Before You Read   You can also listen to this assignment!

Imagine you are in a deep-sea submersible skimming along

the ocean floor. What do you think you might see? How

deep do you think the water might be?


Discuss this with your partners!!



The Ocean Basins

While on your imaginary trip along the ocean floor, the

lights of your vessel may have shown a mountain range. You

may have seen a huge opening in the seafloor that is so

deep you canŐt see the bottom.


What is the continental shelf?

Ocean basins are low areas of Earth that are filled with

water. They have many different features. Beginning at the

ocean shoreline is the continental shelf. The continental

shelf is the gradually sloping end of a continent that extends

under the ocean.


On some coasts, the continental shelf extends a long

distance. This is true for North AmericaŐs Atlantic and Gulf

coasts. The continental shelves on these coasts stretch 100

km to 350 km into the sea. This is equal to 60 to about 200 miles.

Here in Georgia we have a very wide continental shelf which is one reason we have such good shrimp fishing.


On the Pacific Coast, the coastal range mountains are

close to the shore. The shelf there is only 10 km to 30 km

wide. The ocean covering the continental shelf can be as

deep as 350 m.  Off the shore of California, the ocean drops down very deep rather quickly. 


  What other features are found in the ocean?

The figure below shows that beyond the shelf, the ocean floor drops more steeply, forming the continental slope. The continental slope extends from the outer edge of the continental shelf down to the ocean floor. Beyond the continental slope lie the trenches, valleys, plains, mountains, and ridges of the ocean basin.

Ocean Floor Diagram 1


How do abyssal plains form?

In the deep ocean, sediment that comes mostly from land, settles constantly on the ocean floor. This sediment fills in valleys in the oc

ean floor and creates flat seafloor areas called  abyssal plains (uh BIH sul plains)  .  The figure above shows an abyssal plain. Abyssal plains are from 4,000 m to 6,000 m below the surface of the ocean and are the flattest places on our planet.

In the Atlantic Ocean, some abyssal plains are large and extremely flat. The Canary Abyssal Plain has an area of approximately 900,000 km2. Sometimes abyssal plains have small hills and seamounts. Seamounts are underwater, inactive volcano peaks. Seamounts are most commonly found in the Pacific Ocean.  Some seamounts are so tall that they stick up out above the surface of the ocean.  We call these very tall seamounts islands.  The Hawaiian islands are all seamounts formed by volcanoes.  Sometimes, a seamount that used to stick up above the oceanŐs surface gets cut back down to the level of the ocean by waves.  A cut down seamount is called a guyot. 


Ridges and Trenches

A  mid-ocean ridge is a part of an ocean basin area where new ocean floor is formed. Mid-ocean ridges are found at the bottom of all oceans and form a continuous underwater ridge about 70,000 km long. In fact, the mid-ocean ridge forms the worldŐs longest mountain range.  It just happens to be under the ocean. 

Crustal plates, large sections of  EarthŐs crust and upper mantle, are constantly moving. As these plates move, the ocean floor changes. When plates separate, hot magma from inside Earth erupts through small cracks, forming new crust. This process is called seafloor spreading. The hot magma is cooled by ocean water and hardens to solid rock.


In the picture below, we see a mid-ocean ridge.  Letter A shows the bottom of the ridge which is the rift valley.  It is filled with magma.  As the Earth splits open, the split or rift fills with molten rock.  The cold seawater begins cooling the magma and quickly seals the rift.  The cooling magma forms a ridge, sort of like a scab on your hand is raised above the skin around it.  This ridge is what we call the mid-ocean ridge.  Letter B shows the edge of the ridge.  To make things clear, as the seafloor spreads, a rift valley forms and fills with magma.  The cooling magma forms the ridge.  As the seafloor continues to spread, the ridge splits down the middle and fills with more magma.



Ocean Floor Diagram 2



What are subduction zones?

While seafloor is being formed along mid-ocean ridges, it is being destroyed in other parts of the ocean. Areas where old ocean floor slides beneath another plate and moves into EarthŐs mantle are called subduction zones.

On the ocean floor, subduction zones are found within deep ocean trenches, as shown in Ocean Floor Diagram 1 at the top of page. A  trench is a long, narrow, steep-sided depression where one plate sinks beneath another. Most trenches are found in the Pacific Basin. Ocean trenches are usually longer and deeper than any valley on land. The Mariana Trench reaches about 11 km below the ocean surface and is the deepest place in the Pacific Ocean. The Mariana Trench is so deep that Mount Everest, the tallest mountain on Earth, could easily fit into it. The figure below shows what it would look like if Mt. Everest were placed in the Mariana Trench. 


















Resources can be found in many places in the ocean.

Some deposits on the continental shelf are easy to remove. Other resources can be found only in the deep abyssal regions of the ocean floor. People are still trying to figure out how to get these valuable resources to the surface.


What resources are found on the continental shelf?

Many organisms live along continental shelves. When organisms die, they sink to the bottom and decay. Over millions of years, accumulations of organic matter form deposits of useful resources. Oil and natural gas are found under the seafloor. Wells are drilled into the seafloor to remove these resources.


What other resources can be found?

Other deposits on the continental shelf include phosphorite, which is used to make fertilizer, and limestone, which is used to make cement. Sand and gravel, both important in construction, also can be removed from the continental shelf. Rivers that flow into oceans carry important minerals to the continental shelf.  These rivers also dissolve salt from the soil around them and carry it into the ocean.  The oceans get their salinity from these rivers. Sometimes ocean waves and currents cause denser mineral grains to come together in one place. These deposits, called placer (PLAHS ur) deposits, can occur in coastal regions where rivers entering the ocean suddenly lose energy, slow down, and drop their sediment. Metals such as gold and titanium, and gems such as diamonds, are mined from placer deposits.



What are deep-water deposits?

Hot water streams out into surrounding seawater through holes and cracks along mid-ocean ridges. As the extremely hot water cools, minerals deposits sometimes form. Elements such as sulfur, iron, copper, Zinc, and silver can be found in these areas. Today, no one mines these areas because it is too expensive. However, in the future, these deposits could become important.


Other mineral deposits can precipitate, or form solids, from seawater. Minerals dissolved in seawater come out of solution and form solids on the ocean floor.


Manganese   Manganese nodules are small, darkly colored lumps found across large areas of  the ocean basins. Manganese nodules form around a core, such as old sharksŐ teeth or whale bone. These nodules grow very slowly— perhaps as little as 1 mm to 10 mm every million years. Manganese nodules are rich in manganese, copper, iron, nickel, and cobalt. These minerals are used to make steel, paint, and batteries. Most of these modules lay thousands of meters deep in the ocean and are not currently being mined. Suction devices similar to huge vacuum cleaners have been tested to collect them.