High School Earth Science/Seafloor Spreading

Perhaps surprisingly, it was World War II that gave scientists the tools to find the mechanism for continental drift that had eluded Wegener and his colleagues. Scientists used maps and other data gathered during the war 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. Scientists realized that seafloor spreading could be the mechanism for continental drift that they had been looking for.

Lesson Objectives

• List the main features of the seafloor: mid-ocean ridges, deep sea trenches, and abyssal plains.
• Describe what seafloor magnetism tells scientists about the seafloor.
• Describe the process of seafloor spreading.

Seafloor Bathymetry

During the war, battleships and submarines carried echo sounders to locate enemy submarines (Figure 6.8). Echo sounders produce sound waves that travel outward in all directions, bounce off the nearest object, and then return to the ship. The round-trip time of the sound wave is then recorded. By knowing the speed of sound in seawater, scientists can calculate the distance to the object that the sound wave hit. During the war, the sound wave rarely encountered an enemy submarine, and so most of the sound waves ricocheted off the ocean bottom.

Figure 6.8: A ship sends out sound waves to create a picture of the seafloor below it. The echo sounder pictured has many beams and as a result it creates a three dimensional map of the seafloor beneath the ship. Early echo sounders had only a single beam and created a line of depth measurements.

After the war, scientists pieced together the bottom depths to produce a map of the seafloor. This is known as a bathymetric map and is similar to a topographic map of the land surface. While a bathymetric map measures the distance of the seafloor below sea level, a topographic map gives the elevation of the land surface above sea level. Bathymetric maps reveal the features of the ocean floor as if the water were taken away.

The bathymetric maps that were produced at this time were astonishing! Most people had thought that the ocean floor was completely flat but the maps showed something completely different. As we know now, majestic mountain ranges extend in a line through the deep oceans. Amazingly, the mountain ranges are connected as if they were the seams on a baseball. These mountain ranges are named mid-ocean ridges. The mid-ocean ridges and the areas around them rise up high above the deep seafloor (Figure 6.9).

Figure 6.9: A modern map of the eastern Pacific and Atlantic Oceans. Darker blue indicates deeper seas. A mid-ocean ridge can be seen running through the center of the Atlantic Ocean. Deep sea trenches are found along the west coast of Central and South America and in the mid-Atlantic east of the southern tip of South America. Isolated mountains and flat featureless regions can also be spotted.

Another astonishing feature is the deep sea trenches that are found at the edges of continental margins or in the sea near chains of active volcanoes. Trenches are the deepest places on Earth. The deepest trench is the Marianas Trench in the southwestern Pacific Ocean, which plunges about 11 kilometers (35,840 feet) beneath sea level. Near the trenches, the seafloor is also especially deep.

Besides these dramatic features, there are lots of flat areas, called abyssal plains, just as the scientists had predicted. But many of these plains are dotted with volcanic mountains. These mountains are both large and small, pointy and flat-topped, by themselves as well as in a line. When they first observed the maps, the amazing differences made scientists wonder what had formed these features.

Seafloor Magnetism

In the previous lesson, you learned that magnetometers used on land were important in recognizing apparent polar wander. Magnetometers were also important in understanding the magnetic polarity of rocks in the deep sea. During WWII, magnetometers that were attached to ships to search for submarines discovered a lot about the magnetic properties of the seafloor.

In fact, using magnetometers, scientists discovered an astonishing feature of Earth's magnetic field. Sometimes, no one really knows why, the magnetic poles switch positions. North becomes south and south becomes north! When the north and south poles are aligned as they are now, geologists say the polarity is normal. When they are in the opposite position, they say that the polarity is reversed.

Figure 6.10: Scientists found that magnetic polarity in the seafloor was normal at mid-ocean ridges but reversed in symmetrical patterns away from the ridge center. This normal and reversed pattern continues across the seafloor.

Scientists were surprised to discover that the normal and reversed magnetic polarity of seafloor basalts creates a pattern of magnetic stripes! See Curie's temperature of demagnetization for understanding why. There is one long stripe with normal polarity, next to one long stripe with reversed polarity and so on across the ocean bottom. The ridge crest is of normal polarity and there are two stripes of reversed polarity of roughly equal width on each side of the ridge. Further distant are roughly equal stripes of normal polarity, beyond that, roughly equal stripes of reversed polarity, and so on. The magnetic polarity maps also show that the magnetic stripes end abruptly at the edges of continents, which are sometimes lined by a deep sea trench (Figure 6.10).

The scientists used geologic dating techniques to find the ages of the rocks that were found with the different magnetic polarities. It turns out that the rocks of normal polarity are located along the axis of the mid-ocean ridges and these are the youngest rocks on the seafloor. The ages of the rocks increases equally and symmetrically on both sides of the ridge.

Scientists also discovered that there are virtually no sediments on the seafloor at the axis, but the sediment layer increases in thickness in both directions away from the ridge axis. This was additional evidence that the youngest rocks are on the ridge axis and that the rocks are older with distance away from the ridge (Figure 6.11). The scientists were surprised to find that oldest seafloor is less than 180 million years old while the oldest continental crust is around 4 billion years old. They realized that some process was causing seafloor to be created and destroyed in a relatively short time.

Figure 6.11: Seafloor is youngest near the mid-ocean ridges and gets progressively older with distance from the ridge. Orange areas show the youngest seafloor. The oldest seafloor is near the edges of continents or deep sea trenches.

The scientists also discovered that the seafloor was thinner at the ridge axis and grew thicker as the crust became older. This is because over time, additional magma cools to form rock. The added sediments also increase the thickness of the older crust.

The Seafloor Spreading Hypothesis

Scientists brought all of these observations together in the early 1960s to create the seafloor spreading hypothesis. They suggested that hot mantle material rises up toward the surface at mid-ocean ridges. This hot material is buoyant and causes the ridge to rise, which is one reason that mid-ocean ridges are higher than the rest of the seafloor.

The hot magma at the ridge erupts as lava that forms new seafloor. When the lava cools, its magnetite crystals take on the current magnetic polarity. The polarity is locked in when the lava solidifies and the magnetite crystals are trapped in position. Reversals show up as magnetic stripes on opposite sides of the ridge axis. As more lava erupts, it pushes the seafloor that is at the ridge horizontally away from ridge axis. This continues as the formation of new seafloor forces older seafloor to move horizontally away from the ridge axis.

The magnetic stripes continue across the seafloor. If the oceanic crust butts up against a continent, it pushes that continent away from the ridge axis as well. If the oceanic crust reaches a deep sea trench, it will sink into it and be lost into the mantle. In either case, the oldest crust is coldest and lies deepest in the ocean.

It is the creation and destruction of oceanic crust, then, that is the mechanism for Wegener's drifting continents. Rather than drifting across the oceans, the continents ride on a conveyor belt of oceanic crust that takes them around the planet’s surface.

One of the fundamental lines of evidence for continental drift is the way the coastlines of continents on both sides of the Atlantic Ocean fit together. So let’s look at how seafloor spreading moves continents in the Atlantic by looking more closely at Figure 6.11 above. New oceanic crust is forming at the mid-ocean ridge that runs through the center of the Atlantic Ocean basins, which is called the Mid-Atlantic Ridge. Stripes of different magnetic polarity are found on opposite sides of the Mid-Atlantic Ridge. These stripes go all the way to the continents, which lie on opposite sides of the Atlantic. So new seafloor forming at the Mid-Atlantic Ridge is causing the Americas and Eurasia to move in opposite directions!

Lesson Summary

• Using technologies developed for World War II, scientists were able to gather data that allowed them to recognize that seafloor spreading is the mechanism for Wegener's drifting continents.
• Bathymetric maps revealed high mountain ranges and deep trenches.
• Magnetic polarity stripes give clues as 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 has no ocean crust been located that is older than about 180 million years when the oldest continental crust is about 4 billion years old?
3. Describe the major features of mid-ocean ridges, deep sea trenches, and abyssal plains and their relative ages.
4. Describe why continents move across the ocean basins as if they are on a conveyor belt rather than as if they are drifting, as was Wegener's original idea.
5. Explain why the following scenario is impossible: Oceanic crust is not destroyed at oceanic trenches, but new crust is still created at mid-ocean ridges.
6. 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?
7. 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?

Vocabulary

abyssal plains

Very flat areas that make up most of the ocean floor.

bathymetric map

A map of the seafloor created from the measurement of water depths.

echo sounder

A device that uses sound waves to measure the depth to the seafloor.

mid-ocean ridge

The location on the seafloor where magma upwells and new seafloor forms. Mid-ocean ridges are the dominant feature of divergent plate boundaries found in the oceans.

seafloor spreading

The mechanism for moving continents. The formation of new seafloor at spreading ridges pushes lithospheric plates on the Earth's surface.

trench

A deep hole in the seafloor where subduction takes place. Trenches are the deepest places on Earth.

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?
• The features of the Atlantic Ocean basin are described in terms of seafloor spreading and continental drift. Now look at the features of the North Pacific Ocean basin and explain them in those terms as well.