[SOUND] [MUSIC] So Wegener dies in 1930, his idea of Pangea and continental drift sits in the back water for many years to follow but it's not like geologists were sitting still. The next 30 years, the 1930's through 1960, represent an epic era of discovery about the way our Earth works. During the interim period of time other geologists did continue to think about what continued to drive continents. One geologist Arthur Holmes British geophysicist came up with the idea that the mantle of the earth the interior convected like molasses in a pot on a stove. And he thought that somehow these convection cells could pull continents apart and cause them to move. Holmes is on the right track but wasn't quite there yet. We'll see that, that his thinking is only part of a larger story. One of the major contributions of that period of time was the study of the seafloor. Because up until that point, it would be fair to say that we knew less about the seafloor than we knew about the surface of the moon. At least we could see the surface of the moon. But the seafloor is covered by kilometers of water. Well in the 1930s, new technologies, that curiously were developed, because of World War II, and because of the need to map the seafloor in detail, to allow submarine warfare, these new technologies, allowed geologists for the first time, to develop an image, of what the seafloor looked like. One of the first vessels such as the Vema which crisscrossed the oceans for decades, collected vast amounts of data about the bathymetry, meaning the topography of the seafloor, as well as about the magnetic field of the seafloor, about the thickness of sediments, about the heat flow, the amount of heat coming out of the seafloor at a given location, and many other characteristics of the ocean. Some of the most important new contributions to the study of the seafloor came from documenting bathymetry, the shape of the seafloor. In the pre-electronic world, this was done by dropped a weight on the end of a cable to the seafloor. You can imagine that that takes a long time to get a single point measurement. With the invention of sonar, things changed radically. Because sonar can keep a constant record of the depth to the seafloor as a ship is moving. Well, with sonar measurements from cruises going back and forth and back and forth across the ocean floor geologists were eventually able to compile a map of what the sea floor looked like. In recent years, there are more advanced technologies for documenting the bathymetry of the seafloor. For example, it's now possible to map a swath of a given width electronically as a ship sails across the sea. And even more recently, it's possible to use satellite data to derive the bathymetry of the seafloor. So let's look, for example, at the North Atlantic Ocean. What these bathymetric studies revealed was that as you go from the coast of the United States to the coast of Africa you cross first an area of fairly shallow water called the Continental Shelf. Then the seafloor drops to a deeper level at about four to five kilometers and you cross a vast plane called the abyssal plain. And then, in the middle of the Atlantic Ocean, suddenly the typography or the bathymetry gets a little rougher and the water gets a little bit shallower and you climb up the sides of what is now known as the Mid-Atlantic Ridge. It's a mountain belt that lies still submerged beneath about two or two and a half kilometers of water but rises about two kilometers above the depth of the abyssal plain. Similar features as we now know, occur in other oceans as well. Some of them are in the middle of the ocean floor, some of them are off to one side. We'll understand why that is in a bit. Here's a profile of the seafloor, and again you can see the continental shelf, the abyssal plain, and the Mid-Atlantic Ridge. Well another discovery that followed soon after was the realization that the heat flow, meaning the amount of heat coming up from the earth is much greater along the axis of the Mid-Atlantic Ridge than it is off to the sides. In fact it's very, its its relatively high along the axis of the ridge, it decreases progressively away from the ridge, out until the seafloor is relatively cold beneath the abyssal plains. If you go to other parts of the ocean, for example, the west coast of South America, geologists found, that as you went from the abyssal plains towards the continent. The seafloor dropped off to form a narrow, relatively narrow, but long and very deep trough, called a trench. The deepest of these occurs in the Western Pacific along the edge of the Marianas Islands and it reaches a depth of almost 11 kilometers beneath the seafloor. In other words, you could take the highest mountain in the planet, Mount Everest and submerge it in the Marianna Trench without a trace with kilometers of water over the top. So, the seafloor goes from a maximum depth of almost 11 kilometers at the floor of a trench to about 4 to 5 kilometers over the vast expanses of the abyssal plains up to a shallower level of about two kilometers, or 2 and a half kilometers, along the crest of the mid-ocean ridges. In addition to these features, the ocean floor is dotted with individual oceanic islands, some of which emerge from the sea and are islands covered with palm trees and sandy beaches. Some of which are actually below sea level and occur as seamounts. So now, let's look at the broader example of the Mid-Atlantic Ridge and realize that there's another very distinctive feature. And these features are called fracture zones. Fracture zones are long linear belts that appear to be composed of broken up rock in which there's a bathymetric feature, sometimes it's a very narrow little indentation surrounded by ridges on either side. If you look at these you'll see that these fracture zones are right angles, or nearly right angles, to the axis of the Mid-Atlantic Ridge. Here's one just coming off to the east of where the Amazon enters the Atlantic Ocean, and you can see this long, linear belt that runs from the east coast of South America almost as far east as the west coast of Africa. There are many, many, many of these. They interrupt the axis of the Mid-Atlantic Ridge, at distances of a few kilometers to tens of kilometers, and in some cases to hundreds of kilometers. But they occur and they are typical of all mid-ocean riches. So we've seen that there is some very interesting observations concerning the nature of the seafloor. The seafloor includes not only abyssal plains and cotton shells, but also distinct mid-ocean riches, distinct trenches and distinct fracture zones. The next major discovery had to do with recognizing the distribution of earthquakes. By mid-1950s, seismologists were able to map the locations of earthquake epicenters on the globe. And when the points representing the epicenters were plotted It turned out that they aligned in very distinct belts. This was recognized as early as 1959. One of these belts follows the trace of the Mid-Atlantic Ridge, another belt occurs down the middle of the Indian ocean, other belts occur along the trenches that occur on the edges of Indonesia and there are intense belts of seismicity along the west coast of South America, along Japan, and other areas of the planet where trenches occur. So it seems that there's an association between earthquakes and these distinctive bathymetric features, namely mid ocean ridges, trenches, and fracture zones. Well, about 1960, an American geologist named Harry Hess first proposed an idea that in fact what was happening was that new seafloor was forming along mid ocean ridges and that it is the new seafloor formed, then continents on either side moved apart. A contemporary Robert Deets coined the term seafloor spreading for this process and it's still known by that name today. Now clearly there's a bit of a problem. If new ocean basins are being formed and are getting wider over time, what happens to the old ocean basins. Or, is it possible that the earth is actually getting bigger with time? Well most geologists, in fact, almost all geologists today, conclude that the earth is staying the same circumference over time. You can't create new matter on the inside. So basically, there's got to be a process by which the old seafloor, is able to be consumed. And Hess recognizes and this process is now known as subduction. So there are places where ocean floors are growing by the process of seafloor spreading and there are places where old ocean floor is being consumed meaning it's sinking back into the interior of the earth by a process now known as subduction. So this was a radical idea, the concept of seafloor spreading because in one fell swoop, it explained why continents could drift, something that Wegener was never able to do. So geologists began to think about this in detail, and began to wonder is there any way to prove whether or not seafloor spreading really takes place. Well, the answer came from an unexpected source. It came in the study of the record of the Earth's magnetic field, as preserved in rocks, what we now refer to as paleomagnetism. [MUSIC]
