Seafloor Spreading

SONAR During WWII, the use of the technology of SONAR (SOund Navigation And Ranging) was heavily used to find submarines and also to make maps of the sea floor. SONAR works by sending out soundwaves and measuring the amount of time it takes for those waves to bounce off the seafloor and return. Because the speed at which the waves travel is known, the depth of the seafloor at any given point can be determined (Figure 1.) Figure 1. Diagram showing SONAR.

Topography of the Seafloor Harry Hess (Figure 2) was a captain of a US transport ship during WWII. To help with landing his ship on the islands in the Pacific ocean, his ship was equipped with sonar technology. With this technology he had collected a tremendous amount of data about the topography of the sea floor. Before this time it was thought that the ocean was completely flat and covered with sediment. What Hess had discovered is that the seafloor was full of mountains he called guyots (flat mountains) and seamounts and huge canyons called trenches (Figure 3.) After the war more Hess became a professor of geology at Princeton University and more information was gathered in the Pacific and Atlantic oceans. Figure 2. Harry Hess, considered the “father” of the theory of plate tectonics.

trench seamount Figure 3. Map of the Pacific basin showing seamounts (volcanic mountains and sea floor trenches.

Theory of Sea-Floor Spreading During the mid to late 1800’s the transatlantic telegraph cable was being laid down in the Atlantic ocean to allow North America to more quickly communicate with European countries. As a result of this cable being laid down, people noticed that the middle of the Atlantic was much more shallow. About a century later, sonar technology revealed that running down the middle of the Atlantic ocean and all other ocean basins is a large mountain range called mid-ocean ridges. Also, running down the middle of the mid-ocean ridges is a narrow, deep depression called a rift valley (Figure 4). Figure 4. Diagram of the Atlantic Ocean basin. Running down the middle of this basin is a large ridge that rises up from the ocean floor about 10,000 feet. This is known as the Mid-Atlantic Ridge. rift valley

Theory of Sea-Floor Spreading During the 1950’s and 1960’s Harry Hess developed a theory to help explain the discovery of mid-ocean ridges and deep sea trenches. His theory stated that the Earth’s crust was being pulled apart at mid-ocean ridges causing magma to come out and solidify creating new sea floor. As more magma comes out at these rift valleys, it pushes the seafloor outwards like two giant conveyor belts moving away from each other. However, Hess understood that something else must be going on, because if the earth was constantly creating new sea floor at mid-ocean ridges the earth would be getting larger, and it isn’t. Therefore, he hypothesized that old seafloor must be being destroyed at ocean trenches in a process called subduction. This process of the seafloor spreading (Figure 5) is the driving force that causes the continents to move, which Wegener’s theory of Continental Drift couldn’t explain. Figure 5. This diagram is showing the process of seafloor spreading causing mid-ocean ridges, rift valleys, trenches, and volcanoes.

Evidence – Magnetic Reversals The ocean floor is made up a volcanic rock called basalt (Figure 6). Basalt is made of iron-rich minerals that act like little compasses. When this rock is still molten, the iron-rich minerals align them selves with magnetic field and point north (like a compass). When the magma cools and solidifies that magnetic record becomes locked in the rock. It is also important for this discussion to remember that as new rock is formed at mid-ocean ridges it is split in half and each half is pushed away from each other by the process of sea floor spreading. Scientists have analyzed the magnetic orientation of the rocks of the seafloor and have discovered that the earth’s magnetic field has flipped, or reversed itself. These reversals have are shown in Figure 7 as positive (normal magnetic orientation). You may be thinking that what does this have to do with seafloor spreading and how does this support the theory put forth by Harry Hess? If you look at Figure 7, you will see that the pattern of magnetic reversals on one side of the mid-ocean ridge is exactly the same as the other side of the ridge. The fact that the pattern of magnetic reversals on one side of the ridge is the exact same as the other side of the ridge shows supports the theory of sea-floor spreading. Figure 6. Volcanic rock called basalt. Figure 7. Diagram showing the evidence of Earth’s magnetic reversals in the past.

Evidence – Age of Rocks In the 1960’s samples of the ocean floor were obtained and the age of the seafloor was determined. Using these samples the map seen in Figure 8 was made. This shows three important pieces of evidence that supported the theory of seafloor spreading. As you can see from the map, the age of the rocks on the seafloor get progressively older as you move away from a mid-ocean ridge. Also, like the magnetic anomalies on the seafloor, the pattern of the ages of the rocks on either side of the ridge is the same. Finally, the oldest rocks on the seafloor are about 175 million years old. Scientists knew from other forms of evidence, including fossils, that the oceans, like the continents, have existed for billions of years. Why then can no ocean rock on the seafloor