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Plate Tectonics Help

By — McGraw-Hill Professional
Updated on Aug 31, 2011

Introduction to Plate Tectonics

Have you ever looked out of the window of an airplane and seen the widely different shapes, colors, and textures of the land below? Depending on the plane’s height, the ground appears as an intricate carpet of every shade and texture. From rich browns and blacks to yellow, red, and every shade of green, the Earth’s landmass is a mosaic of color. If you travel from the Midwest to the Northwest you will see everything from long stretches of grasslands with their circles of irrigated crops to salty deserts (ancient seas) of the west to the beautiful mountains formed from the clash of the North American plate with the Pacific plate. And that is just in the United States!

Travel across Africa, Australia, or Asia and see the variety and combinations of surface features there. Our world is a geological “gold mine” of diversity and mysteries waiting to be uncovered. The wide selection of lands and oceans seems beyond definition. New elements, mineral forms, and mechanisms are described nearly every year. It is an exciting field of study with tools that include things like axes, picks, compasses, and camping tents. It’s a lot more fun than the standard bench top laboratory work. Remember, in Earth Sciences, the world is your lab; jeans and t-shirt your lab clothes!

Continental Drift

In the mid-1600s, Francis Bacon noticed that there seemed to be an odd, almost puzzle piece fit between Africa and South America. Not knowing what that meant, he put it down to remarkable coincidence. He told a lot of people about his theory and they thought it was interesting too, but nothing more than that.

Since the Principle of Uniformity , the idea that the Earth’s past is based on its current form was firmly accepted, everyone figured that since the landmasses were anchored now, they must have always been anchored in about the same spots.

In 1858, after comparing the coastlines of Africa and South America closely, Antonio Snider-Pellegrini published his ideas of how the world looked before and after Africa and South America were pulled apart. Some people thought it was possible, while others rolled their eyes at the idea.

It wasn’t until 1915, that Alfred Wegener, professor of meteorology and geophysics, in Graz, Austria, suggested the Africa/South America fit was the result of continental drift .

Wegener knew that scientists studying the fossil record of certain plants and animals were finding fossils in a narrow strip that stretched across several continents. Wegener was also studying the changes in world climate over time. He became aware of reports that fossils of plants that once grew in humid, hot climates were being discovered in polar areas and that fossils from colder climates had been found in hot, equatorial climates.

For example, Glossopteris , is an ancient fossilized plant found in southern Africa, Australia, South America, India, and Antarctica. Its huge pattern of dispersion seemed impossible to explain. Although some paleontologists thought the wind might have carried the seeds a long way, Wegener had a different idea. He thought the major continents had all been joined together in one piece at the time Glossopteris lived. Then, after the crust broke up and pulled apart, the continents drifted long distances from the places where Glossopteris first grew and then later turned into fossils.

Wegener decided that fossil rocks from climates we know today as having cold conditions were formed when their early land location was next to a geographical pole. He thought this was true even though some were now positioned at the equator.

Wegener studied many similarities of different landforms and was convinced that the original supercontinent, Pangea, developed many crossways fractures and drifted apart about 200 million years ago. To describe this in more detail, he published the Origin of Continents and Oceans . In it Wegener described how Africa and South America must have first split during the Cretaceous period, while much later, during the Quaternary period, Europe and North America, as well as South America and Antarctica, broke apart. He thought that even later in the Eocene period, Australia and Antarctica separated. Wegener could not explain what caused the original continental breakup and died while on an expedition to the Greenland icecap, but is considered to be the Father of Continental Drift .

One of Wegener’s supporters, American scientist F. B. Taylor, published a paper in 1910, describing ancient movement of the mountain ranges in Asia from north to south. He thought mountain building was a lot more than just some in-place adjustments of the crust. He also thought the Mid-Atlantic Ridge was a crack that remained from the first pulling apart of Africa and South America. His ideas seemed to fit so well with Wegener’s that some geologists called the whole splitting and drifting idea, the Taylor–Wegener theory .

In 1924, Swiss Alps and tectonics expert, Émile Argand spoke before the International Geological Congress in Brussels. In his talk he explained how it might be possible that the entire Alpine system, the mountains from the western Alps to the Himalayas, were formed from the drift of the Gondwana continent against Eurasia. He invented the word, mobilism , to explain sideways crustal movements and formation of mountain ranges.

A symposium (a meeting of experts) on continental drift was held by the American Association of Petroleum Geologists in 1926. Although the main organizers of the meeting were in favor of the continental drift idea, there were a lot of heated arguments about the existing data and what it meant. (That happens sometimes when you get a lot of scientists together!) By the end of the meeting, since no one could prove how continental drift took place, the majority of people didn’t think continental drift was correct.

Finally, in 1928, Scottish geologist and Professor at the University of Edinburgh, Arthur Holmes, came up with the idea of an “engine” that might be providing the energy source for continental drift. Some people thought volcanic activity was the answer to continental drift, but Holmes didn’t think so. He thought that a much higher energy source was needed to release the amount of heat produced by radioactive elements in the deepest layers of the Earth. He suggested that convection currents , the constantly circulating movements of heat and magma in the deepest layers of the Earth, provided enough energy to power continental drift.

Holmes knew that granites that make up a lot of the continents are high in radioactive elements. So he hypothesized that the temperature beneath the continents was probably higher than the temperature under the oceans.

If this was true, then convection currents would rise under the continents and spread out horizontally toward the continent’s edges. After reaching the continent/ocean edge and lower temperatures, the cooler currents would turn and sink back downward.

Holmes knew that when a liquid was heated from below, the temperature increases upward until a critical temperature gradient is formed. When the temperature is increased even higher, the gradient is interrupted and thermal circulation currents form.

Have you ever seen a “lava lamp?” Picture the lazy blob movement in lava lamps. The waxy “lava” is heated by a bulb in the base of the lamp and when it gets hot enough, it rises upward. After reaching the top of the lamp, the lava starts to cool and eventually sinks downward again. When it gets back to the bottom and the hot light bulb, the lava heats and the whole cycle begins again. Figure 4-1 gives you an idea of how the Earth’s thermal convection currents work. The expanding, newly formed lithosphere is also shown.

Plate Tectonics

 

Fig. 4-1. Thermal convection currents play a main part in the Earth’s magma movement.

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