User:Earl Mitchelle Pelegre

The Pangaea and Seafloor Spreading

Pangea, also spelled Pangaea, was a supercontinent that existed on the Earth millions of years ago and covered about one-third of its surface. A supercontinent is a very large landmass that is made up of more than one continent. In the case of Pangea, nearly all of the Earth's continents were connected into one large landmass. It is believed that Pangea began forming about 300 million years ago, was fully together by 270 million years ago and began to separate around 200 million years ago.

The name Pangea is ancient Greek and means "all lands." The term began being used in the early 20th century after Alfred Wegener noticed that the Earth's continents looked like they fit together like a jigsaw puzzle. He later developed his theory of continental drift to explain why the continents looked the way they did and first used the term Pangea at a symposium in 1927 focused on that topic.

Formation of Pangea

Due to mantle convection within the Earth's surface, new material constantly comes up between the Earth's tectonic plates at rift zones, causing them to move away from the rift and toward one another at the ends. In the case of Pangea, the Earth's continents were eventually moved so much over millions of years that they combined into one large supercontinent.

Around 300 million years ago the northwestern part of the ancient continent of Gondwana (near the South Pole), collided with the southern part of the Euramerican continent to form one very large continent. Eventually, the Angaran continent, located near the North Pole, began to move south and it collided with the northern part of the Euramerican continent to form the large supercontinent, Pangea, by about 270 million years ago.

It should be noted however that there was another separate landmass, Cathaysia, which was made up of north and south China that was not a part of the larger Pangea landmass.

Once it was completely formed, Pangea covered around one-third of the Earth's surface and it was surrounded by an ocean that covered the rest of the globe. This ocean was called Panthalassa.

Break-Up of Pangea

Pangea began to break up about 200 million years ago as a result of the movement of the Earth's tectonic plates and mantle convection. Just as Pangea was formed by being pushed together due to the movement of the Earth's plates away at rift zones, a rift of new material caused it to separate. Scientists believe that the new rift began due to a weakness in the Earth's crust. At that weak area, magma began to push through and create a volcanic rift zone. Eventually, the rift zone grew so large that it formed a basin and Pangea began to separate.

In the areas where Pangea began to separate, new oceans formed as Panthalassa rushed into the newly opened areas. The first new oceans to form were the central and southern Atlantic. About 180 million years ago the central Atlantic Ocean opened up between North America and northwestern Africa. Around 140 million years ago the South Atlantic Ocean formed when what is today South America separated from the west coast of southern Africa. The Indian Ocean was the next to form when India separated from Antarctica and Australia and about 80 million years ago North America and Europe separated, Australia and Antarctica separated and India and Madagascar separated.

Over millions more years, the continents gradually moved to their current positions.

Evidence for Pangea

As Alfred Wegener noticed in the early 20th century, the Earth's continents seem to fit together like a jigsaw puzzle in many areas around the globe. This is the significant evidence for the existence of Pangea millions of years ago. The most prominent place where this is visible is the northwestern coast of Africa and the eastern coast of South America. In that location, the two continents look like they were once connected, which they, in fact, were during Pangea.

Other evidence for Pangea includes fossil distribution, distinctive patterns in rock strata in now unconnected parts of the world and the distribution of the world's coal. In terms of fossil distribution, archaeologists have found matching fossil remains if ancient species in continents are separated by thousands of miles of ocean today. For example, matching reptile fossils have been found in Africa and South America indicating that these species at one time lived very close to each other as it is not possible to them to have crossed the Atlantic Ocean.

Patterns in rock strata are another indicator of the existence of Pangea. Geologists have discovered distinctive patterns in rocks in continents that are now thousands of miles apart. By having matching patterns it indicates that the two continents and their rocks were at one time one continent.

Finally, the world's coal distribution is evidence for Pangea. Coal normally forms in warm, wet climates. However, geologists have found coal under Antarctica's very cold and dry ice caps. If Antarctica were a part of Pangea it is likely that it would have been in another location on the Earth and the climate when the coal formed would have been very different than it is today.

Many Ancient Supercontinents

Based on the evidence scientists have found in plate tectonics, it is likely that Pangea was not the only supercontinent to exist on the Earth. In fact, archaeological data found in matching rock types and searching for fossils shows that the formation and break-up of supercontinents like Pangea are a cycle throughout the Earth's history (Lovett, 2008). Gondwana and Rodinia are two supercontinents that scientists have discovered that existed prior to Pangea.

Scientists also predict that the cycle of supercontinents will continue. Currently, the world's continents are moving away from the Mid-Atlantic Ridge toward the middle of the Pacific Ocean where they will eventually collide with one another in about 80 million years (Lovett, 2008). To see a diagram of Pangea and how it separated, visit the United States' Geological Survey's Historical Perspective page within This Dynamic Earth.

https://youtu.be/Y3Rduq9Es6g

People and their Biographies

• Alfred Wegener

• Rober Dietz

• Harry Hess

• Maurice Ewing

• Bruce Heezen

 Alfred Wegener 

Alfred Wegener, in full Alfred Lothar Wegener, (born November 1, 1880, Berlin, Germany—died November 1930, Greenland), German meteorologist and geophysicist who formulated the first complete statement of the continental drift hypothesis.

German meteorologist and geophysicist Alfred Wegener was the first person to formulate a complete statement of the continental drifthypothesis. Previous scientists had explained the separation of the modern world’s continents as having resulted from the subsidence, or sinking, of large portions of an ancient supercontinent to form the oceans.

Wegener noticed the similarity in the coastlines of eastern South America and western Africa and speculated that those lands had once formed a supercontinent, Pangaea, which had split and slowly moved many miles apart over geologic time. He also pointed to closely related fossil organisms and similar rock strata that occurred on widely separated continents.

Wegener published his theory in full in 1915, but his contemporaries mostly found it implausible. By 1930 it had been rejected by most geologists, and it sank into obscurity for the next few decades. Wegener’s impact was finally felt when his theory was resurrected as part of the theory of plate tectonics during the 1960s.

 Robert S. Dietz's Biography 

Robert S. Dietz was an American Geologist who was known for his substantial contributions to and for the promotion of Theory of Plate Tectonics. Dietz was born in Westfield, NJ on September 14, 1941 and died in Tempe, Arizona in 1995. Dietz was educated at the University of Illinois where he received his BS., MS. and Ph.D. in Geology. Originally, Dietz wanted to study the surface of the moon for his dissertation but was told that there was no way to confirm his findings; instead he completed his work at the Scripps Institute of Oceanography in San Diego where he mapped the sea floor canyons off the coast of California.

After completing his graduate work in 1941, Dietz worked as a government scientist for thirty years. At that time, marine geology was so young that academic positions where almost unheard of. As a government scientist, Dietz could pursue questions of science that interested him because, working in government, no one was paying attention to what he published. According to Dietz, "science was simple then." This  resulted in substantial contributions made by Dietz to meteor impact theory as well as plate tectonics. He worked in sea floor studies at the United States Navel Electronics Laboratory from 1946 - 1963 after which he took a position at the USCGS in Washington DC and then at the National Oceanographic and Atmospheric Administration until 1976. He closed his career with a tenured position at the University of Arizona in planetary sciences.

 Robert Dietz's Interests 

Dietz's primary interests in plate tectonics were in the area of subduction and continental slopes as rift scars and and as prisms for accretion (wedge-shaped bodies of folded and faulted sea floor that abut a continental margin at areas of subduction or compression of plates). He found that guyots appeared tilted and were moving into the deep trenches of the Pacific Ocean. Dietz spent the early sixties, speaking about the 'Commotion under the Ocean' describing how sea floor spreading supported the idea of motionism and refuted questions about an expanding earth and the fixed sea floor, beliefs that were held by many scientists.

Dietz's interest in a mobile sea floor began after he had been involved in the following findings:


 * There were few sediments on the sea floor compared with the continents.


 * In 1952, Dietz, along with Menard, discovered fracture zones in the Pacific Ocean floor which led to his ideas about sea floor spreading.
 * Image of the Pacific Fracture zones 2
 * The sea floor, based on fossil evidence, was much younger (Cretaceous as opposed to Cambrian) than the continents. This evidence was found by Hamilton, a graduate student working under Dietz at the Scripps Institute of Oceanography.
 * That guyots (flat-topped seamounts) were tilted toward and appeared be moving into the deep trenches. A guyot begins as a volcano on the ocean floor.  As the oceanic plate moves the volcano moves as well. Without the magma from the plume to fuel the volcano it becomes dormant. Over time the erosional forces from the atmosphere and the hydrosphere cause the volcano's top to become flat.
 * In 1953, during his Fulbright Fellowship in Japan, he analyzed the Japanese-made soundings of the North Pacific where he mapped and named the Emperor Seamount chain which trace northwest from Midway to the Kamchatka Trench.  He noted in 1953 to fellow scientist, Robert Fisher, over lunch,  that this line of seamounts 'must be on some sort of conveyor belt.'

 Harry Hammond Hess' Biography 

Harry Hess was born on May 27, 1906 in New York City. He received a Bachelor of Science degree from Yale in 1931 and a Ph.D. from Princeton in 1932. Upon graduating, Hess taught for one year at Rutgers University and then spent a year as a research associate in the Geophysical Laboratory of the Carnegie Institution of Washington. In 1934, Hess became a professor at Princeton University, where he became the head of the Geology department in 1950 and in 1964  the sixth Blair Professor of Geology. Harry Hess died of a heart attack on August 26, 1969 in Massachusetts.

 Harry Hess's Contributions 

Dr. Hess' most significant contribution to the plate tectonic theory began in 1945 when he was the commander of the U.S.S. Cape Johnson. While traveling from one destination to another, Hess would leave the sounding equipment on that would take measurements of the sea floor. It was at this time that Dr. Hess discovered features on the floor of the ocean that appeared to be mountains with the tops flattened. He called these features guyouts after the first Geology professor at Princeton.

In 1953, Maurice Ewing and Bruce Heezen discovered the Great Global Rift and inspired Hess to reexamine the data that he collected while at sea. History of Ocean Basins was published in 1962 and explained the mechanism behind Alfred Wegener's continental drift theory. In the paper Hess described how hot magma would rise from under the crust at the Great Global Rift. When the magma cooled, it would expand and push the tectonic plates apart. This theory addressed many unanswered questions in the field of geology. One such question was why no marine fossils found in the ocean were more than 180 million years old, but older marine fossils could be found on land. Another question that seemed to be explained my Hess' theory was why there was so little sediment deposited on the ocean floor, even though the ocean was 4 billion years old. Hess explained that new crust was created at the Great Global Rift and was pushed under the continental crust about 300 million years later where it would melt and turn into magma. This motion also explained the formation of the guyots that were found at the bottom of the ocean. Hess theorized that these were once volcanic mountains that were moved and eroded over time.

 Seafloor spreading Theorized 

Both Hess and Dietz theorized new crust formation at the mid-ocean ridges and subduction of older crust at the trenches; the whole sea floor was moving outward from the mid-ocean ridges there by widening the sea floor. Both stated that upwelling in the convection currents caused magma to reach the ocean bottom forming new rock and that downwelling of the convection currents dragged the sea floor downward at the trenches, thus holding the size of the Earth constant.

One important way in which Dietz's theory of sea floor spreading differed from Hess', was that Dietz correctly surmised that the lithosphere was moving on top of the plastic Asthenosphere with the convection currents, where as Hess suggested the the base of the crust was moving with currents. Dietz described the ocean floor as the "exposed and outcropping limbs of convection" (Dietz, 1961). Hess and Dietz also disagreed about the fracture zones that ran perpendicular to the ridges. Hess thought that fracture zones were unrelated to the mid-ocean ridges; Dietz, meanwhile believed that the fracture zones were a result of uneven convection motion in the mantle thereby moving parts of the ocean floor at different rates.

Theories

• Theory of plate tectonics or continental drift

• Seafloor spreading

 Seafloor Spreading 

Seafloor spreading, theory that oceanic crust forms along submarine mountainzones, known collectively as the mid-ocean ridge system, and spreads out laterally away from them. This idea played a pivotal role in the development of plate tectonics, a theory that revolutionized geologic thought during the last quarter of the 20th century.

Shortly after the conclusion of World War II, sonar-equipped vessels crisscrossed the oceans collecting ocean-depth profiles of the seafloor beneath them. The survey data was used to create three-dimensional reliefmaps of the ocean floor, and, by 1953, American oceanic cartographer Marie Tharp had created the first of several maps that revealed the presence of an underwater mountain range more than 16,000 km (10,000 miles) long in the Atlantic—the Mid-Atlantic Ridge.

The seafloor spreading hypothesis was proposed by the American geophysicist Harry H. Hess in 1960. On the basis of Tharp’s efforts and other new discoveries about the deep-ocean floor, Hess postulated that molten material from Earth’s mantle continuously wells up along the crests of the mid-ocean ridges that wind for nearly 80,000 km (50,000 miles) through all the world’s oceans. As the magma cools, it is pushed away from the flanks of the ridges. This spreading creates a successively younger ocean floor, and the flow of material is thought to bring about the migration, or drifting apart, of the continents. The continents bordering the Atlantic Ocean, for example, are believed to be moving away from the Mid-Atlantic Ridge at a rate of 1–2 cm (0.4–0.8 inch) per year, thus increasing the breadth of the ocean basin by twice that amount. Wherever continents are bordered by deep-sea trench systems, as in the Pacific Ocean, the ocean floor is plunged downward, underthrusting the continents and ultimately reentering and dissolving in Earth’s mantle, from which it had originated.

A veritable legion of evidence supports the seafloor spreading hypothesis. Studies conducted with thermal probes, for example, indicate that the heat flowthrough bottom sediments is generally comparable to that through the continents except over the mid-ocean ridges, where at some sites the heat flow measures three to four times the normal value. The anomalously high values are considered to reflect the intrusion of molten material near the crests of the ridges. Research has also revealed that the ridge crests are characterized by anomalously low seismic wave velocities, which can be attributed to thermal expansion and microfracturing associated with the upwelling magma.

https://youtu.be/k-_Z6p5cjKg