Earth Science Lesson 3
Dynamic Earth Processes (Grades 8-12 )
Instruction 3-2
Features of the Ocean Floor
www.etap.org
What do you think the ocean floor is like? Smooth and sandy like the beach?
It's not.
Beneath the oceans lie rugged mountains, vast plateaus, active volcanoes and
almost bottomless trenches.
Here is a diagram.
Nobody has actually seen most of this, but oceanographers know these features
exist because they have spent years mapping the ocean bottom.
In the early days, sailors measured the depth of the ocean with a long piece of
rope with a lead weight at the end of it. This rope was marked off in fathoms (sixfoot sections) and dropped into the ocean.
Today's oceanographers use sonar instruments. These instruments generate a
sound signal that is bounced off the ocean floor. How long it takes for the sound
to travel to the bottom and back tells how deep the ocean is.
Manned and unmanned submersibles (small submarines) can also now take
photographs at depths no human diver could survive.
You can see incredible TV programs about underwater exploration on PBS, the
Discovery Channel and the National Geographic Channel.
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California Content Standards Earth Science 3.a.
B. J. Subbiondo © 2004
Earth Science Lesson 3
Dynamic Earth Processes (Grades 8-12 )
Instruction 3-2
Features of the Ocean Floor
www.etap.org
What's Down There
As you can see by the preceding diagram, most of the continents are surrounded
by shallow seas. These shallow seas cover gently sloping areas called
continental shelves -- which slope down to about 650 feet (200 m).
Where these continental shelves end, continental slopes begin -- which slant
more steeply down to the deeper parts of the ocean.
Beyond the continental slope is the abyss. Within the abyss there are plains,
long mountain ranges called ocean ridges, isolated mountains call seamounts
and ocean trenches.
Ocean trenches are the deepest parts of the oceans. Some have rift valleys in
the middle of them, where earthquakes and volcanic eruptions take place. The
tops of some volcanoes appear above the surface of the ocean as islands.
Other mountain ranges are made up of extinct volcanoes. Some of these, called
guyots, have tops that are very flat. It is thought that these guyots were once
mountains whose tops were worn away by waves.
Now might be a good time to study the diagram again.
The Changing Sea
The face of the Earth is always changing -- and this includes the oceans.
New oceans are constantly being created and destroyed, although this happens
very slowly.
Today, scientists think that the ocean bottom is moving from 1/2 to six inches a
year through a process called plate tectonics -- which we'll tell you more about in
our next Instruction.
200 million years ago, all the continents on Earth were grouped together in one
giant supercontinent called Pangaea.
The rest of the Earth was covered by one single ocean called Panthalassa.
Slowly Pangaea broke apart and the pieces began to move all over Earth's
surface. They were driven by convection currents in the molten rocks beneath
Earth's outer layers, which we told you about in our last Instruction.
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California Content Standards Earth Science 3.a.
B. J. Subbiondo © 2004
Earth Science Lesson 3
Dynamic Earth Processes (Grades 8-12 )
Instruction 3-2
Features of the Ocean Floor
www.etap.org
This is called continental drift. For a diagram of how it happened, click:
http://www.mos.org/oceans/planet/change.html
By 35 million years ago, the land and sea looked pretty much like they do today.
But, as we said, change is still going on.
The Atlantic and Indian Oceans are getting a few inches wider every year.
And the Red Sea is growing, too -- as it has been doing for 25 million years. In
200 million years, it will be as wide as the Atlantic is today.
But where does all the water for these oceans come from?
Where the Water Comes From
Originally, there were no oceans on Earth. Its surface was so hot that the
water simply boiled away.
But as volcanoes poured steam into the atmosphere and the Earth cooled down,
the steam began to turn to water vapor.
This water vapor condensed and began to fall as rain in a downpour that lasted
for thousands of years.
This downpour filled hollows in the land and formed oceans.
The Water Cycle
Today, oceans gain and lose water in a never-ending process called the
Water Cycle.
The sun and wind lift tiny particles of moisture from the ocean's surface.
This water vapor is mixed with air.
As the air cools , clouds form.
Eventually they shed their moisture as rain or snow.
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California Content Standards Earth Science 3.a.
B. J. Subbiondo © 2004
Earth Science Lesson 3
Dynamic Earth Processes (Grades 8-12 )
Instruction 3-2
Features of the Ocean Floor
www.etap.org
Most of this precipitation falls directly back into the sea. And the rain and snow
that fall on land runs into rivers that flow back to the sea. So the oceans never
dry up.
For a diagram of The Water Cycle and instructions on how to build a model of it,
click http://www.most.org/oceans/planet/cycle.html
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California Content Standards Earth Science 3.a.
B. J. Subbiondo © 2004