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 (six-foot 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.
 
 
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 called 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.

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.
 
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://ga.water.usgs.gov/edu/watercycle.html

Experiments for Home and Classroom

With depth, of course, comes pressure. In this hands-on experiment, students can make their own ""Deep Sea Diver" from a balloon or medicine dropper and study what happens as pressure increases and decreases. Click: http://www.amnh.org/education/resources/rfl/web/oceanguide/activities/diver.html

In this exercise, students learn from a science teacher's adventures when she joins a research expedition to study deep sea vents in the Pacific Ocean. Teachers are encouraged to go to the link shown below, read the instructions and the teacher's "Dispatch from the Deep" and guide students though a fascinating experience. Click:
http://www.amnh.org/education/resources/rfl.php?set=b&topic_id=4&subtopic_id=60 

This activity, designed for grades 5 through 10, demonstrates how temperature and pressure affect deep ocean circulation. This activity is also appropriate for the next Lesson -- Lesson 4, Energy in the Earth System. Click: http://www.carolina.com/calendar_activities/2002/0205.asp

In this experiment, students create their own "plumes" -- and see how hot water behaves when it comes in contact with cold water (which is how ocean currents are formed). This experiment requires 4 participants. Click: http://www.amnh.org/education/resources/rfl/pdf/dsv_a04_plume.pdf

In this experiment (also about currents), students are invited to model deep-ocean currents to see how temperature and convection influence the movement of water. Click: http://www.carolina.com/calendar_activities/2002/0205.asp

Understanding the Water Cycle is necessary if a student is to understand the role oceans play in our weather. Evaporation is an important part of the Water Cycle, and in this experiment (designed for younger children with adult supervision) students discover whether evaporation is a heating or a cooling process. Click:
http://www.carolina.com/earth/evaporation.asp

Now let's do Practice Exercise 3-2 (top). Choose printer friendly or online exercises. Printer friendly version requires the Adobe Acrobat Reader 5. Click HERE to obtain a free copy.