Unit 5: The Dynamic Earth Chapter 17: Plate Tectonics     A Theory Waiting for the Right Technology The Dutch mapmaker Abraham Ortelius recognized the puzzle-like fit of the coastlines of Africa and South America in the late 1500s. Alfred Wegener proposed the theory of continental drift in 1912. So why did it take so long to find evidence of seafloor spreading and to support the theory of plate tectonics? Most of the evidence was hidden on the ocean floors. It took a world war to develop the technology to study the ocean floor. Submarines Provide the Key Prior to the 1940s, most scientists theorized that the ocean floors were flat feature-less plains. Advances in technology during the 1940s and 1950s allowed scientists to study the ocean floor in greater detail. As you learned in the text, these advances in technology produced sonar and the magnetometer. Why were these technologies developed? They were not developed by scientists interested in Wegener's hypothesis of continental drift. His hypothesis was considered to be impossible by most scientists, since Wegener could not explain how the continents moved. So what caused the interest in studying the ocean floors? In a strange twist, several of the necessary technological advances stemmed from the use of submarines during World War II. Submariners needed good maps of the ocean floor in order to operate safely. Navigators needed detailed maps so they would not run into any underwater obstructions. In addition, a good map could help them locate their position underwater without surfacing. This need for better maps pushed the development of echo sounding as a mapping tool. The technique of echo sounding or sonar uses sound waves to measure water depth. This was much faster than previous methods which used weights and cables, and oceanographers working for the both the Allies and the Germans quickly developed detailed maps of the ocean floor. To the surprise of most scientists, the ocean floors were not flat featureless plains. They found deep canyons and rugged mountains. Another technological offshoot from World War II was the use of magnetometers-devises that can detect small changes in the magnetic field. Magnetometers were designed to locate enemy submarines and later were adapted to study the magnetic patterns of rocks on the continents and the ocean floor. The first major breakthrough was evidence that Earth's magnetic poles wandered throughout geologic time. Magnetic data showed that the continents' magnetic poles had wandered significantly through time. If magnetic data from a single location is analyzed from rocks of different ages, the magnetic poles appear to wander significantly. There are two ways to explain these data-either Earth's magnetic poles moved or the continents moved. Wegener had explained the apparent polar wandering as being due to the continents moving. As scientists gathered more data after World War II, they began to come to the same conclusion. Oceanographers began detailed magnetic surveys of the world's oceans by towing magnetometers behind ships. They found what would be one of the keys to the theory of plate tectonics-the series of stripes of magnetic anomalies parallel to the mid-ocean ridges. The technology for the last piece of the puzzle would again not be developed by scientists, but during the exploration for oil. The Relentless Search for Energy To fuel the post war demand for oil, the oil and gas industry began to extend their search under the oceans along the continental shelves. While drilling for oil and gas, samples of the rock that were drilled were analyzed. As the oil and gas industry moved into deeper water in the exploration for energy, they developed drilling platforms, submersible drilling rigs, and drilling ships. Offshore drilling techniques were adapted for retrieving core samples of deep-sea sediments and bedrock for the scientific investigation of the ocean floor. The sediment and rock samples obtained from the ocean floor were then dated. Scientists found that the ocean floor was youngest near the mid-ocean ridges, and got older as the distance from the ridge increased. Using these data, Harry Hess proposed the theory of seafloor spreading. The theory of seafloor spreading states that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches. This was the mechanism for plate motion that Wegener was searching for over 60 years earlier. It took the technology developed to win a world war and to search for undersea oil and gas to provide the information needed to propose the theory of plate tectonics. Plate tectonics has become one of the most important theories, not just in geology, but in all of science because it can explain so many of Earth's features. LINK-UP: Find out more information about the latest technology for ocean floor exploration. Activity Research how oceanographers use a variety of technologies to study the ocean, including ships, submersibles, remote vehicles, and remote instruments such as satellites, magnetometers, seismometers, and gravimeters. Write a short report and present it to the class.