>>Today we will discuss how the continents
and the oceans are different. Let's start
with the oceans. The oceans have thin crust,
and they're composed principally of basalt
and gabbro. And there's a large mountain range
in the center of many of the oceans like the
Mid-Atlantic Ridge. Why is there a mountain
range there? Well, that's a divergent plate
boundary where hot magma is coming up and
is adding crust. Heat rises, it causes the
mountain range to be high.
We also have ocean trenches. These are the
deepest locations in the ocean, and they occur
at subduction zones. Remember, where one plate
goes underneath another plate. Now these trenches
are deep because of the subduction zone. But
that's also a big hole, and it tends to fill
with sediment. And one of the most important
types of sediment is called a turbidity current.
It's a density current composed of sediment
and water that moves downslope. And there's
actually a very interesting story about how
we recognized the importance of turbidity
currents. And it came from study of the 1929
Grand Banks earthquake off the northeastern
part of the United States.
In the 1950s several geologists studied the
records from the 1929 Grand Banks Earthquake.
In 1929 if you wanted to call someone in England,
you did it over a cable that stretched across
the Atlantic Ocean, and there were phone operators
always on these cables constantly connecting
people back and forth. Well, after the 1929
earthquake, a lot of those cables broke and
we lost the connection. And the operators
always noted the time. Well, the geologists
in the 1950s looked at those records, and
what they noted was that the cables that were
in shallow water broke first and the ones
that were in deeper water broke later. So
they hypothesized that an underwater current,
a density current, had moved downslope and
broke the cables sequentially. They then went
out and tested that idea by collecting cores
in the area in deeper water from where the
cables broke. And what they found was a grated
bed of sand. It went from coarse sand, to
fine sand, to mud. And they interpreted that
as a density flow deposit, which they called
a turbidity current, which is one way that
sand gets out into deeper water.
Well, let's talk about another type of deposit.
Let's talk about atolls. Atolls are circular
islands that are found in nice warm tropical
climates like in the Southwest Pacific. They
form above an extinct volcano. As the volcano
cools down, it sinks. But the corals and reefs
want to stay in shallow water, so they grow
up. So what happens is you produce a circular
ring of islands, but there's an old volcano
deep underneath it.
Now let's talk about the continents. The continents
have thicker crust than the oceans, and the
average composition is granite. There's one
interesting thing about continents. If you
look at the edge of continents, some of the
continents have very steep and narrow continental
shelves, like out in California. These are
called active margin continental shelves.
They're above subduction zones or along transform
fault margins. On the other hand, some shelves
are very broad and gentle, like on the east
coast of the United Sates. These are called
passive continental margins. There's not a
subduction zone there. Continents contain
the oldest rocks on the earth. The oceans
are much younger. Why? Well, it's all about
subduction. All of the old oceanic crust has
been subducted, and it's gone, so we don't
have any old oceanic crust.
