Plate Tectonics Theory - Plate Boundaries and Associate Landform Features: Geography UPSC

The plate tectonic theory is a fundamental concept in the field of geology that explains the movement and interaction of the Earth's lithospheric plates. It is the overarching framework used to understand various geological phenomena, including the formation of continents, ocean basins, mountains, earthquakes, and volcanic activity. This theory was developed over many decades and has significantly contributed to our understanding of the Earth's geological processes.

 

The term "plate tectonics" was initially coined by Tuzo Wilson, a researcher at the University of Toronto. However, the theory of plate tectonics was formally introduced to the scientific community by W.J. Morgan of Princeton University in 1962. Plate tectonics is a scientific framework that explains the extensive movements of seven major plates and numerous smaller plates within the Earth's lithosphere over hundreds of millions of years. This theoretical model is built upon the earlier concept of continental drift, which had been developed in the early 20th century. The geo-scientific community widely embraced the theory of plate tectonics following the confirmation of seafloor spreading in the late 1950s and early 1960s.

 

 BASIC IDEA  about Plate Techtonics 

 

Plate tectonics theory is a unified theory of geology that explains the large-scale motions of tectonic plates and the resulting geological phenomena. The theory was developed in the 1960s and is now widely accepted by the scientific community.

 

Plate tectonics theory is based on two key hypotheses:

❏      Arthur Holmes' convection current hypothesis: This hypothesis states that the Earth's mantle is constantly moving due to convection currents. Convection currents are caused by the heating of the Earth's core, which causes the mantle to rise and then cool and sink back down.

❏      The concept of seafloor spreading: This concept states that the ocean floor is constantly being created at mid-ocean ridges and destroyed at convergent boundaries.

Lithosphere, Asthenosphere and Convection Current (Source: Britannica)

 

Plate tectonics theory explains a wide range of geological phenomena, including:

Ø  Earthquakes

Ø  Volcanic eruptions

Ø  Mountain building

Ø  Ocean trench formation

Ø  The distribution of fossils and rocks around the world

 

The theory also explains why the Earth's continents are constantly moving and changing shape.

Plate tectonics theory is one of the most important scientific theories of our time. It has revolutionized our understanding of the Earth's dynamic processes.

 

The Earth's Interior According to Mechanical Rigidity

 

The Earth's interior can be classified into three layers based on their mechanical rigidity:

❏      Lithosphere: The lithosphere is the rigid outer shell of the Earth, consisting of the crust and upper mantle. It is about 100 kilometers thick and is less dense than the material below it, so it "floats" on the asthenosphere.

❏      Asthenosphere: The asthenosphere is a plastic layer below the lithosphere. It is about 200 kilometers thick and is able to flow over time. The lithospheric plates float on the asthenosphere.

❏      Mesosphere: The mesosphere is the solid lower mantle. It is about 2,800 kilometers thick and is more rigid than the asthenosphere.

 

The theory of plate tectonics rejects the ideas of SIAL and SIMA, which were earlier classifications of the Earth's crust based on their chemical composition.

 

 Principles of Plate Tectonics 

 

Here is a more detailed explanation of the key principles:

 

◍       Tectonic plates: The tectonic plates are the rigid outer shell of the Earth, consisting of the crust and upper mantle. There are seven major tectonic plates and many minor plates.

◍       Asthenosphere: The asthenosphere is a plastic layer below the lithosphere. It is able to flow over time, which allows the tectonic plates to move.

◍      Convection currents: Convection currents are caused by the heating of the Earth's core. The hot mantle rises to the surface, cools, and then sinks back down. This creates a cycle of rising and sinking mantle material, which drags the tectonic plates along with it.

◍      Plate margins: The margins of the plates are where most geological activity occurs. When two plates collide, one plate may be forced under the other, a process known as subduction. This can lead to the formation of mountains, volcanoes, and earthquakes. When two plates move apart, a new crust is formed. This can lead to the formation of ocean ridges and rift valleys.

 

 Major and Minor Plates  

 Major Tectonic Plates 

 

1. Antarctica and the surrounding oceanic plate – (Surrounded by divergent boundaries.)
2. North American plate – (shifting westwards, velocity 4-5 cm/year. It is half oceanic—half continental)
3. South American plate – (shifting westwards, Half continental — half oceanic. 3-4 cm/year)
4. Pacific plate – (Truly oceanic plate, Shifting NW 2- 3cm/year)
5. India-Australia-New Zealand plate
6. Africa with the eastern Atlantic floor plate
7. Eurasia and the adjacent oceanic plate – (mostly continental, shifting eastwards. Velocity -2-3cm/year)

 

 Minor Tectonic Plates 

 

1. Cocos Plate
2. Nazca Plate
3. Persian Plate
4. Caribbean Plate
5. China Plate
6. Arabian Plate: Mostly the Saudi Arabian landmass
7. Bismark Plate (North Bismarck Plate & South Bismarck Plate)
8. Carolina Plate [located north of New Guinea]
9. Juan de Fuca Plate (between Pacific and North American plates)
10. Philippine Plate: Between the Asiatic and Pacific plate
11. Anatolian Plate or the Turkish Plate is a continental tectonic plate comprising most of the Anatolia (Asia Minor) peninsula (and the country of Turkey)
12. Fiji Plate (located b/w the Pacific Plate and the Indo-Australia Plate)

 

 Plate Motion and Boundaries 

 

There are three types of plate motions are observed along the plate boundaries:

1. Divergent or Separation or Constructive Plate Margins
2. Convergent or Closing together or Destructive Plate Margins
3. Transform or conservative plate margin

 

Different types of Plate motion and boundaries (Source: Science Facts)

 Divergent or Constructive Plate Boundary 

◍       Divergent boundaries are where two tectonic plates move away from each other.

◍       Along these boundaries, lava spews from long fissures and geysers spurt superheated water.

◍       Earthquakes are also common along divergent boundaries.

◍       Beneath the divergent boundary, magma, or molten rock, rises from the mantle.

◍       The magma oozes up into the gap and hardens into solid rock, forming a new crust on the torn edges of the plates.

◍       The magma from the mantle solidifies into basalt, a dark, dense rock that underlies the ocean floor.

◍       For example, the Great Rift Valley in Africa is a divergent boundary. The African Plate is splitting in two, and a new ocean is slowly forming.

 

Features or Landforms:  Divergent boundaries are important because they play a role in many geological processes, including earthquakes, volcanoes, and the formation of new oceans, Mid-oceanic ridges, and Rift Valley.

 

 Convergent or Destructive Plate Boundary 

◍       Convergent boundaries are where two tectonic plates come together.

◍       When two plates collide, one plate may be forced under the other, a process known as subduction.

◍       This can lead to the formation of mountains, volcanoes, and earthquakes.

◍       The subducting plate is usually made of denser oceanic crust, which sinks below the lighter continental crust.

◍       As the oceanic crust sinks, it melts and releases water vapour. This water vapour rises and weakens the continental crust, causing it to buckle and fold. This process forms mountains.

◍       Magma from the melting oceanic crust also rises to the surface, forming volcanoes.

◍       Earthquakes are common along convergent boundaries because of the movement of the plates and the release of water vapour.

 

Examples: The Pacific Ring of Fire, which is a horseshoe-shaped belt of mountains and volcanoes around the Pacific Ocean. The Himalayas, which was formed by the collision of the Indian Plate and the Eurasian Plate

 

Landform features associated with convergent boundaries: Mountains, Volcanoes, Ocean trenches/Subduction zone, Accretionary wedges/prism, Fold belts

 

The three types of convergent boundaries are –

 

1. Oceanic–continental convergence,
2. Oceanic–oceanic convergence,
3. Continental–continental convergence

 

 Oceanic-Continental Convergence 

◍       When oceanic crust collides with a continent, the denser oceanic crust is forced under the continent in a process called subduction. The oceanic plate sinks into the mantle, which is the layer of hot, flowing rock beneath the crust.

◍       The denser oceanic crust sinks under the continent. As the oceanic crust sinks, it melts and releases water vapour. This water vapour rises and weakens the continental crust, causing it to buckle and fold. This process forms mountains.

◍       Magma from the melting oceanic crust also rises to the surface, forming volcanoes. Earthquakes are common along convergent boundaries because of the movement of the plates and the release of water vapour.

◍       Oceanic-continental convergence is an important process in plate tectonics. It is responsible for the formation of mountains, volcanoes, and earthquakes.

Pacific Ring of Fire along the Oceanic-Continent Plate Margins (Source: iStock)

 

 Oceanic-Oceanic Convergence 

◍       When two oceanic plates collide, the denser plate sinks under the less dense plate. This is called subduction. As the plate sinks deeper into the Earth, it releases water vapour. This water vapour rises and weakens the overlying plate, causing it to buckle and fold. This process forms an oceanic trench.

◍       Magma from the melting oceanic crust also rises to the surface, forming volcanoes. Over time, a chain of volcanic islands forms called a volcanic island arc.

◍       Examples of volcanic island arcs include the Aleutian Islands and the Mariana Islands. With time, volcanic island arcs can become more mature island arc systems, such as in Japan and the islands of Sumatra and Java in Indonesia.

 

 Continental-Continental Convergence 

◍       Continental-continental convergence is when two continental plates collide. The continental crust is too buoyant to subduct, so the plates are forced into the air, forming mountains instead. The Himalayan Mountains are a classic example of continental-continental convergence.

◍       Continental-continental convergence is responsible for some of the tallest and most beautiful mountains on Earth. It is also a major cause of earthquakes and volcanic eruptions. The Alps, the Andes, and the Rocky Mountains are all examples of mountains formed by continental-continental convergence.

 Transform Fault Boundaries 

◍       A transform fault boundary is a type of plate boundary where two plates slide past each other horizontally. There is no creation or destruction of landform at a transform fault boundary, but the existing landform can be deformed.

◍       Transform faults are often found in oceans, where they connect segments of mid-ocean ridges. They can also be found on continents.

Some examples of transform faults include:

◍       The San Andreas Fault in California, USA

◍       The North Anatolian Fault in Turkey

◍       The Dead Sea Fault in the Middle East

Transform faults can cause earthquakes, as the plates slide past each other. They can also cause landslides and other types of ground deformation.

 

 Significance of Plate Tectonics: 

 

Here are some important aspects and natural events related to plate tectonics.

• Plate tectonics is responsible for the formation of many of the Earth's major landforms, including mountains, volcanoes, and ocean trenches.

When two plates collide, one plate may be forced under the other, a process known as subduction. This can lead to the formation of mountains. Volcanoes are also formed at plate boundaries, where magma rises to the surface and erupts. Ocean trenches are formed when one plate subducts beneath another.

• Plate tectonics plays a role in the formation of mineral deposits.

When plates collide, they can force mineral-rich rocks to the surface. This is why many economically valuable minerals, such as copper and uranium, are found near plate boundaries.

• By studying how plate tectonics works, scientists can predict how the Earth's land masses will change in the future.

For example, scientists predict that North and South America will eventually separate and that a piece of land will separate from the east coast of Africa. Australia is also moving closer to Asia.

In addition to the above points, plate tectonics is also important for the following reasons:

• It helps us to understand the Earth's climate and weather patterns.
• It is essential for managing natural hazards, such as earthquakes and volcanic eruptions.
• It can help us to develop sustainable resources, such as geothermal energy.

Plate tectonics is one of the most important scientific theories of our time. It has revolutionized our understanding of the Earth's dynamic processes.