Hello marine biology students.
In this video we're going to be learning about
ocean basins, the ocean floor, and how plate
tectonics relates to ocean formation.
[Intro Music]
So let's talk about the oceans on the surface
of the planet Earth.
It turns out that these oceans are actually
all connected to each other and we call this
collected feature the world ocean.
The world ocean is a dominant feature on the
surface of planet Earth, in terms of total
area.
In fact sixty-one percent of the total area
of the Northern Hemisphere
is covered in ocean and about 80 percent of
the southern hemisphere is also covered in
ocean.
The world ocean is divided up into four large
basins and those are: the Pacific, the Atlantic,
the Indian, and the Arctic oceans.
Now, you may be saying “Well what's all
this stuff about the seven seas?
And sailing the seven seas?
Where does that come from?”
Well, our modern understanding of what the
seven seas are include the Arctic Ocean, the
North and South Atlantic, the North and South
Pacific, the Indian Ocean and the Southern
Ocean.
But, it turns out that this phrase “the
seven seas” has been around in many different
cultures and from many different times, and
in each of those cultures, the seven seas
refer to different bodies of water.
So there are different bodies of water for
different cultures at different times.
It's a bit surprising that this phrase has
been around for so long and in so many places.
So here we see a map of the Earth, one way
of representing the Earth, and we can see
the major ocean basins.
We can see that the Pacific Ocean is the largest
of the current ocean basins, but it is also
an ocean basin that is shrinking, whereas
the Atlantic Ocean basin is growing.
Now, the Southern Ocean may not … may not
be one you've really considered or thought
about much, but if we look at our planet from
the South Pole, we can see that surrounding
Antarctica, there's an entirely open expanse
of ocean that actually connects the Indian
Ocean, the Atlantic Ocean, and the Pacific
Ocean altogether.
Now, looking at the North Pole, we can see
that the Arctic Ocean actually covers over
the North Pole itself and has connections
both to the Pacific and the Atlantic oceans.
Now, another thing you may not be aware of
is the general shape of Antarctica and I remember
hearing the shape of Antarctica described
to me as being a duck-shape and since I've
heard that, now it I can't unsee it.
So there you go, Antarctica is in the shape
of a duck, and the Southern Ocean entirely
surrounds it.
Now, I'd like to talk a bit about plate tectonics.
So, when we look at the Earth itself, the
Earth is thought to have originated 4.5 billion
years ago from dust accumulated from the Big
Bang.
Now due to the heat associated with these
events, the early Earth was likely molten,
meaning that the major components at that
point were liquid.
This allowed the materials of the earth to
settle by density
as the materials cooled, with the more dense
material settling down into the core.
So, the heavier materials 
settle deep into the Earth, whereas the lighter
components ended up forming a thin crust on
the top, and as that crust cooled, it became
solid.
Eventually, the Earth's oceans and atmosphere
began to form.
Now, the location of the Earth relative to
the Sun allows for water to stay liquid, an
essential substance for sustaining life as
we know it.
When we look at the layers of the Earth we
have the core, which is the innermost layer.
It's an inner solid core and the liquid outer
core.
It's iron-rich, iron and nickel.
It's a very dense material.
And then, we have the mantle, which are the
middle layers and they're semi plastic compositions.
What that means is that they're movable.
They're flexible.
They aren't fully liquid and normally what
we think of as lava, that's not exactly the
same material that the mantle is made out
of, but the mantle is thick and somewhat movable.
That's what we mean by semi-plastic.
And then lastly, we have the crust which are
the outermost layers, they are the thinnest
portion of the Earth.
So here, on this next slide, we can see a
diagram of the Earth and its layers.
With the core including both an inner core
and an outer core, a lower mantle an upper
mantle, on which these lithospheric plates
are found.
And then, we have the continental crust and
the oceanic crust.
These crust or plates are what make up the
surface of the Earth.
So, the crust of the Earth is actually different
below the oceans as compared to on the continents.
The oceanic crust 
is made up primarily of a darker colored mineral
known as basalt.
It is denser than the continental crust, thinner
than the continental crust, and also younger
than the continental crust.
With most parts of the oceanic crust being
less than 200 million years old.
Conversely, the continental crust
is light-colored, it's mostly composed of
granite, as I'd mentioned it's less dense,
and actually thicker than the oceanic crust.
Some continental crust is believed to be as
old as 3.8 billion years old and this is where
our oldest fossils and oldest evidence of
life are found.
In this table, we can see some of the characteristics
comparing oceanic crust and continental crust.
With the oceanic crust being denser, thinner,
younger, darker, and possessing different
elemental composition than the continental
crust.
Now, in 1912, the theory of continental drift
was proposed by a German geologist named Alfred
Wegner.
Wegner suggested that all continents have
been joined in a single supercontinent which
he named Pangaea.
He proposed that Pangaea began breaking up
180 million years ago.
Now, when he proposed this idea, it was not
widely accepted at the time and a large part
of that is due to a lack of mechanism.
He couldn't explain why it was that continents
were moving especially, when to everyone's
observation they seemed to be so solid and
stationary and not moving in reference to
each other.
Well, plate tectonics explains the “How”
behind Wegner’s continental drift theory.
Some of the major features of plate tectonics
is that the Earth's surface is covered by
a series of lithospheric plates.
This is the crust that we were just mentioning.
Also, that ocean floors are constantly moving,
spreading in the center and sinking at the
edges and being regenerated.
Within the mantle, there are convection currents
beneath the plates, and this is what is causing
these lithospheric plates to move on the surface
of the mantle.
It's the heat from the mantle that drives
these currents.
When we look at the map of the Earth, we can
see some of these major lithospheric plates.
At the boundaries between plates, there can
be subduction, there can be moving together.
These are going to be the sites of volcanism,
of earthquakes.
Also at these plate boundaries, the plates
may be moving apart from each other, and this
is the case along most of the oceans.
When we look at the seafloor, there are mid-oceanic
ridges in most of the major oceans.
And a mid-oceanic ridge, a chain of submarine
volcanic mountains that rise from the ocean
floor.
Now, while these chains of submarine mountains
are mostly continuous, at somewhat regular
intervals, the ridge is displaced 
by faults in the Earth's crust called transform
faults.
So instead of the plates moving directly away
from each other, in this case the plates slide
past each other.
And so, what was a continuous mountain range
can end up being broken up.
Now, in the oceans, on the seafloor, sediment
accumulates over time.
What we see is that the sediment is thicker
away from these mid-ocean ridges and thinner,
in fact of the point of being non-existent,
at the mid-ocean ridge itself.
This means that the crust further away from
the ridge is older.
Here we see a map of the age of oceanic crust
and we can see these brighter colors, the
reds and oranges and yellows, that denotes
a younger age for those pieces of the crust.
And so, we can see, along the mid-ocean ridge
we have younger pieces of oceanic crust and
the farther you get from these mid-ocean ridges
the older the oceanic crust gets.
Another thing that we can use to date oceanic
crust are geomagnetic anomalies.
At random intervals in the Earth's past, the
Earth's magnetic field reverses.
It switches polarity.
What we would call magnetic North ends up
becoming the South and the South becomes the
North.
Now, this hasn't happened during recorded
human observation, while we've known about
the magnetic field, but we can see clear evidence
of this in the geologic past and one of the
places we can see that evidence is in the
oceanic crust forming by these mid-ocean ridges.
New rock forms from magma and it records the
orientation
of the Earth's magnetic field at the time
that the rock cools and forms.
Studies of the seafloor have revealed stripes
of alternating magnetization parallel to the
mid-oceanic ridges.
We can see in this figure what we would consider
our normal magnetic orientation versus reversed
magnetic orientation and as the magnetic orientation
of the world flips, the polarization or the
direction of the magnetic particles within
the oceanic crust also flips.
One of the last lines of evidence that supports
plate tectonics are island arcs.
These are chains of islands that are found
throughout the oceans, especially in the western
Pacific.
These island arcs are usually situated along
the continental side
of deep sea trenches.
These deep sea trenches form when oceanic
plate moves underneath neighboring plate,
whether it's another oceanic plate or a continental
plate.
As these plates move under the neighboring
plate, this subduction causes the trench,
but it also generates earthquakes, it generates
magma, it generates volcanic activity that
can result in these island arcs.
So these observations, along with many other
studies of our planet support the theory that
underneath the Earth's crust is a layer of
heated rock driving the creation of new ocean
floor.
So here we can see an oceanic plate subducting
beneath a continental plate and this is along
the border of South America.
We see the Andes are formed, the volcanic
activity of that mountain range is because
the Nazca plate, which is an oceanic plate,
is subducting below the continental plate.
We can also see this in the Aleutian Islands,
which are off the end of Alaska.
This island chain is caused by the Pacific
plate moving beneath the North American plate.
And again, earthquakes and magma formation,
and volcanic activity is caused by the subduction
zone and it's right on the continental side
of an oceanic trench.
So when we look at a map of the Earth's oceans
and some of these major features such as trenches
and mid-ocean ridges, we can see that this
theory of seafloor spreading at the mid-ocean
ridges is accounted for and the movement of
these major lithographic plates also causes
the movement of our continents.
So this is the last video for this week.
Thanks for your attention and I'll see you
in the next video.
