When we discussed
the structure of the earth
there was a diagram showing the continents floating on top of
the rest of the planet like foam on water. This page will describe
some of the features of the ocean floor.
lie on the continental plates. The edges of these plates
are under water, and they form the continental shelves.
These shelves slope outward very gently towards the ocean depths.
In some places these shelves go out a long way, up to 900 miles:
In other places the shelves are much narrower. These shelves
are fairly smooth because debris from the land is falling onto
them from the water. This debris contains nutrients washed down
from the land, and these nutrients, plus the shallow water, contribute
to the abundance of life forms near the continents.
This is a very productive area of
Where the continental
shelves end, there is a steep drop downwards. This is called
the continental slope. It plunges down into the dark,
cold waters of the ocean. At the bottom of the continental slope
there is a an area that slopes gradually downward to blend into
the abyssal plain. This is the bottom of the ocean as
we think of it.
The ocean floor
seems like a different world. There is no sun light down here.
The water is very cold. The pressure of the weight of miles of
water above this area would easily crush a submarine. Tiny flecks
of material from the sunlit world float downward and eventually
settle on the bottom. There is almost nothing to eat, and the
animals that live here are few and far between. There are many
different kinds of animals, but they are small and widely scattered.
They are able to survive on very little.
is not all silence and darkness here. The earth's crust is thinnest
in the ocean basins, and volcanic eruptions are part of this
environment. Here and there isolated peaks called sea mounts
rise above the flat abyssal plain. However, the most active part
of deep ocean geology occurs in the deep sea trenches, where
the ocean floor is spreading apart. Lava comes up through the
fissures, building up mountains and deep sea vents, called
The constantly up-welling lava pushes the sea floor away from
the trenches and out towards the
continents, pushing the continents
slowly along and causing the
movement called continental
drift. . When the sea floor
reaches the continental plates, it subducts, or slides underneath
them, carrying the cold rocky floor back into the hot mantle
of the earth.