Hawaiian islands formation | Cosmology & Astronomy | Khan Academy

We've talked a lot about
the formations of mountains

and volcanoes when plates
are running into each other,

or when one plate is being
subducted under another.

But that isn't
the only place, it

is the dominant place
or the most likely place

to find mountains and volcanoes
on the surface of the Earth,

but that's not the only place
that mountains or volcanoes can

form.

And probably the biggest
example of volcanic activity,

or the most popular
one-- this might

be a slightly American,
Amerocentric point of view,

but the most often cited
example of volcanic activity

away from a plate
boundary is Hawaii.

So this right here, these
are the Hawaiian Islands.

This is the big
island of Hawaii,

and it is experiencing
an active volcano.

Lava or magma is flowing
from underneath the ground,

and once it surfaces
we call it lava.

And that lava is actively
making the island bigger.

So where is that volcanic
activity coming from?

And then how can we think
about that volcanic activity

or that kind of heat rising from
below the surface of the Earth

to explain some of the
geological features we

see around Hawaii?

So what we think is happening,
and once again, this

is all theory right
here, is that Hawaii

is sitting on top of a hot
spot, and in particular,

the big island of Hawaii is
sitting on top of the hot spot

right now.

And this hot spot,
there's different ways,

different theories on
how it might emerge.

But we think that at the
mantle core boundary--

and I don't know in this
diagram whether they intended

this white area to be
the core, but let's just

say that this is the
outer core down here.

Let's just say that
this is the outer core

for the sake of
explaining things.

It's possible that just based
on the fluid dynamics of what's

happening at that mantle
outer core boundary,

that plumes of really hot
material can kind of rise up.

Let me do this in
a darker color.

They could rise up
from the outer core,

and then create hot spots
underneath the moving

lithospheric plates.

Now, we don't know for sure
whether the hot spots are being

created by these mantle plumes,
these material formed or heated

up at the outer core
mantle boundary.

But what we do feel
pretty confident about

is that there is
this hot spot here,

and it's independent of any
of those convection patterns

that we saw.

I shouldn't say independent.

It's obviously all
related because we

have all this fluidic motion
going on in the mantle,

but it's separate on some degree
from all of those convection

patterns that we talked
about that would actually

cause the plates to move.

And to a large
degree, or the way

we think about it right
now, this is stationary,

this hot spot is stationary
relative to the plates.

And the reason why we feel
pretty good about thinking

that it's stationary
relative to the plates

is we see this
notion right here,

if you look at the volcanic
rock in Kauai, which

is one of the older inhabited
Hawaiian Islands, the oldest

rocks that we've observed
there is 5.5 million years old,

and it's all volcanic rock.

Now, the oldest
volcanic rock that we've

observed on the big island
is about 700,000 years old.

We also know that
the Pacific Plate,

you could look at this
diagram right over here,

is moving in this
general direction.

We know it from
GPS measurements.

It's moving exactly
in the direction

that the Hawaiian Islands
are kind of a distributed in.

So frankly, the only
good explanation

for why we see this pattern,
why we see newer land here,

and then as we go further and
further up the Hawaiian Island

chain we see older
and older land,

and actually if
we keep going, we

have the Leeward
Islands over here.

And as we keep measuring the
rock on the Leeward Islands

they get older and older
as you go to the Northwest.

And then if you even look
at what's below the ocean,

this is the big
island of Hawaii,

these are the main
Hawaiian Islands,

these are the Leeward Islands.

But you see even beyond that
submersed under the Pacific

Ocean you continue to
see a chain of islands.

So the explanation for
what's happening here

is that you have a
stationary hot spot that

is right now underneath
the big island of Hawaii.

And I just want to be
clear, the big island

is called the island of Hawaii.

It is one of the islands
in the state of Hawaii.

So I don't want to
cause you confusion.

I'll just call it the big
island from here on out.

So the hot spot is right
under the big island.

But if you were to rewind
5 million years ago,

the entire Pacific
Plate was probably

on the order of about
150 to 200 miles,

however far Kauai is
from the big island,

it was probably shifted
that much to the southeast

if you go back 5
million years ago.

So 5 million years
ago, when all of this

was shifted down and
to the right, then

Kauai was on top
of the hot spot.

And so this is how each of
these islands are formed.

If you rewind a ton of years
then maybe this area over here

on the Pacific Plate
was over the hot spot.

An island formed there.

Then the Pacific Plate kept
moving to the Northwest.

It kept moving to the Northwest,
and new islands, new volcanoes

kept forming.

Those volcanoes would release
lava that would keep piling up,

keep piling up, keep
piling up, eventually go

above the surface of
the water and form

this whole chain of islands.

And as the whole Pacific Plate
kept moving to the Northwest,

it kept forming new islands.

Now, the one question
you might ask

is, well, how come the
big island is bigger?

Has a plate kind of
paused over there?

Is it spending more
time over the hot spot

so that more lava can kind
of form there to form this?

Essentially, it's an
underwater mountain

that's now also above the water.

And actually if you go from
the base of the Pacific Ocean

to the top of the
big island of Hawaii,

it's actually 50% higher
than Mount Everest.

So you could really just
view it as a big mountain.

But the question is this looks
so much bigger than Kauai,

and they keep getting
smaller as you

keep going to the Northwest.

Is it somehow the
Pacific Plate slowing?

Is it spending more time here?

And the answer is it's
probably not slowing.

What's happening is at one time
Kauai was also probably also

a relatively large island.

If you rewind maybe
5 million years ago

Kauai also might have
been about that big.

But over 5 million
years it's just

experienced a ton of erosion.

Remember, once it moved over
the hot spot and new land

wasn't being created it's in
the middle of the Pacific Ocean.

It's experiencing weather.

5 million years is a
long period of time.

And so it just got
eroded over that time.

So the older the island is, the
more eroded it's going to be,

and the smaller
it's going to be.

So if you go to these
underwater mountains

up here that don't even surface
above the ocean, at one time

they might have surfaced, but
due to the ocean and weather

and whatnot they've just
been eroded over time

to become smaller and smaller
kind of remnants of volcanoes.

So anyway, I thought
you would find

that entertaining of how the
Hawaiian Islands actually got

formed, and how we
can actually have

these hot spots, and
this volcanic activity,

and actually even
earthquake activity

outside of actual
plate boundaries.

Actually, while we're
looking at this diagram,

we talked about the trenches
at plate boundaries.

You can actually see it here
because this shows the depth.

And the really dark,
dark, dark, dark blue

is really deep
parts of the ocean.

So this right here is
the Mariana Trench.

And you can see over here
the Pacific Plate just

getting abducted.

Or not abducted, getting
subducted into other plates

underneath and forms
these trenches here.

Anyway, hopefully you
found that entertaining.