>>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.