting.
One last thing that we can actually look at to help us um... prove that
this is this
seafloor is actually spreading is by looking at the age of the rocks.
And we can do that through
using magnetic polarity
can be one way.
But also looking at the sediments that build up on the ocean floor and actually
figuring out how old they are.
Here's a map showing you uh... gray areas as the continents
and then the age of the seafloor, the reds being very young, the blues
being much older.
And see that along those divergent plate boundaries like in the
middle of the atlantic ocean
also in the pacific ocean what we see very young rock right at the very center
where things are
actively pulling away from each other and creating new crust.
And as we move further further away from that plate boundary, the older older
the rocks get.
And the more and more sediment that gets built up on
those
uh... bits of crust.
So another thing that we can use are satellites and gps units. So
satellites
are
in orbit around the earth. They can
connect to gps points on the earth's surface and actually measure
how far
continents have moved
and also how fast they're moving so we can record that data
with more accuracy with this new technology that we have. So
that's some of the evidence that exists to support this theory. Next we'll come
back and talk about the different plate
boundaries
that exist.