This webpage introduces middle school students to the ocean floor by focusing on  three areas:  (1) the factors that make ocean-floor research difficult, (2) the features of the ocean floor and (3) the processes that have shaped the ocean floor.

INTRODUCTION
What are the highest and lowest places on Earth?  If you named Mt. Everest and the Grand Canyon, you are incorrect! The tallest mountains and deepest canyons are actually found in the ocean.  If placed side by side, the Hawaiian Islands of the Pacific Ocean would tower over Mt. Everest.  The Pacific is also home to the deepest trench on the Earth, the Mariana Trench. It measures eleven kilometers below the sea's surface.  This is seven times the depth of the Grand Canyon. How do we know so much about the ocean floor?  What other features are on the ocean floor?  What processes have shaped the ocean floor?  Read on to find out!

EXPLORING THE OCEAN
People have explored the oceans for thousands of years.  Until recently, however, the ocean floor had not been studied and much was unknown about the deep oceans.  Why is this?  Studying the ocean floor is difficult; conditions are very harsh in the deep ocean.   First, it is dark since sunlight does not penetrate far below the surface.  Second,  the water is cold, only a few degrees above freezing.  Finally, there is tremendous pressure due to the mass of the water pushing down from above.  Because of the darkness, cold, and extreme pressure, scientists have had to develop technology to enable them to study the deep ocean floor.  That technology includes sonar, scuba, submersibles, satellites, remote underwater manipulators, and gravity mapping.  The development of sonar was a major break through for mapping the ocean floor. "Sonar, which stands for sound navigation and ranging, is a system that uses sound waves to calculate the distance to an object. The sonar equipment on a ship sends out pulses of sound that bounce off the ocean floor.  The equipment then measures how quickly the sound waves return to the ship.  Sound waves return quickly if the ocean floor is close.  Sound waves take longer to return if the ocean floor is farther away" (Simons & Wellnitz, 2000, p. 149).  Using sonar, scientists were able to develop detailed maps of the ocean floor.

FEATURES OF THE OCEAN FLOOR AND THE PROCESSES THAT HAVE SHAPED IT
Once scientists were able to map the ocean floor, they discovered something surprising. The bottom of the ocean was not a flat, sandy plain stretching between the continents, as many people once thought.  In fact, the ocean floor was rocky and dramatically uneven.  Many previously unknown features of the seafloor were discovered.

Extending out from a continent's edge is a gently sloping, shallow area called the continental shelf (F).  At the edge of the shelf, the ocean floor drops off in a steep incline called the continental slope (A).  The continental slope marks the true edge of the continent, where the rock that makes up the continent stops and the rock of the ocean floor begins.  Beyond this slope is the abyssal plain (C), a smooth and nearly flat area of the ocean floor.  In some places, deep, steep-sided canyons called trenches (G) cut into the abyssal plain.  A continuous range of mountains called the mid-ocean ridge (D) winds around Earth.  There are mountains on the abyssal plain, too.  Some reach above the ocean surface to form volcanic islands (E).  Others, called seamounts (B), are completely under water.

To understand how ocean-floor features are formed, it is important to understand the Earth's structure.  The Earth consists of layers around its center, or core.  The outer layer, or crust, is thin and rocky.  The thick layer between the crust and the core is the mantle.  It contains hot, liquid rock called magma.  Magma flows out of the mantle and onto the surface through cracks in the crust.  Magma on the surface of the Earth is called lava, which hardens to form new crust.

Earth's crust is broken into large plates that float on the mantle.  As these plates slowly move, they create different landforms.  The mountain ranges of the mid-ocean ridge, trenches, and underwater volcanoes are all formed by interactions of Earth's plates.  At the mid-ocean ridge, plates are diverging or moving apart.  Magma squeezes up though cracks between the plates and hardens to form new rock.  Newer eruptions push the older rock away from the ridge in a process called sea-floor spreading.  Over millions of years, sea-floor spreading created the ocean floor.

Even though new ocean floor is constantly being created at the mid-ocean ridge, Earth always stays the same size.  That is because while divergence is occurring at the mid-ocean ridge, convergence, where two plates come together, is occurring at a different location on the plate.  When two plates converge, one plate sinks under the other plate and old rock sinks into a trench and back into the Earth's mantle.

Lesson Plans
 

Related Links
Ocean Planet - The Smithsonian Institution created the exhibition Ocean Planet to share with the public what recent research has revealed about the oceans and to encourage ocean conservation. This online booklet of lessons and activities adapts several themes of the exhibition for use in the middle and high school classroom.

JASON Project - The JASON Foundation for Education, sponsors an annual scientific expedition with curriculum developed for grades 4 through 8. During the expedition, students "take part in live, interactive programs which are broadcast using state-of-the-art technology to a network of educational, research, and cultural institutions in the United States, Mexico, Bermuda, and the United Kingdom.

OAR On-Line - The goal of this site is "to provide middle school science students and teachers with research and investigation experiences using on-line resources." Topics include El Nino, storms (tornadoes, lightning, hurricanes, and forecasting), atmosphere, fisheries, Great Lakes, and oceans, and each topic walks students through an introduction, information gathering, data gathering and analysis, application, and enrichment. From the the Office of Oceanic and Atmospheric Research (OAR) and the College of Education at the University of South Alabama.

NOAA Home Page - The National Oceanic and Atmospheric Association's home page.  A wealth of information can be found here.

NOAA Ocean Explorer - Explore the ocean realm with NOAA without getting wet!

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References
Ford, Brent A., Project Earth Science:  Geology. Arlington:  National Science Teachers Association, 1996.
Simons, Barbara Brooks, and Thomas R. Wellnitz.  Science Explorer:  Earth's Waters. Upper Saddle River: Prentice Hall, 2000.