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Geological Time Scale

Seafloor Spreading, a Quick Look

Okay, so you want to learn more about the concept of seafloor spreading, and the evidence for it. Well, you've come to the right place. The basic idea behind seafloor spreading is rather simple. A mid-oceanic ridge will form in the middle of an ocean (hence the name). These form as hot magma forces its way through the thin oceanic crust. When it reaches the surface, it pushes aside the already cooled rocks to one side or the other. Wash, rinse, repeat. Over time, you'll see that there is a pattern of rocks forming on either side of this ridge. If it exists on a passive margin (where the oceanic plate isn't being subducted under a continental plate, like the eastern coast of the U.S.), it tends to push continents away from each other, exactly like what happened between South America and Africa. Above is a cartooned diagram of how seafloor spreading happens, as well as other tectonic processes.

The Role of Paleomagnetism in Seafloor Spreading

The notion of seafloor spreading was conceptualized by Arthur Holmes in the 1920's. Using the newly discovered ideas regarding radioactivity in the Earth's core, he proposed that the continents moved by convection of hot molten rock. He pretty much got it right, too. Now, when molten rock (or magma) wells up in mid-oceanic ridges, it is quickly cooled off by the water. This igneous rock, which is highly magnetic, gets magnetized in the direction of the prevailing strongest pole. Today, the strongest of the two poles is at the North Pole, so rocks forming today, and since the last magnetic reversal, will be oriented to the north. Rocks that formed when the strong pole was at the South Pole will be magnetized to the opposite direction. Keeping this in mind, geologists can make a map of magnetic reversals along the sea floor, using core samples taken from rocks formed along the bottom. If we think about normal magnetism (like we have now) as being some random color, and reversed as white, a definite zebra-striped pattern soon appears on the ocean floor. The diagram to the left shows an example of what we might see. In this instance, normal polarity is orange-colored.

In this color-enhanced cartoon, we can apply what we've learned about seafloor spreading and magnetic reversals. Although the scale is difficult to see, each color represents a new period of reversals, and they span from about 10 million years ago up until the present. The youngest material is near that black dotted lines. As you can see, there is a parallel banding as you move away from the ridge. This banding, tied in with the ages of the rocks studied, wrote the story of seafloor spreading and paleomagnetic reversals. With the understanding of seafloor spreading, geologists were then able make preditions and theories regarding other geological features, such as volcanoes and earthquakes. In a nutshell, that is the concept of seafloor spreading.