Lesson Objectives

• Describe the main features of the seafloor.
• Explain what seafloor magnetism tells scientists about the seafloor.
• Describe the process of seafloor spreading.

Vocabulary

• abyssal plains
• echo sounder
• seafloor spreading
• trench

Introduction

World War II gave scientists the tools to find the mechanism for continental drift that had eluded Wegener. Maps and other data gathered during the war allowed scientists to develop the seafloor spreading hypothesis. This hypothesis traces oceanic crust from its origin at a mid-ocean ridge to its destruction at a deep sea trench and is the mechanism for continental drift.

Seafloor Bathymetry

During World War II, battleships and submarines carried echo sounders to locate enemy submarines (Figure below). Echo sounders produce sound waves that travel outward in all directions, bounce off the nearest object, and then return to the ship. By knowing the speed of sound in seawater, scientists calculate the distance to the object based on the time it takes for the wave to make a round-trip. During the war, most of the sound waves ricocheted off the ocean bottom.

This animation shows how sound waves are used to create pictures of the sea floor and ocean crust: http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_sonar.html.

After the war, scientists pieced together the ocean depths to produce bathymetric maps, which reveal the features of the ocean floor as if the water were taken away. Even scientist were amazed that the seafloor was not completely flat (Figure below).

The major features of the ocean basins and their colors on the map in Figure above include:

• mid-ocean ridges: rise up high above the deep seafloor as a long chain of mountains; e.g. the light blue gash in middle of Atlantic Ocean.
• deep sea trenches: found at the edges of continents or in the sea near chains of active volcanoes; e.g. the very deepest blue, off of western South America.
• abyssal plains: flat areas, although many are dotted with volcanic mountains; e.g. consistent blue off of southeastern South America.

When they first observed these bathymetric maps, scientists wondered what had formed these features.

Seafloor Magnetism

Sometimes — no one really knows why — the magnetic poles switch positions. North becomes south and south becomes north.

• Normal polarity: north and south poles are aligned as they are now.
• Reversed polarity: north and south poles are in the opposite position.

During WWII, magnetometers attached to ships to search for submarines located an astonishing feature: the normal and reversed magnetic polarity of seafloor basalts creates a pattern.

• Stripes of normal polarity and reversed polarity alternate across the ocean bottom.
• Stripes form mirror images on either side of the mid-ocean ridges (Figure below).
• Stripes end abruptly at the edges of continents, sometimes at a deep sea trench (Figure below).

The characteristics of the rocks and sediments change with distance from the ridge axis as seen in the Table below.

A map of sediment thickness is found here: http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_sedimentthickness.html.

The oldest seafloor is near the edges of continents or deep sea trenches and is less than 180 million years old (Figure above). Since the oldest ocean crust is so much younger than the oldest continental crust, scientists realized that seafloor was being destroyed in a relatively short time.

This 65 minute video explains “The Role of Paleomagnetism in the Evolution of Plate Tectonic Theory”: http://online.wr.usgs.gov/calendar/2004/jul04.html.

The Seafloor Spreading Hypothesis

Scientists brought these observations together in the early 1960s to create the seafloor spreading hypothesis. In this hypothesis, hot buoyant mantle rises up a mid-ocean ridge, causing the ridge to rise upward (Figure below).

The hot magma at the ridge erupts as lava that forms new seafloor. When the lava cools, the magnetite crystals take on the current magnetic polarity. As more lava erupts, it pushes the seafloor horizontally away from ridge axis.

These animations show the creation of magnetic stripes of normal and reversed polarity at a mid-ocean ridge: http://www.nature.nps.gov/GEOLOGY/usgsnps/animate/A49.gif; http://www.nature.nps.gov/GEOLOGY/usgsnps/animate/A55.gif.

The magnetic stripes continue across the seafloor.

• As oceanic crust forms and spreads, moving away from the ridge crest, it pushes the continent away from the ridge axis.
• If the oceanic crust reaches a deep sea trench, it sinks into the trench and is lost into the mantle.
• The oldest crust is coldest and lies deepest in the ocean because it is less buoyant than the hot new crust.

Seafloor spreading is the mechanism for Wegener’s drifting continents. Convection currents within the mantle take the continents on a conveyor-belt ride of oceanic crust that over millions of years takes them around the planet’s surface.

The breakup of Pangaea by seafloor spreading is seen in this animation: http://www.scotese.com/sfsanim.htm.

The breakup of Pangaea with a focus on the North Atlantic: http://www.scotese.com/natlanim.htm.

An animation of the breakup of Pangaea focused on the Pacific Ocean: http://www.scotese.com/pacifanim.htm.

Seafloor spreading is the topic of this Discovery Education video: http://video.yahoo.com/watch/1595570/5390151.

The history of the seafloor spreading hypothesis and the evidence that was collected to develop it are the subject of this video (3a): http://www.youtube.com/watch?v=6CsTTmvX6mc (8:05).

Lesson Summary

• Using technologies developed to fight World War II, scientists were able to gather data that allowed them to recognize seafloor spreading as the mechanism for Wegener’s drifting continents.
• Bathymetric maps revealed high mountain ranges and deep trenches in the seafloor.
• Magnetic polarity stripes give clues to seafloor ages and the importance of mid-ocean ridges in the creation of oceanic crust.
• Seafloor spreading processes create new oceanic crust at mid-ocean ridges and destroy older crust at deep sea trenches.

Review Questions

1. Describe how sound waves are used to develop a map of the features of the seafloor.
2. Why is the oldest seafloor less than 180 million years when the oldest continental crust is about 4 billion years old?
3. Describe the major features and the relative ages of mid-ocean ridges, deep sea trenches, and abyssal plains.
4. Describe how continents move across the ocean basins as if they are on a conveyor belt.
5. If you were a paleontologist who studies fossils of very ancient life forms, where would be the best place to look for very old fossils: on land or in the oceans?
6. Imagine that Earth’s magnetic field was fixed in place and the polarity didn’t reverse. What effect would this have on our observations of seafloor basalts?
7. Look at a map of the Atlantic seafloor with magnetic polarity stripes and recreate the history of the Atlantic Ocean basin.

Further Reading / Supplemental Links

• A basic description of sea floor spreading with animations: http://www.pbs.org/wnet/savageearth/hellscrust/index.html.

Points to Consider

• How were the technologies that were developed to fight World War II used by scientists for the development of the seafloor spreading hypothesis?
• In what two ways did magnetic data lead scientists to understand more about continental drift and plate tectonics?
• How does seafloor spreading provide a mechanism for continental drift?
• Look at the features of the North Pacific Ocean basin and explain them in seafloor spreading terms.
• What would have to happen if oceanic crust was not destroyed at oceanic trenches, but new crust was still created at mid-ocean ridges?