Plate Tectonics

Wegener- Fossil evidence

As Wegener studied the fossils from the various continents, he discovered that they (the southern continents) shared many identical fossil organisms. Fossils of a group of plants called the Glossopteris flora are found scattered on these continents. These are temperate climate plants whose remains are now found in deserts, and even in tropical areas. They simply could not have grown there nor could they have moved from one continent to another unless they were connected. He also found a group of reptiles (Mesosaurus, Lystrosaurus and Cynognathus) dispersed on the various continents that simply could not have swam across oceans. If they are found on different continents, Wegener reasoned, they must have walked there on a previously connected continental mass.

Wegener- Similarity of Rock Sequences and Mountain Ranges

If the continents were once joined together, then rock types, rock sequences and even mountain ranges must have counterparts on the other continents to which they were first joined. Wegener found ample evidence of this in Africa and South America and even between rocks and mountain ranges in Europe and North America. One group of rocks on the southern continents, called the Gondwanan Sequence is especially wide spread and easy to recognize on the now separated continental masses.

Alfred Wegener and Continental Drift

In the 1920's, a German glaciologist by the name of Alfred Wegener once again sought to explain the occurrences of fossils and other aspects of the world's continents. He proposed what is now universally known as the Theory of Continental Drift. In his books, Wegener cited much evidence that he believed proved that a supercontinent called Pangaea, consisting of all of the world's continents, had once existed. All of the current continents were the result of Pangaea breaking up and the pieces drifting away from one another. Wegener used several lines of evidence to support his ideas.

Wegener- Jigsaw Puzzle Fit

It had been know for many years that the east coast of South America was very similar in shape to the west coast of Africa. Wegener argued that all of the continents could be reassembled like a jigsaw puzzle, using the ocean/land boundary outlines, into one large supercontinent.

Pangaea

Like Wegener's continental drift, plate tectonics had it supercontinent. Pangaea was composed of two older continental pieces: Laurasia, which included North America, Greenland, Europe, and Asia, and Gondwanaland, which included South America, Africa, India, Australia, and Antarctica. The difference between the two theories is that Pangaea was just the latest in a series of supercontinents to have existed in the earth's long history of plate movements.

Wegener- Modern Plate Tectonics Theory

Modern Plate Tectonics Theory - For many years after Wegener first published his theory, it was hotly debated in the geologic community. Unfortunately, the final bits of evidence to prove drifting and lead to the plate tectonics theory would have to wait until after the end of the Second World

Evidence Supporting the Theory of Plate Tectonics

Paleomagnetism, Discovery of the Mid-ocean Ridge System , Seafloor Spreading and Magnetic Reversals , Age of the Seafloor , Polar wandering , Refined Jigsaw Puzzle Fit

Plate Tectonics

Perhaps the most important ideas in Geology over the last 100 years.... clarified the relationships between the various disciplines of earth science. With the development of plate tectonics theory, we were finally able to explain the relationship between volcanoes, earthquakes and areas of mountain building, unravel mysteries involving the evolution and distribution of life on the planet. The "Unified Theory" of geology. The concepts behind plate tectonics theory are actually very old and predate modern geology.

How does convection move the plates?

Push or "ridge push" , Pull or "slab pull", sliding, conveyor belt or piggy back

Edward Suess

Suess was an Austrian geologist who, in 1831, wrote what is considered the first book on tectonic theory. He noted the similarities between the fossil organisms found on all of the southern continents. He proposed the name Gondwanaland for this supercontinent composed of Australia, India, South America and Antarctica. He didn't see this as a fully conjoined continental mass however. He believed that some type land bridge connected the continents together and that over time these land bridges formed and then were destroyed.

Frank Taylor

Taylor was an American geologist and in 1910 he published a small pamphlet concerning his ideas of continental drift. He proposed that mountain ranges were the result of lateral movements by continents. He also envisioned a world with two large polar supercontinents that then broke up to form the continents that we have today. He suspected that the driving force behind the movement were the tidal forces of the moon and the sun.

Heat Flow and Thermal Convection - the Driving Force of Plate Tectonics

The force behind plate tectonics seems to be the movement of heated rock in the earth's mantle. The plates of the lithosphere are "rafts" that "float" on the semiplastic ocean of the asthenosphere. Heat from the earth's core is basically passed through the layers of the earth by conduction however; the mantle behaves in a plastic, almost liquid manner and passes the heat upward through the process of convection. Convection is essentially the rising of material towards the crust where it is deflected to the sides, cools and then sinks back into the mantle. Two questions remain, however, as to how this convection occurs: How deep is convection? How does this convection cause the plates to move?

Sliding

The mid-ocean ridge is a prominent mountain range on the ocean floor. It is possible that new ocean crust, added at the top of the ridge, slides downhill under the influence of gravity. The molten mantle material below the crust supplies the slippery surface to allow the sliding to occur.

Plate Tectonics and the Rock Cycle

The plate tectonics cycle involves the creation and destruction of crust. New crust forms as mantle material is pushed into the mid-ocean ridges and old crust is consumed at the subduction zones. The process includes the production of mountains, intrusions, and metamorphism. In this manner, the plate tectonics cycle and rock cycle intersect, particularly with the production of igneous rocks. A close look also reveals that the production of mountains, by plate collisions, encourages the erosion of mountains and the production of sedimentary rocks. The link between these two cycles is more complex than it first appears as both cycles move rock through the geosphere.

Conveyor Belt or Piggy Back

The plates may simply ride on top of the convection cells in much the same way as one might ride on a conveyor belt or on an escalator.

Plate Boundaries

The surface of the earth is currently covered by 7-12 major plates. As the plates move about on the asthenosphere, they interact with one another. This interaction is what causes earthquakes, volcanoes and mountain building. By plotting the locations of earthquake and volcanoes, it is easy to see the outlines of the plate boundaries.

Wegener- Cause of Continental Drifting

Ultimately, even though he felt that the evidence for drift was overwhelming, Wegener was faced with the task of explaining a mechanism to cause the drifting. Life Taylor, Wegener thought that maybe tidal forces might cause the drifting. He also thought that centrifugal forces caused by the spinning of the earth might cause the drifting. Neither explanation satisfied detractors of the theory.

Wegener- Glacial evidence

Wegener found much evidence of glaciation on the southern portion of his Pangaea. In particular he looked for two particular bits of evidence that can be used to determine the direction that the glaciers were moving - end moraines and glacial grooves. End moraines can tell you the last position of a glacier as they form in front of the glacier as it moves. Glacial grooves, scratches on rocks made by the passing glacier, can actually be used to determine the direction that a glacier came from and was moving towards. When Wegener looked at this evidence, it seemed to indicate that the ice was moving FROM THE WATER ON TO THE LAND in many places. Since glaciers move under the force of gravity, it is impossible for ice to move from land to water. In order to fully explain the locations of glacial deposits and the apparent direction of movement, the land masses had to be connected together with the southern tip of Africa near the South Pole.

Pull or "slab pull

While ocean floor is constructed at the mid-ocean ridges, it is consumed or returned to the mantle in a process called subduction. It may be that this subduction of the cold ocean plate acts to pull the crust down into the mantle and at the same time, pull open the mid-ocean ridge allowing new material to come to the surface.

Push or "ridge push"

as convection brings molten material to the surface, it is added to the crust at the mid-ocean ridge where it pushes the ocean apart and creates new seafloor

Types of Plate Boundaries

divergent, convergent ( cont- cont, cont- ocean, ocean- ocean), transform/ transverse, triple point/ joint junctions, hot spots

Shallow convection

only the upper most part of the mantle convects. The rest passes heat upward by conduction only.

Whole mantle convection

the entire mantle convects as a single convection cell. Heat from the core rises upward all the way to the crust and then cycles back down to the core.

Layered convection

two separate convection cells may exist in the mantle. One operates in the lower mantle and brings heat from the core partway up towards the surface. A second shallow convection cell then operates in the upper mantle to bring the heat to the surface