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DEVELOPMENT OF THE THEORY OF PLATE TECTONICS
Wegener died in 1930 on an expedition in Greenland. In his lifetime, he was poorly respected, and his ideas of moving continents seemed destined to be lost to history as a fringe idea. However, starting in the 1950s, evidence started to trickle in that made continental drift more viable. By the 1960’s, there was enough evidence supporting Wegener’s missing mechanism, seafloor spreading, allowing the hypothesis of continental drift to develop into the Theory of Plate Tectonics. Widespread acceptance among scientists has transformed Wegener’s hypothesis to a Theory. Today, GPS and earthquake data continue to back up the theory. Below are the pieces of evidence that allowed the transformation.
MAPPING THE OCEAN FLOORS
Starting in 1947 and using an adaptation of SONAR, researchers began to map a poorly-understood topographic, and thermal high in the mid-Atlantic . Bruce Heezen and Marie Tharp were the first to make a detailed map of the ocean floor, and this map revealed the mid-Atlantic Ridge, a basaltic feature, unlike the continents. Initially, this was thought to be part of an expanding Earth or a mechanism for the growth of the ocean. Transform faults were also added to explain movements more completely. When it was later realized that earthquake epicenters were also located within this feature, the idea that this was part of continental movement took hold.
Another way the seafloor was mapped was magnetically. Scientists had long known of strange magnetic anomalies (magnetic values that differ from expected values) associated with the ocean floor. This tool was adapted by geologists later for further study of the ocean depths, including strange alternating symmetrical stripes on both sides of a feature (which would be discovered later as the mid-ocean ridge) showing reversing magnetic pole directions. By 1963, these magnetic stripes would be explained in concordance with the spreading model of Hess and others.
Seafloor sediment was also an important feature that was measured in the oceans, both with dredging and with drilling. Sediment was believed to have been piling up on ocean floors for a very long time in a static model of accumulation. Initial studies showed less sediment than expected, and initial results were even used to argue against continental movement. With more time, researchers discovered thinner sediment close to ridges, indicating a younger age.
As the video below explains, today scientists are also able to use satellite imagery to map the ocean floor.
Around the same time that mid-ocean ridges were being investigated, ocean trenches and island arcs were also being linked to seismic action, thus explaining the opposite sides of the movement of plates. A zone of deep earthquakes that lay along a plane trending from the surface near the trenches to inside the Earth beneath the continents and island arcs were recognized independently by several scientists. Today called the Wadati-Benioff zone; it was an essential piece of the puzzle.
Magnetic field mapping, as mentioned above, was not the only way magnetism was used in the development of plate tectonics. In fact, the first new hard evidence that supported plate motion came from paleomagnetism. Paleomagnetism is the study of magnetic fields frozen within rocks, basically a fossil compass. This is typically most useful with igneous rocks where magnetic minerals like magnetite crystallizing in the magma align with the Earth’s magnetic field and in the solid rock point to the paleo-magnetic north. The earth’s magnetic field creates flux lines surrounding the magnetic north and south poles (like a bar magnet) which are both close to the Earth’s rotational north and south poles. In igneous rocks, magnetic minerals align parallel with these flux lines as shown in the figure. Thus both magnetic inclination, related to latitude, and declination related to magnetic north are preserved in the rocks.
Scientists had noticed for some time that magnetic north, to which many rocks pointed, was nowhere close to current magnetic north. This was explained by implying the magnetic north pole moved over time. Eventually, scientists started to realize that moving continents explained the data even better than moving the pole around alone.
SEAFLOOR SPREADING HYPOTHESIS
World War II gave scientists the tools to find the mechanism for continental drift that had eluded Wegener. Maps and other data gathered during the war allowed scientists 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 and is the mechanism for continental drift.
During World War II, battleships and submarines carried echo sounders to locate enemy submarines. Echo sounders produce sound waves that travel outward in all directions, bounce off the nearest object, and then return to the ship. By knowing the speed of sound in seawater, scientists calculate the distance to the object based on the time it takes for the wave to make a round-trip. During the war, most of the sound waves ricocheted off the ocean bottom. This animation shows how sound waves are used to create pictures of the seafloor and ocean crust.
After the war, scientists pieced together the ocean depths to produce bathymetric maps, which reveal the features of the ocean floor as if the water were taken away. Even scientists were amazed that the seafloor was not completely flat. What was discovered was a large chain of mountains along the deep seafloor, called mid-ocean ridges. Scientists also discovered deep-sea trenches along the edges of continents or in the sea near chains of active volcanoes. Finally, large, flat areas called abyssal plains we found. When they first observed these bathymetric maps, scientists wondered what had formed these features.
Scientists brought these observations together in the early 1960s to create the seafloor spreading hypothesis. In this hypothesis, hot buoyant mantle rises up a mid-ocean ridge, causing the ridge to rise upward. The hot magma at the ridge erupts as lava that forms new seafloor. When the lava cools, the magnetite crystals take on the current magnetic polarity and as more lava erupts, it pushes the seafloor horizontally away from ridge axis.
The magnetic stripes continue across the seafloor. As oceanic crust forms and spreads, moving away from the ridge crest, it pushes the continent away from the ridge axis. If the oceanic crust reaches a deep sea trench, it sinks into the trench and is lost into the mantle. Scientists now know that the oldest crust is coldest and lies deepest in the ocean because it is less buoyant than the hot new crust.
THE UNIFYING THEORY OF PLATE TECTONICS
Using all of the evidence mentioned, the theory of plate tectonics took shape. In 1966, J. Tuzo Wilson was the first scientist to put the entire picture together of an opening and closing ocean. Before long, models were proposed showing the plates moving concerning each other with clear boundaries between them, and scientists had also started to piece together complicated tectonic histories. The plate tectonic revolution had taken hold.
Seafloor and continents move around on Earth’s surface, but what is actually moving? What portion of the Earth makes up the “plates” in plate tectonics? This question was also answered because of technology developed during the Cold War. The tectonic plates are made up of the lithosphere. During the 1950s and early 1960s, scientists set up seismograph networks to see if enemy nations were testing atomic bombs. These seismographs also recorded all of the earthquakes around the planet. The seismic records could be used to locate an earthquake’s epicenter, the point on Earth’s surface directly above the place where the earthquake occurs. Earthquake epicenters outline these tectonic plates. Mid-ocean ridges, trenches, and large faults mark the edges of these plates along with where earthquakes occur.
The lithosphere is divided into a dozen major and several minor tectonic plates. The plates’ edges can be drawn by connecting the dots that mark earthquakes’ epicenters. A single plate can be made of all oceanic lithosphere or all continental lithosphere, but nearly all plates are made of a combination of both. Movement of the plates over Earth’s surface is termed plate tectonics. Plates move at a rate of a few centimeters a year, about the same rate fingernails grow.
LAYERS OF THE EARTH
Introduction to Physical Geography by R. Adam Dastrup is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
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