User:Kim Early Manguera
Pangea, also spelled Pangaea was a supercontinent that existed on the Earth million of years ago. In here, the continents were connected into a one large landmass. IT is believed that he Pangea began forming 300 million years ago and began to separate around 200 years ago. The term Pangea or Pangaea is came from the Greek mythology Godess Gaia. Alfred Wegener, a German meteorologist and geophysicist, was the first person who proposed the concept that the continents on Earth once existed as a continuous land mass. He also came up with the continental drift theory but the geologist didn't believed his theory for two reasons: One, his theory of continental drift was just too weak for most geologists to accept. Even though he believed the supercontinent that broke up into different continents moved, he did not have a clear explanation to how the continents moved. The other reason is that some of his explanation clashed with ideas that were widely accepted in the science communities. He used similar fossils from different continents to back up his theory of continental drift. However, at that time, many scientists that had observed similarities in fossils in places like South America and Africa believed there were similar fossils in different continents because of a land bridge that were formed by two continents.
He published "The Origin of Continents" in 1912. Later on, Wegener expanded his theory in another book he published called the "The Origin of Continents and Oceans". It was here in 1915 when he hypothesized that before the continental drift, the continents existed as a single supercontinent of which he named Urkontinent. The term Pangea first entered the English and German scientific literature in 1926 and 1922 respectively.
Alfred Wegener
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Alfred Wegner was a German Meteorologist in the early 1900s who studied ancient climates. Like most people, the jigsaw puzzle appearance of the Atlantic continental margins caught his attention. He put together the evidence of ancient glaciations and the distribution of fossil to formulate a theory that the continents have moved over the surface of the Earth, sometimes forming large supercontinents and other times forming separate continental masses. He proposed that prior to about 200 million years ago all of the continents formed one large land mass that he called Pangea
The weakness of Wegner's theory, and the reason it was not readily accepted by geologists was that he proposed that the continents slide over ocean floor. Geophysicists disagreed, stating the ocean floor did not have enough strength to hold the continents and too much frictional resistance would be encountered.
Sea-Floor Spreading
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During World War II, geologists employed by the military carried out studies of the sea floor, a part of the Earth that had received little scientific study. The purpose of these studies was to understand the topography of the sea floor to find hiding places for both Allied and enemy submarines. The topographic studies involved measuring the depth to the sea floor. These studies revealed the presence of two important topographic features of the ocean floor
Oceanic Ridges- long sinuous ridges that occupy the middle of the Atlantic Ocean and the eastern part of the Pacific Ocean.
Oceanic Trenches- deep trenches along the margins of continents, particularly surrounding the Pacific Ocean.
Plate Tectonics
[edit]By combining the sea floor spreading theory with continental drift and information on global seismicity, the new theory of Plate Tectonics became a coherent theory to explain crustal movements. Plates are composed of lithosphere, about 100 km thick, that "float" on the ductile asthenosphere. While the continents do indeed appear to drift, they do so only because they are part of larger plates that float and move horizontally on the upper mantle asthenosphere. The plates behave as rigid bodies with some ability to flex, but deformation occurs mainly along the boundaries between plates. The plate boundaries can be identified because they are zones along which earthquakes occur. Plate interiors have much fewer earthquakes.
Types of Plate Boundaries
[edit]- Divergent Plate boundaries, where plates move away from each other.
- Convergent Plate Boundaries, where plates move toward each other.
- Transform Plate Boundaries, where plates slide past one another
Divergent Plate Boundaries- These are oceanic ridges where new oceanic lithosphere is created by upwelling mantle that melts, resulting in basaltic magmas which intrude and erupt at the oceanic ridge to create new oceanic lithosphere and crust. As new oceanic lithosphere is created, it is pushed aside in opposite directions. Thus, the age of the oceanic crust becomes progressively older in both directions away from the ridge.
- Because oceanic lithosphere may get subducted, the age of the ocean basins is relatively young. The oldest oceanic crust occurs farthest away from a ridge. In the Atlantic Ocean, the oldest oceanic crust occurs next to the North American and African continents and is about 160 million years old
- In the Pacific Ocean, the oldest crust is also Jurassic in age, and occurs off the coast of Japan.
- If the spreading rate (relative velocity) is high, magma must be rising rapidly and the lithosphere is relatively hot beneath the ridge. Thus for fast spreading centers the ridge stands at higher elevations than for slow spreading centers. The rift valley at fast spreading centers is narrower than at slow spreading centers. As oceanic lithosphere moves away from the ridge, it cools and sinks deeper into the asthenosphere. Thus, the depth to the sea floor increases with increasing age away from the ridge.
Convergent Plate Boundaries - When a plate of dense oceanic lithosphere moving in one direction collides with a plate moving in the opposite direction, one of the plates subducts beneath the other. Where this occurs an oceanic trench forms on the sea floor and the sinking plate becomes a subduction zone. The Wadati-Benioff Zone, a zone of earthquakes located along the subduction zone, identifies a subduction zone.
- Earthquakes may extend down to depths of 700 km before the subducting plate heats up and loses its ability to deform in a brittle fashion.
- As the oceanic plate subducts, it begins to heat up causing the release water of water into the overlying mantle asthenosphere. The water reduces the melting temperature and results in the production of magmas. These magmas rise to the surface and create a volcanic arc parallel to the trench.
Transform Plate Boundaries - Where lithospheric plates slide past one another in a horizontal manner, a transform fault is created. Earthquakes along such transform faults are shallow focus earthquakes.
- Most transform faults occur where oceanic ridges are offset on the sea floor. Such offset occurs because spreading takes place on the spherical surface of the Earth, and some parts of a plate must be moving at a higher relative velocity than other parts One of the largest such transform boundaries occurs along the boundary of the North American and Pacific plates and is known as the San Andreas Fault. Here the transform fault cuts through continental lithosphere
Evolving Plate Boundaries
[edit]Plate boundaries can evolve. New plate boundaries can form where mantle upwelling results in creating a rift in the crust and plate boundaries can die when two plates of continental lithosphere collide.
- Continental Rifting- A new divergent plate boundary can form when continental lithosphere stretches, and thins to form a rift valley. As the rift widens and thins, upwelling asthenosphere can melt to produce magmas that start to create new oceanic lithosphere and spread the new plates apart. An example of an where rifting may be forming a future diverging plate margin is an area of northeastern Africa, called the East African Rift Valley. Another area where this is apparently occurring is the Basin and Range Province of the Western U.S.
- Continental Collisions- When two plates that have low density continental lithosphere collide with one another subduction ceases because the continental lithosphere has too low of a density to be subducted. As the plates continue to collide fold - thrust mountain belts that develop along the zone of collision.
- Currently the highest mountains in the world, the Himalayas represent this kind of event. The Himalayas resulted from a collision of the plate containing India with the plate containing Eurasia. This collision is still taking place and results in joining the two formerly separate plates. The occurrence of ancient fold -thrust mountain belts such as the Appalachian Mountains of the Eastern U.S., the Urals of Central Russia, and the Alps of southern Europe, are evidence of ancient continental collision margins.
What Causes Plate Tectonics?
[edit]- From seismic wave velocities we know that the asthenosphere behaves in ductile manner, that is even though it is solid it can flow under stress and behave like a liquid. If this is the case, then it can also convect. Convection is a mode of heat transfer wherein the heat moves with the material. Convection is caused when material that occurs at a deeper level is heated to the point where it expands and becomes less dense than the material above it. When this occurs, the hot less dense material rises. In a confined space, rising hot material will eventually cool and become denser than its surroundings. This cool dense material must then sink. This gives rise to convection cells, with hot rising currents and cool descending currents.
- If the asthenosphere is in fact moving as a result of convection, then convection could be the mechanism responsible for plate tectonics. Hot rising currents would occur beneath oceanic ridges.
- Magma intruding into the ridge would push lithosphere apart at the ridge. As the new lithosphere cools, it will slide off the topographic high that results from the upwelling of the mantle and will eventually become cold and dense. This dense lithosphere will tend to pull the rest of the lithosphere downward.
- A combination of dragging the lithosphere along the top of the convection cell, ridge push, sliding, and slab pull all appear to be contributing factors to the cause of plate tectonics. There is still some debate as to whether asthenospheric convection drives the plates or the plates themselves drive plate tectonics. Until we have a better idea of what is happening in the mantle this debate will not likely be resolved. At least for now it appears that both convection and slab pull are the major factors (note that your textbook comes to a bit different conclusion.