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A Short History of the Development of Plate Tectonics

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1 A Short History of the Development of Plate Tectonics

2 A Short History of the Development of Plate Tectonics
The Appearance of the Early Earth Plate Tectonic Theory was accomplished by the collection and interpretation of clues found by the multi-year investigative efforts of many individuals working in diverse science disciplines. The buttons to the right will introduce you to these steps. It is best to work through the sections sequentially. Within each section next and back buttons in the lower right corner provide navigation. At the end of each section the home button in the lower left will return you to this page and allow you to move on to the next section Continental Drift The Discovery of the Oceanic Ridge The Discovery of the Deep Sea Trench The Thickness of Deep Sea Sediments The Age-Dating of the Oceanic Crust Discovery of Magnetic Banding on the Ocean Bottom Seismic Evidence for Earth’s Internal Structure Convection Cells Within the Asthenosphere Plate Tectonics: The Final Solution Back

3 The Appearance of the Early Earth
From the very beginnings of earth science in the 16th century, it was taught that the present appearance of Earth in terms of the number, shape, and location of the continents was determined once water filled the ocean basins and did not change thereafter. However, by the 16th century, map makers were preparing maps of the continents that showed their shapes with a reasonable degree of accuracy, certainly with sufficient accuracy that individuals began to observe certain similarities in the shapes of continental margins; the most obvious being the similarity in the eastern coastline of South America and the western coastline of Africa. The outlines of the Atlantic coastlines of these two continents were so similar that the obvious question was soon to arise: “Were they once joined into a larger continent?” What do you think? Back

4 The answer would not come for about 300 years.
Does your opinion about the coastlines of South America and Africa fit in with the scientists ideas? Read on to find out! The answer from the geo-scientists of the day was that they were not, that any similarity was purely co-incidental. But questions persisted; could they have once been joined, broken apart and “drifted” to their present locations? The persistent questioning was put to rest by two questions posed by the earth-scientists: Where is a source of energy large enough to accomplish the task? What mechanism exists by which this source of energy could be applied to tear the supposed “super-continent” apart? The answer would not come for about 300 years. Back

5 Continental Drift If one person must be given credit for formulating our present understanding of the history of Earth’s crust it is Alfred Wegener. Wegener was a German climatologist who saw the same apparent fit of the continental margins of South America and Africa that had been discussed for hundreds of years. However, in 1915, Wegener took the idea several steps further by proposing that up until about 200 million years ago, all of the present continents were joined together into a super-continent he called Pangea. If Pangea existed, what happened to it? Back

6 What happened to Pangea? Where did today’s continents come from?
Wegener hypothesized that Pangea broke into pieces that drifted away from each other to create the continents that adorn the present face of Earth. He presented what we would consider today to be irrefutable evidence for Pangea. His ideas were resoundingly denied by the geologic community of the day and scientists comfortable in the status quo. Wegener’s real problem was that he could not come up with a scientifically-acceptable source of energy and a mechanism to explain HOW and WHY Pangea was ripped apart. Wegener was to die before he saw his ideas vindicated by answers obtained from oceanographers studying the ocean bottom. Back

7 The Discovery of the Oceanic Ridge
Until World War II, very little was known of the topography of the ocean bottom, primarily because there was no easy way to study it. The only depth measurements were scattered soundings taken with a weighted cable. Based on these data, with the exception of the occasional, equally unexplainable volcanic island, the ocean bottom was perceived as a near-perfectly flat surface that extended from one continent to the other. All this was to change. Back All this was to change.

8 Beginning in the early 1940’s the U. S
Beginning in the early 1940’s the U.S. Navy constructed sonar profiles of the Atlantic Ocean bottom. This investigation found that the deep ocean bottom occurred at an average depth of about 12,000 feet. This process also unveiled the mid-ocean ridge in the middle of the Atlantic Ocean. Back This process also unveiled a mountain range that ran the entire length of the Atlantic.

9 What clue might have revealed this fact?
For the most part the submarine mountain range remained about 2,000 feet below the ocean surface. However, a few islands, such as Iceland and the Azores, are actually summits of the few parts of the Atlantic ridge that rise above sea level. The range became known as an oceanic ridge. It was soon discovered that every ocean had an oceanic ridge and that they all interconnected for a total length of about 40,000 miles. In time it would be discovered that the oceanic ridges were volcanically active. What clue might have revealed this fact? Back

10 The sonar operators and oceanographers also took samples from the ocean floor. They were surprised to find that the most common rock they pulled up was basalt. Basalt is an igneous rock. Igneous rocks such as basalt and granite are a cooled version of molten rock we call magma and lava. The scientists had discovered that the ocean floor and the oceanic ridge were formed by eruptions of submarine volcanoes and fissures. But, more importantly, what was the explanation for the existence of the submarine volcanoes along the oceanic ridge? No one knew. Magma is molten rock below the ground surface or below the ocean surface. Lava is molten rock that is above the ground surface or land surface. This distinction is important because it provides a descriptive method of describing a rocks origin. For the most part, lava will cool faster than magma because the magma is insulated by the surrounding country rock. The difference in cooling rates is a significant factor in determining the size of the mineral crystals that are found in the cooled rock. Remember: Mineral = building blocks for making rocks. Definition: A mineral is a naturally occurring, inorganic element or compound having an orderly internal structure of atoms and characteristic elemental composition, crystal form, and physical properties. Rock = composed of minerals Back

11 The Discovery of the Deep Sea Trench
It wasn’t long before another deep-ocean bottom feature was discovered, this time in the Pacific: deep sea trenches. Again, with no explanation as to why they existed, certain observations seemed to hold true for the trenches: 1. They always paralleled the margin of the continent. 2. They were located either immediately offshore or a few hundred miles from shore. 3. They were always associated with a volcanic mountain chain. What could this mean? Back

12 Could they somehow be related?
Two more observations about the trenches allowed scientists to classify them into two broad groups (HINT: READ CAREFULLY!!). A. When a oceanic trench was immediately offshore, there was a volcanic mountain range on the continental margin. Examples of this would be South American Andes and the North American Cascades. B. If the oceanic trench was a few hundred miles off shore, there was NOT a volcanic mountain range on the continent. Instead, there was a volcanic mountain range that rose from the ocean bottom. These ranges appeared as chains of volcanic islands. Examples of these would be the Aleutians or the Japanese Islands. A B Even with these observations no reasonable explanation existed for the deep sea trenches and their relationship with different volcanic mountain ranges and volcanic island chains. Could they somehow be related? Read on. Back

13 The Thickness of Deep Sea Sediments
According to pre-1960s concepts, the rocks of the ocean floor were created during the primeval days of Earth history. Therefore, the ocean floor rocks should be about 4 billion years old. BUT THEY WEREN’T! THEY WERE MUCH YOUNGER!!!! If the ocean floor rocks were 4 billion years old the amount of fine sediments, in the form of microscopic remains of animals and atmospheric dust, that settled on top of them should have produced a very thick accumulation of deposits. When techniques were developed to measure the thickness it was found that they were very thin, NOT thick as hypothesized! Scientists were faced with two conclusions: they had over-estimated the sediment accumulation rate Or B. the ocean floor wasn’t as old as they thought. The scientists were very confident that they had not over-estimated the rate at which fine-grained materials were settling to the ocean bottom. The only alternative was that the rocks of the ocean bottom were much younger than they had thought. But how could this be? The answer was soon to come. Which of these choices, A or B, do you think is most plausible? Back

14 Age-Dating of the Oceanic Crust
Ocean-going research vessels such as the Glomar Challenger drilled into the basaltic oceanic crust at several places in the Atlantic Ocean. It came as a great shock to find that the oldest rocks in the Atlantic were only 200 million years old. Even more stunning was the fact that the seafloor basalt were older at the continental margins and younger at the summit of the oceanic ridge. Can you put some of the pieces of the puzzle together now? Click here to see if you’re on the right track. Back At this point, some scientists remembered Wegener’s hypothesis. Wegener had suggested that the Atlantic Ocean began to form 200,000,000 years ago as Pangea broke up. Could Wegener have been right all along? Continental drift was back into the news! Using the continental drift ideas of moving continents the observed variation in the age of the oceanic crust could be explained. New oceanic crust formed at the summit of the oceanic ridge while the older rocks moved away to make room for the new. There seemed to be little doubt that the Atlantic Ocean was opening…just as Wegener had said. But several more discoveries were to come that would confirm this “new” and radical idea.

15 The Discovery of Magnetic Banding on the Ocean Bottom THE NEXT CLUE!
We have seen evidence pointing to the opening of the Atlantic Ocean. Scientists also found that magnetic characteristics of the ocean floor basalt provided another important piece of information about the nature of the Earth’s crust. Click on the buttons to learn more about this important new evidence. Origin of magnetism in ocean floor basalt Implications of magnetic reversal Magnetic reversal Back

16 Magnetism in ocean floor basalt
Scientists studying a series of basaltic lava flows of the Columbia Plateau in eastern Oregon and Washington noticed grains of the mineral magnetite within the basalt. More importantly, they noticed that all of the grains seemed to be oriented magnetically in the same direction. How could this happen? Basalt is an iron-rich rock formed by the cooling of lava. As the lava cools, magnetite crystals (Fe3O4) form. As the temperature of the basalt cools below a temperature known as the Curie Point, the individual magnetite crystals develop a magnetic field. This field is oriented with Earth’s existing magnetic field. In other words, each magnetite crystal becomes a compass that allows the direction to the magnetic North Pole to be determined. More interesting to the scientists was the fact that their measurements showed that the orientation of the ocean floor magnetite grains changed from sample to sample! Back

17 Implication of Magnetic Reversals
In the early 1960s, oceanographers found symmetrical bands of magnetic intensity on opposite sides of the oceanic ridge. Based on the findings of the magnetite grain orientation study within basalt, this data could only be interpreted as a record of changing Earth magnetism! This data showed a symmetry of magnetic intensity on opposite sides of an oceanic ridge, We have already seen how scientists had proven that the age of oceanic crust increases away from the summit of the oceanic ridge. The magnetic banding data implied that the Atlantic Ocean must have opened as new oceanic crust formed at the summit of the oceanic ridge and then moved away from the ridge summit. There was no longer any doubt that Wegener was right. Pangea did exist. It did break up to form all of the present-day continents. The process also created the oceans basins. North Where did the energy come from and how was it applied to accomplish such a deed? The answer came from the seismologists. One huge question remained Back

18 What is “magnetic reversal?”
The data from consecutive layers of basalt showed that the orientation of the magnetic field switch back and forth with each particular orientation lasting for varying periods of time. What the scientists discovered was a series of “magnetic reversals” between individual layers of basalt. Apparently, for unknown reasons, Earth’s magnetic field switches polarity. That is, the north pole becomes the south pole and the south pole becomes the north pole. This transition is apparently quite fast, perhaps taking place within a few hundred or a few thousand years. Back

19 This plastic layer became the key to explaining how continents moved.
The Energy Dilemma: Seismic Evidence for Earth’s Internal Structure Reveals a Power Source During the early 1960s, seismologists studying earthquake seismic waves detected the layer configuration of the Earth’s crust, mantle, outer and inner core. They found that the crust and the upper most portion of the mantle actually acted together as a single brittle layer. They called this the lithosphere. More importantly, they discovered that under the lithosphere, a several hundred mile thick portion there existed a layer of rock with fluid-like properties. They called this layer the asthenosphere. Although the rocks were solid, they acted as though they were plastic. This plastic layer became the key to explaining how continents moved. Back

20 Convection Cells Within the Asthenosphere
Convection cells operating in the asthenosphere result in the creation of the forces of compression and tension within the rigid overlying lithosphere. Note that in the case of multiple convection cells, the location of the zones of tension and compression alternate. How would the rigid lithosphere be expected to react to these compressive and tensional forces? This understanding was the key to the development of the plate tectonic theory. Explore the effects of lithospheric compression and tension due to convection cells by clicking on the buttons below. TENSION COMPRESSION Back Next

21 long-sought source of energy?
With a greater understanding of the asthenosphere geologists recalled a theory presented in the 1920's. This idea proposed heat-driven convection cells within the mantle as a possible source of energy and mechanism to drive continental drift. It was rejected at the time because of the belief that all of the rocks above the core were rigid and brittle. Now geologists knew there the athenosphere reacted plastically. Could heat from Earth’s interior be the Let’s consider convection cells and how they operate. long-sought source of energy? Could convection cells within the asthenosphere be the equally long-sought mechanism to break-up Pangea? Back Next

22 How a Convection Cell Operates
Heat-driven convection cells are everywhere from a pot of boiling water to the highs and lows seen on the evening weather map. Because of its decreased density, hot masses rise to some upper level at which point they spread out laterally during which time, they cool. In time the mass has cooled to the point where increased density causes the mass to sink. As it approaches the source of heat the mass becomes reheated, the density decreases once more and the cycle is repeated. Such a single convection cell can be easily demonstrated by heating water in a beaker. Multiple heat sources will result in multiple convection cells with the only limit in the number of convection cells being the number of heat sources. A convection cell is a function of matter of energy (heat) and density! Hotter area (red) is less dense so it rises. Cooler area (blue) is more dense and sinks. Back

23 The Response of the Lithosphere to Tensional Forces
Think of the lithosphere as “Earth’s brittle outer layer.” With this in mind, we would expect it to break (because it is brittle) under both tensional and compressive forces. We also need to consider that the rising portion of a convection cell may occur beneath a continent or the ocean floor. Explore the sequence of features that develop in either circumstance by clicking on the appropriate button. RIFT ZONES RIFT VALLEYS LINEAR OCEAN OPENING OCEAN Back

24 Continental Rift Zones
As tensional forces begin to build within the continental lithosphere, cracks will begin to form at the base of the lithosphere and work their way toward the surface. Eventually, a linear zone of fractures will begin to form on the continental surface signaling that the continent is beginning to be ripped apart. At some point in time, rock at the top of the asthenosphere actually liquefies forming a mass of basaltic magma. Being lower in density than the surrounding rock, the basaltic magma begins to rise toward the surface along the fractures. If the magma reaches the surface it erupts in basaltic lava flows and in some cases, small cinder cone volcanoes. The Rio Grande Rift can be traced from Mexico to southern Colorado. Whether the presence of the Rio Grand Rift signals the breakup of North America remains to be seen. Back

25 RIFT VALLEYS Rift Valleys
Continued application of tensional forces will result in the conversion of some of the fractures into normal faults. These faults will result in the collapse of the rocks within the interior of the ever-widening rift zone. In time the depression of the surface will result in the conversion of the rift zone into a rift valley. Back

26 Linear Ocean The best example of a modern rift valley is the East African Rift Valley that extends from Ethiopia to Mozambique. The East African Rift Valley is the location of the headwaters of the Nile River, Lake Albert, Lake Victoria, and Mount Kilimanjaro. The East African Rift Valley is also an example of a linear sea in the making. In time, one end of a rift valley will reach the ocean and begin to fill with water. The northern end of the East African Rift Valley is already beginning to fill with water from the Red Sea. If the filling process were to continue to the point where the entire valley fills, the rift valley would be converted into a linear ocean. The Red Sea is a modern example of a linear ocean. Back

27 The Opening Ocean Eventually when the land-locked end of the linear ocean is breached (c), the linear ocean becomes an opening ocean (d-e) which will continue to open as long as the tensional forces continue to be applied. As the ocean opens, new oceanic crust is constantly being created along the summit of the oceanic ridge; the oceanic ridge can be looked upon as the birth scar of the ocean. The sequence of events from the initial formation of the rift zone to the creation of the opening ocean is referred to as divergence and the edges of the lithosphere are referred to as divergent margins. Back

28 The Response of the Lithosphere to Compressional Forces
Lithospheric tension is produced by the upward rising portion of the convection cell. It is important to remember that there is also a downward component to the convective motion. This portion of the cell is responsible for development of compressive forces. Explore the sequence of features that develop in either circumstance by clicking on the button below. ZONES OF SUBDUCTION Back

29 Subduction Up To 100’s Of Miles Off Shore Of The Continent
Zones of Subduction As the oceanic crust is compressed it achieves a brittle status and breaks. If compression forces remain active, one portion of the broken crust is forced down and under the other portion of the crust. This action is called subduction. The ultimate controlling factor for subduction are the density differences in rock that make up the crust. Subduction zones always parallel the continental margin and develop under two scenarios. Subduction and Density: Integration of an Important Science Concept Note that because of the higher density of the oceanic crust (3.0 versus 2.95) the oceanic lithosphere always subducts below the continental lithosphere, never the other way. Off Shore Subduction Subduction Up To 100’s Of Miles Off Shore Of The Continent Back

30 Subduction Just Off Shore
As shown in the illustration, a chain of volcanic mountains forms on land along the margin of the continent. Examples of this type of subduction zone and the mountains formed by it can be found in the Andes Mountains of South America and the Cascade Mountains of American Pacific Northwest. Back

31 Subduction Up to a Few Hundred Miles Off Shore
In the second scenario, the volcanoes form a chain of islands, usually volcanic. Examples are the Aleutian Islands and the string of islands along the western margin of the Pacific Ocean from the Japanese Islands to New Zealand. Back

32 We Have Finally Reached The Point Where The Idea Of
Tectonic Plates Was Born. Read On For The Final Installment On The Development Of The Plate Tectonic Theory. Tension Convergent Compression Divergent Tensional and compressive forces created by convection cells act upon the lithospheric crust causing subduction and divergence. Notice anything? Only oceanic crust is being subducted. Continental crust cannot be subducted. (WHY? Review density discussion) The breaking of the lithosphere along zones of subduction creates convergent plate margins. The breaking of the lithosphere along zones of divergence creates divergent plate margins. The processes of convergence and divergence break the lithosphere into pieces called plates. Back Next

33 The Lithospheric Plates
The lithosphere is broken into pieces called plates that fit together but at the same time are moving relative to each other. All told, there are about a dozen plates, some of which are very large while others are quite small. The “drifting” of the continents is the result of the continental crust being carried as a passive passenger on a plate. Note that the continent isn’t moving, the plate on which the continent resides is doing the moving. Note also as one ocean opens, another is closing. As an ocean closes, two continents approach each other and if the movement continues, the two continents may collide, weld together into a larger continent and create a mountain range in the act. As we speak, India is colliding with Asia. The collision actually started about 40 million years ago as part of the breakup of Pangea. The presence of earthquakes along the trend of the mountains indicates that the collision has not yet come to an end. It takes a lot to stop a continent. Wegener’s Pangea was a super-continent created by the collision of a number of smaller continents and the present continents were created as Pangea broke up and the Atlantic, Indian, and Arctic oceans were created as the continents “drifted” apart. This creation and movement of the lithospheric plates is the core of the most significant theory ever set forth in the 250-year history of the science of geology, namely, the Theory of Plate Tectonics. The Theory of plate tectonics has explained more geologic processes than any other single idea from the distribution of volcanoes and earthquakes to the similarity of continental margins. We now know that Wegener was right about Pangea and that the current continents are indeed fragments of a super-continent that existed up until about 200 million years ago. However, Wegener’s hypothesis lacked the mechanism required to actually move the plates. Back

34 The Lithospheric Plates Wegener’s idea about Pangea was correct!!
After all of this exploration, discovery, and problem solving, geologists finally realized that Earth’s Lithosphere in not solid, static, and immobile. Instead, it is broken into plates that fit together but at the same time are moving relative to each other. Wegener’s idea about Pangea was correct!! Once the mechanism for making it work was discovered Plate Tectonics became a useful and unifying theory! Pangea can now be explained as a super-continent created by the collision of a number of smaller continents forced together by the compressive convective cell mechanism. Pangea broke up and the Atlantic, Indian, and Arctic oceans were created by the tensional forces making continents “drift” apart. Back

35 The Theory of Plate Tectonics has enabled Earth scientists to provide
rational and scientific explanations for the location and processes of volcanoes, earthquakes, the location and formation of mountain ranges, ocean basins, etc. All of this has occurred since the mid-1960’s! There is much more to investigate and learn about our dynamic Earth. Perhaps you might be the one to unveil new understandings of our Earth. Back

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