Plate Tectonic Theory- A Brief History of Plate Tectonic Theory

Plate Tectonic Theory

Plate tectonics, theory dealing with the dynamics of Earth’s outer shell—the lithosphere—that revolutionized Earth sciences by providing a uniform context for understanding mountain-building processes, volcanoes, and earthquakes as well as the evolution of Earth’s surface and reconstructing its past continents and oceans.
Plate Tectonic Theory
Plate Tectonic Theory


➽ Theory of Plate Tectonics
When the concept of seafloor spreading came along, scientists recognized that it was the mechanism to explain how continents could move around Earth’s surface. Like the scientists before us, we will now merge the ideas of continental drift and seafloor spreading into the theory of plate tectonics.

➽ The History of Plate Tectonics
Plate tectonics is a theory that was first proposed in the early 1900s by scientist Alfred Wegener, but was not said to be true until the 1960s.

When Alfred Wegener first proposed the theory of continental drift, he described the tectonic plates, or the continents, as puzzle pieces fitting together. He continued to explain his theory after realizing the Earth's surface changing significantly over time while noticing that continents which, at one point, were not separated, were. This was also noted by paleontologists when fossils of the same species were found on different continents, now separated. Their combined observations determined that the Earth's lithosphere had, in fact, been moving over time.


➽ How Plates Move
If seafloor spreading drives the plates, what drives seafloor spreading? Picture two convection cells side-by-side in the mantle, similar to the illustration in figure
1. Hot mantle from the two adjacent cells rises at the ridge axis, creating new ocean crust.
2. The top limb of the convection cell moves horizontally away from the ridge crest, as does the new seafloor.
3. The outer limbs of the convection cells plunge down into the deeper mantle, dragging oceanic crust as well. This takes place at the deep sea trenches.
4. The material sinks to the core and moves horizontally.
5. The material heats up and reaches the zone where it rises again.

➽ Evidence in Support of Plate Tectonics
1. Evidences for both See Floor Spreading and Plate tectonics are complimentary (almost same evidences).

2. Paleomagnetic rocks are the most important evidence. The orientation of iron grains on older rocks shows an orientation which points to the existence of the South Pole, once upon a time, somewhere between the present-day Africa and Antarctica (Paleomagnetism).

3. Older rocks form the continents while younger rocks are present on the ocean floor. On continents, rocks of upto 3.5 billion years old can be found while the oldest rock found on the ocean floor is not more than 75 million years old (western part of Pacific floor). As we move, towards ridges, still younger rocks appear. This points to an effective spread of sea floor (See floor spreading is almost similar to plate tectonics except that it examines the interaction between oceanic plates only) along oceanic ridges which are also the plate margins.

4. The normal temperature gradient on the sea floor is 9.4°C/300 m but near the ridges it becomes higher, indicating an upwelling of magmatic material from the mantle.

5. In trenches, where subduction has taken place (convergent edge), the value of gravitational constant ‘g’ is less. This indicates a loss of material. For instance, gravity measurements around the Indonesian islands have indicated that large gravity anomalies are associated with the oceanic trench bordering Indonesia.

6. The fact that all plate boundary regions are areas of earthquake and volcanic disturbances goes to prove the theory of plate tectonics.
Major Earth Plates


➽ Major Earth Plates – 7
Pacific Plate
North American Plate
Eurasian
African Plate
Antarctic Plate
Australian Plate
South American Plate

➽ Minor plates:
Carribbean Plate, Cocos Plate, Caroline Plate, Juan de Fuca Plate, Juan Fernandez micro Plate, Iranian Plate, South sandwich Plate, Myanmar Plate, Anatolian Plate, Nazca Plate, Nubian Plate, Philippines Plate, Okhotsk Plate, Scotian Plate, Eastern micro Plate, Somalian Plate, Arabian Plate, Solomon Plate, Fiji Plate, Bismarck Plate.

➽ Rates of Plate Movement
The rate of plate movement is determined by the bands of normal and reverse magnetic fields that parallel the mid-oceanic ridge. The rates of plate movement have a considerable variation. For example, while the Arctic Ridge has the slowest rate (less than 2.5 cm/yr), the East Pacific Rise in the South Pacific has the fastest rate (more than 15 cm/yr). An interesting fact is that the movement of Indian plate from south to equator was one of the fastest plate movements in history.

➽ Plate Boundaries
Plate boundaries are the edges where two plates meet. Most geologic activities, including volcanoes, earthquakes, and mountain building, take place at plate boundaries. How can two plates move relative to each other?

➧Divergent plate boundaries: the two plates move away from each other.
➧Convergent plate boundaries: the two plates move towards each other.
➧Transform plate boundaries: the two plates slip past each other.

The type of plate boundary and the type of crust found on each side of the boundary determines what sort of geologic activity will be found there.

a. Convergent Boundaries.
When two plates meet underwater and one or both are composed of oceanic crust. The plate which is denser is pushed underneath the less dense tectonic plate, which eventually melts and is destroyed completely.

➧Ocean-Continent
When oceanic crust converges with continental crust, the denser oceanic plate plunges beneath the continental plate. This process, called subduction, occurs at the oceanic trenches (figure 6). The entire region is known as a subduction zone. Subduction zones have a lot of intense earthquakes and volcanic eruptions. The subducting plate causes melting in the mantle. The magma rises and erupts, creating volcanoes. These coastal volcanic mountains are found in a line above the subducting plate (figure 7). The volcanoes are known as a continental arc.
Ocean-Continent
Ocean-Continent


➧Ocean-Ocean
When two oceanic plates converge, the older, denser plate will subduct into the mantle. An ocean trench marks the location where the plate is pushed down into the mantle. The line of volcanoes that grows on the upper oceanic plate is an island arc. Do you think earthquakes are common in these regions (figure)?
Ocean-Ocean
Ocean-Ocean


➧Continent-Continent
Continental plates are too buoyant to subduct. What happens to continental material when it collides? Since it has nowhere to go but up, this creates some of the world’s largest mountains ranges. Magma cannot penetrate this thick crust so there are no volcanoes, although the magma stays in the crust. Metamorphic rocks are common because of the stress the continental crust experiences. With enormous slabs of crust smashing together, continent-continent collisions bring on numerous and large earthquakes.
Continent-Continent


b. Divergent Boundaries.
Divergent boundaries are when two tectonic plates are moving away from one another. This process can occur underwater or on land

c. Transform Plate Boundaries
Transform plate boundaries are seen as transform faults, where two plates move past each other in opposite directions. Transform faults on continents bring massive earthquakes
California is very geologically active. What are the three major plate boundaries in or near California (figure)?
Continent-Continent


A transform plate boundary between the Pacific and North American plates creates the San Andreas Fault, the world’s most notorious transform fault.

Just offshore, a divergent plate boundary, Juan de Fuca ridge, creates the Juan de Fuca plate.

A convergent plate boundary between the Juan de Fuca oceanic plate and the North American continental plate creates the Cascades volcanoes.

➽ Criticism
1. Plate tectonics theory is unable to explain why subduction is limited to the Pacific coast while spreading is found in all the ocean. The length of spreading (ocean ridges) is far greatest than the subduction zone. The rate of construction is more than the rate of destruction.
2. The Benioff Zone (Ring of Fire) is not present equally in all probable places. For example, the intermediate and deep focus earthquakes are absent in North America.
3. There are certain mountain ranges, such as the Eastern highlands of Australia, Drekenburg mountain of South Africa and Sierra-Dalmar of Brazil which can not be related to plate tectonics.
Despite all these omissions and commissions plate tectonic is a revolutionary and comprehensive theory which scientifically explains the present distribution and arrangement of the continents and ocean basin. It also provides as satisfactory explanation of the distribution of volcanoes and earthquakes. It has also confirmed the theory of Continental Drift.

➽ Importance of the theory of Plate Tectonics
1. For geologists, it is a fundamental principle for study. It is the unifying theory of geology, which further explains large-scale geological phenomena, such as earthquakes, volcanoes, and the existence of ocean basins and continents.

2. Plate tectonics theory explains why there are lots of volcanoes in Iceland and Japan, but far fewer in Russia and Africa. This is because Iceland was created by a mid-oceanic ridge. Similarly, Japan is located on a fault line. The constant pressure around the fault line causes many earthquakes and volcanic eruptions.

3. For geographers, the theory of Plate tectonics aids in the interpretation of landforms. It ultimately explains why and where deformation of Earth’s surface occurs.

4. Further, the concept of plate tectonics explains mineralogy. New minerals pour up from the core along with the magmatic ejections. The plate boundaries are the pathways through which rocks from the mantle come out as deposits on lithosphere. These rocks are the source of many minerals. The famous Pacific Ring of fire known for its violent volcanic activity is also a ring of mineral deposits.

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