This is Keith Meldahl.

We're going

to review here some

of the major motions

that happen

at the different tectonic

plate boundaries.

We'll start here

with the divergent boundary.

These are the mid-ocean ridges

of the world.

These are the places

where the plates separate

from each other,

allowing hot magma to come

up from the asthenosphere,

the mantle underneath.

The magma congeals along the

mid-ocean ridges

in volcanic eruptions,

forming pillow lava

or pillow basalt.

There are lots of earthquakes.

And the new ocean floor forms

and spreads away

from the ridges

like two oppositely moving

conveyor belts.

So the seafloor grows

and spreads

from these divergent

boundaries called the

mid-ocean ridges.

Now, the opposite process

happens at the convergent

boundaries of the world.

These are marked

by the deep ocean trenches,

deep creases

on the ocean floor.

And these creases are formed

because one plate dives

down beneath another one

in the process

that we call subduction.

One plate, the ocean floor,

dives beneath the other plate.

As it does that,

it makes frequent earthquakes.

As the plate gets

to a certain depth

in the mantle,

it triggers melting

of the mantle,

which causes magma to rise

and form a line of volcanoes

that we call a volcanic arc:

a line of volcanoes that rises

and runs parallel

to the ocean trench.

So we call these

convergent boundaries.

They are the locations

of subduction, and they occur

at the ocean trenches

of the world.

The third kind

of motion is called transform

motion, or a transform

boundary, and this is

where the plates slide side

by side past each other.

The main thing

that happens here

is earthquakes.

We don't have the creation

of the Earth's crust;

we don't have its

destruction either.

We simply have side

by side sliding.

The San Andreas Fault

of California and a number

of other very large faults,

including on the ocean floor,

are characterized by this kind

of motion.

In this final animation,

what we can see here is

that as the new ocean floor is

created at the mid-ocean

ridges, it spreads

and slides along for a while

and eventually slides

down an ocean trench

and is consumed

in the Earth's mantle.

But not before making lots

of earthquakes and rising

to form magma that erupts

and forms volcanoes.

So that's plate tectonics

in a nutshell.