to subduct some ocean floor

underneath the edge

of a continent.

And as that happens,

as you know,

you get an ocean trench.

But at a certain depth,

about 80 miles

down in the Earth,

the subduction

of the plate triggers melting

of the mantle.

And that magma rises

and a whole series

of volcanoes begins to pop

up along the edge

of the continent parallel

to the trench.

We call this a continental

volcanic arc.

The Andes Mountains are a

classic example

of such a system.

Now, another thing we can do

is have instead

of ocean floor subducting

underneath continent,

we could have ocean floor

subducting

under other ocean floor.

In this case,

the animation is flipped

around, but it doesn't

really matter.

You can imagine either

of these in a mirror image.

Same process.

We're going

to subduct the ocean floor

underneath this other

ocean floor.

We're going to generate magma.

The magma's going to rise

and a whole series

of volcanic islands is going

to pop up in the ocean

parallel to the trench.

You see how it's very similar

to the continental situation.

The only difference is

that the volcanoes are popping

up out of the ocean parallel

to the trench instead

of on land parallel

to the trench, okay.

So an example

of this would be the Aleutian

Islands, say,

or the Philippines

or the Marianas Islands

in the Pacific.

These are all what we call

volcanic island arcs,

of volcanic activity

Hot spots --

there's magma being generated

all along a long line.

where magma forms

in just one local area.

And we think these hot spots

or columns, of magma that rise

from deep in the mantle.

We call these mantle plumes,

And as tectonic plates move

if mantle plumes stay

and there's controversy

But let's assume the plume

stays in one place.

The volcano will get

Another volcano will form;

it'll get carried away

And yet another volcano will

form and it'll get

And you get a chain

of hot spot volcanic islands.

This is a classic example

of how for example the

Hawaiian Islands formed,

all in a line,

one after the other,

as they moved

across a hot spot.

Let's back this up again

and we'll show one volcano,

then another,

and then another forming

over the hot spot.

So those are the main reasons

and processes

by which the Earth makes

volcanic activity.