- All right, in this tutorial,
I'm gonna quickly go over
the different types of plate boundaries
so that you can easily recognize them
when you're thinking
about plate tectonics.
It's important to remember that there are
three major types of plate boundaries.
We have convergent plate boundaries
where plates are coming together.
We have divergent plate boundaries
where plates are moving apart,
and we have transform plate boundaries
where the two plates are
sliding past each other.
So we're gonna start out by looking
at the convergent plate boundaries.
Okay, and we can actually
see that there are three
subtypes of convergent plate boundaries,
and it just tells us what type of plate
is colliding with what
type of other plate.
So the first example that we have would be
looking at an oceanic-continental
convergent plate boundary.
Okay, so here we have an
oceanic lithosphere plate
colliding with another
plate, that on that edge
where the collision is taking place
is going to have that continental
lithosphere, or crust.
So when we see this, we're
gonna, actually, draw it out,
I'll draw it quickly and
then, kind of, explain
everything that you start to see.
So this is our continent, and
this is our oceanic plate.
Okay, we know it's oceanic,
it's underneath the water,
and it's going to be a lot thinner.
Now oceanic lithosphere
is made out of basalt,
okay, it's more mafic in
composition, so it's denser.
This plate, when it collides with
this continental plate, so
these are coming together,
when it meets this continental plate,
which is felsic overall in composition,
that felsic material is more buoyant,
it has a lower density, so when
these two collide together,
the denser point is going
to be forced down below
through the process of subduction
below this continental plate.
Now as it goes down, we're
gonna experience a lot
of earthquakes all through this boundary.
Okay, and those earthquakes, you know,
they would register as
earthquakes at different places
along this continental plate
or even this lithospheric plate,
and so that would create
a lot of activity.
As the plate comes down, it's reaching
higher temperatures and pressures,
we're gonna start to see some
melting that's happening,
okay, so those are the little blobs,
these are called diapirs, they're
material that's melted and rising up.
It sits underneath this continent,
and it causes this continent,
right here, to start to partially melt,
so that will generate some more
magma inside of this plate.
If this magma cools and crystallizes,
it's going to form some
course-grained igneous rocks,
and then some of it will
reach all the way up
to the surface, and it will
cause a volcanic eruption,
so we get the formation of volcano's.
So at an oceanic-continental
plate boundary,
we have a subduction zone, or a trench,
oceanic trench right here, and that's
where this plate is going down.
We experience a lot of earthquakes.
We see mountain forming,
and we also see volcano's.
Okay, so that's the first
of the convergent plate boundaries.
Now the second one that we have
is going to look very similarly,
this is if we have an
oceanic-oceanic convergence.
Okay, so this is where
two plates come together,
and at that point of contact,
the plates are all oceanic.
And it's gonna look very similarly,
except this mountain isn't
gonna be quite so big.
So when these two plates
are coming together,
one of 'em is forced down,
and the other one is gonna override,
okay, and it, kind of, creates a blot
because they're coming
together, this one goes down,
this one's coming against
it, but it, kind of,
gets built up and squished together.
And again, it's oceanic, so we're,
actually, gonna see ocean
on both sides, okay?
Now how do we determine which
plate is going to go down?
Well when we look at the two plates,
the plate that is older,
the rocks tend to be cooler,
they've been cooling for
a much longer period,
then they are denser, so the denser plate
is the one that's gonna be forced down
through that process of subduction,
and the one that is
younger is more buoyant,
it's going to sit up higher,
and that's where we're gonna
start to see the formation
of mountains and volcano's.
Again, same thing, we
see a subduction zone,
an oceanic trench, we get earthquakes
all down this subduction zone,
we're gonna get some partial melting,
and that's gonna create a few
mountains as well as volcano's.
So oceanic-oceanic
convergent plate boundaries,
we do see the formation of island arcs
when these plates come together
because it's converging on a sphere.
In a map view, we actually
are gonna see a nice
curved plate, and we're
gonna see the formation
of the volcano, so we see
these volcanic island arcs.
So, by the end, they're not quite as tall
as we would see in the
oceanic-continental convergence.
And then the third type
that we have is when we have
two continental plates that come together.
Okay, now when you have
two very low-density,
very buoyant pieces of
lithosphere that collide together,
neither one wants to subduct,
so we get this big pressure building up,
we see the formation
of a lot of earthquakes
and really big mountains.
So, kind of, the end product that
you would see as they come together
is gonna look something like that.
Just to, kind of, draw it.
Now, inside of this, we're
gonna see lots of folds,
and this is gonna get a
lot of built up material
'cause those two plates
are coming together.
Neither one wants to go down, a little bit
of one will probably go
underneath the other one,
but when those two pieces
of continent come together,
they're gonna push and, kind of, build up.
If you just imagine pushing
your hands together,
the only place for it to go
is up and a little bit down.
The example that we see here
would be the Himalaya's.
If we go and we look
inside of the Himalaya's,
we get lots and lots of earthquakes
throughout this whole
zone, especially along
that suture zone, where
they're coming together,
but throughout both of the plates,
we're gonna see a lot of
earthquakes happening.
We'll see some faults that are trying
to help offset the motion, okay,
as those come together and
then that allows the rocks
to, kind of, slide up against each other.
The faults are gonna be forming closer
to the surface where the pressure is low,
but deep down inside,
where the temperatures
and pressures are higher, the
rocks don't, actually, deform
brittlely, they're gonna start to flow,
and we'll see the formation
of a lot of folds,
and as that material moves up,
we'll start to see those
folds exposed at the surface.
So at this plate boundary,
we see very big mountains,
we see a lot of internal deformation,
(sneezes) excuse me,
and earthquakes that are all forming here.
So our example here
would be the Himalaya's.
That's, like, our modern analog.
If we want to look at an
oceanic-oceanic plate boundary,
a place like the Philippines
would be a good example.
I'm probably gonna put the
wrong number of l's in there.
And the classic example of an
oceanic-continental
convergent plate boundary
would be looking at the
West Coast of South America.
So this could be down looking at Peru,
and then we have the Pacific Plate,
or the Nazca Plate, actually, off
out in the ocean, so
those two coming together.
So, I would just say West
Coast of South America
'cause it runs the entire
length of the continent
that you would see that happening.
So that's our example of our
convergent plate boundaries.
So the next question is what happens
at a divergent plate boundary.
Real quick, kind of, going
through this quickly.
Okay, so divergent is
the second major type,
and divergent just means the
two plates are moving apart,
so there's only two places
where we see this happening.
The first would be at an oceanic-oceanic
example, we see lots of these.
Okay, so oceanic-oceanic
divergence is happening
at all of our mid-oceanic ridges,
so these are places underneath the ocean.
So there's the water, and
here, we have two plates,
just gonna, kind of, draw 'em like that,
where they're moving apart.
Okay, as these two plates pull
apart, we see the formation
of a lot of faults, okay,
these are, actually,
normal faults caused by extension,
so we get blocks that drop
down right through here.
And as these pull part,
this crust right here,
which was, actually, originally one piece
that just started getting stretched
apart until it had a gap,
well there's not really
a gap that ever forms,
but it gets thin so much,
the rocks start to break
to try to accommodate some
of that motion along them.
The plate starts to thin, and
then we have mantle down below
that's starting to lift
up, and it rises up, okay?
And so what we see is a
little bit of partial melting.
Sorry for the tiny writing,
but partial melting
that's happening in the
mantle, it has the reverse
of Bowen's Reaction
Series that's happening,
so we, actually, start
to see the formation
of mafic magmas out of
this mantle material,
which would be ultramafic,
and that's gonna start to
allow that materials, kind of,
rises up, we'll get some basaltic flows
that start to fill in any open spaces
that happen, and that creates new crust.
So this, at this point,
where we see these faults
that are, kind of, dropped down,
this is a mid-oceanic
ridge, or a rift valley.
Okay, we're gonna see faults
that are happening here.
Anytime there's movement along a fault,
we do see some earthquakes,
and then we're gonna see
the formation of new crust.
If we look overall at the whole ocean,
this rift valley's gonna sit pretty high,
and that's because this new
crust material is very buoyant,
it's very warm, so it sits up higher
than the older, colder, denser plates
that are moving away from it.
So as it moves away,
this plate is cooling,
it becomes denser, and
it starts to sink down,
so the depth of the
ocean will become deeper
as opposed to right
over those rift valleys.
So any example, go out into
any of the oceanic ridges,
the Atlantic would be one good example.
And then I'll just write
m,o,r, mid-oceanic ridge.
Okay, the other type of divergent boundary
would be one where we see a
continental-continental divergence.
Okay, and this is a
place where originally,
I'll just, kind of, draw this out,
we just have a nice big
piece of continental crust,
but it's gonna start to
experience some tension,
so we might have some magma
coming out from the mantle.
It's gonna start to cause this plate
to want to go in two different directions,
it causes a lot of breaking,
so we'll see some faults start
to form at the top,
and those are gonna try
to accommodate this
motion, those are, again,
normal faults that are forming,
we'll get some earthquakes.
Over time this plates gonna
get really thinned out,
and it's gonna break down to allow
some more partial melting to happen
within that continental plate.
So it's very similar, except
the crust is different,
we don't have water sitting on top of it.
And eventually we'll just see
in that big continental crust,
see if I can draw
on a bigger scale for you,
kind of, this large piece of continent.
So the last thing to point out with the
continental-continental
divergent plate boundaries is,
again, these are places
where we see faulting,
we see earthquakes
happening, the formation
of new crustal material,
and these big rift valleys.
All right.
Now the last type of
plate boundary we have is
the transform plate boundary,
and these are places where the two plates
are sliding past each other, horizontally.
And the most common example that we see
is going to be a
continental-continental example.
Okay, now this one, we're
gonna be looking at,
instead of a sideways cross
section like we have before,
we're gonna be looking at the top,
looking down onto the map.
Now this is a place, the example
that we have is the San Andres
fault area, and this was
originally mapped out
as just one little fault,
but we, actually found out,
so if we go down to Southern California,
I'll, kind of, draw a boundary
for you, kind of, the edge.
And we have Baja, and
there's Central Mexico,
so this is just trying to
look at the border of the U.S.
So we have, you know, let's see,
maybe, Washington way up
here, and then Oregon,
and then California, and then this is
Baja California, and this is Mexico, okay?
So now, the place that we're
looking at, actually, runs
right through here where
that purple line is.
That we have a part of, where
we have Southern California
and Baja California, these are, actually,
out here on the Pacific Plate,
and then we have the North American Plate.
Now what's going on in this location is
that the Pacific Plate is
sliding up to the left,
and the North American
Plate, at this location,
when we look at it is,
kind of like, rotating,
but in that process, it's sliding down,
so these two locations
are moving over time.
Now here, although the Pacific Plate
is mostly oceanic lithosphere,
this little sliver
right here that makes up
Baja California and Southern California
is continental, so it's
continental right at this boundary,
okay, and the North American
Plate is also continental.
What we see happening here is we get a lot
of earthquakes all along
this plate boundary.
We might start to see in
places where there is a bend,
we might start to see the
rock getting pushed up
and creating some localized mountains,
but we're not going to
find subduction zones,
we're not going to see rift valleys.
It's mostly just looking at, kind of,
places where we might get
gaps, or valleys created,
or small compressional mountains where
there's bends in this overall zone.
So that would be one example
of a transform fault.
The example of an oceanic-oceanic
transform fault would
be looking at something,
again, in map view, going out into,
we'll just go to the North Atlantic.
And we're going to be looking
around the mid-oceanic ridge.
Okay, so I'm gonna draw this
one a little bit differently,
I'm gonna try to keep the dark
purple as the color to see.
So when we look at the mid-oceanic ridge,
I'm gonna, kind of, show the
divergent boundary in orange,
so here the two plates are moving apart,
and this in the middle would
be the rift zone, okay?
And we, actually, find that
these rift zones get offset
because we are taking
rocks and we're trying to
move them, and it can be,
it, kind of, goes back and forth depending
on what's happening,
but because these plates
are moving apart on a round sphere,
they can't just stretch
continuously as one straight line.
It, actually, gets offset
by these transform faults
and that's just to take up the motion
that's happening along a sphere.
So the transform fault
boundary is, actually,
right in this middle part, right here.
At this location, I'll
draw it with purple,
the plate is moving to
the right, and down here,
it's moving to the left,
so this is the active zone.
We'll get earthquakes that are happening
between the two offset
ridges, but on the other side,
we get no activity 'cause the
plates are moving together,
and so that transform fault boundary,
that transform plate boundary is going to
be inactive as we get farther away
where the two plates are then
moving the same direction.
So, again, the active zone is right here
where it's moving in opposite direction.
So I'll try to draw out one more.
I'll move this one over
here, this plate boundary,
and we'll draw that line in,
so I'm gonna connect them.
So, again, the active
boundary is right here
and right here, and we're
gonna get earthquakes
happening all along this area.
Again, earthquakes happen here,
we're not gonna see the
formation of new crust,
and it's really just
that active earthquakes
that are happening in that area.
Apologize for the noise from upstairs.
So those are, again, are
the three plate boundaries
with some examples where
you would find them.
Do take some time to
make sure you understand
how the plate type relates
to what's showing up in
the different locations.
Make sure you have a modern day
analog for each of the locations,
and that you understand
how the plates are moving
at those areas and the
different geographic features
that are forming at those locations.
