Theory of Plate Tectonics suggests that Earth's outer shell (lithosphere) is divided into several plates that glide over the Earth’s rocky inner layer above the soft core (mantle). The plates act like a hard and rigid shell compared to Earth's mantle. According to Theory of Plate Tectonics earth's outer crust lithosphere is divided into plates (seven major and seven minors) that move over the asthenosphere, the molten upper portion of the mantle. Plate Tectonics is responsible for various geomorphological processes such as folding, faulting etc. The theory of plate tectonics provides the necessary rationale for the distribution of oceans and continents around the globe. In this article, we will discuss the Theory of Plate Tectonics which will be helpful for UPSC/IAS exam preparation.

Theory of Plate Tectonics

• Plate tectonics theory explains the large-scale motions of the earth’s lithosphere. The term plate was first coined by JT Wilson in 1965.
• Though this theory was proposed by Harry H. Hess in 1962 still, it was explained scientifically by other important thinkers like Morgan, Mckenzie, Parker, and Holmes.
• It is considered the most complex and comprehensive hypothesis about the drift of continents and the expansion of sea floors, and it is an improvement over Wegener's continental drift theory.
• According to this theory, the crust of the earth is divided into several big and small fragments called plates. These lithospheric plates are about 100 km thick. These plates are floating over the semi-molten asthenosphere.
• Lithospheric plates (tectonic plates) range in size from small to significant, continental (Arabic plate) to oceanic (Pacific plate), and occasionally a combination of continental and oceanic plates (Indo-Australian plate).
• The oceanic plates are thinner and mostly made up of Simatic crust, whereas the continental plates are heavier and mostly made up of Sialic crust.
• The formation of various landforms and all earth motions are caused by the movement of these crustal plates (due to convection currents in the mantle).
• Convection currents created in the upper mantle are thought to be responsible for the movement of these plates.
• Seafloor spreading, volcanic eruptions, crustal deformation, mountain construction, and continental drift all occur along the plate borders.

Seven Major Plates

1. North American plate (with the western Atlantic floor separated from the South American plate along with the Caribbean islands)
2. South American plate (with western Atlantic floor separated from the North American plate along with the Caribbean islands)
3. Pacific plate (Largest plate)
4. Antarctica and the surrounding oceanic plate
5. Eurasia and the adjacent oceanic plate
6. Africa with the eastern Atlantic floor plate
7. India-Australia-New Zealand plate

Minor Plates

1. Caribbean Plate
2. Cocos Plate
3. Caroline Plate
4. Juan de Fuca Plate
5. Juan Fernandez micro Plate
6. Iranian Plate
7. South Sandwich Plate
8. Myanmar Plate
9. Anatolian Plate
10. Nazca Plate
11. Nubian Plate
12. Philippines Plate
13. Okhotsk Plate
14. Scotian Plate
15. Eastern micro Plate
16. Somalian Plate
17. Arabian Plate
18. Solomon Plate
19. Fiji Plate
20. Bismarck Plate

Rate of Plate Movements

• Harry H. Hess neither explains the features of ocean floors nor the movement of the continents.
• Tectonic plates move at rates that vary from less than 6 feet per 100 years to 66 feet per 100 years (1.83–20.1 m/100 years); and these rates may have been faster in the ancient past.
• At an average rate of 33 feet per 100 years (about 10 cm/year), a tectonic plate can move 62.5 miles (about 100 km) in 1 million years.

Types of Plate Boundaries

There are three types of plate boundaries:

Divergent Plate Boundaries

• Those plates that move away from each other consistently are called Divergent Plate Boundaries.
• This type of interaction results in the formation of mid-ocean ridges (such as the Mid-Atlantic Ridge).
• The basaltic magma erupts and separates here (seafloor spreading).
• The East African Rift Valley, produced by the divergence of the African and Somali plates, is the most important geomorphological feature on the continent.
• Divergent edges are places where the earth crust is formed (hence the name constructive edge), and volcanic earth formations are common along them.
• Along diverging margins, earthquakes (with a shallow focal) are prevalent.

Convergent Plate Boundaries

• When continental and oceanic plates collide, the oceanic plate, which is thinner and denser, is pushed aside by the continental plate, which is thicker and less dense.
• In a process known as "subduction," the oceanic plate is driven down into the mantle.
• The oceanic plate is thrust into higher temperature environments as it lowers.
• Materials in the subducting plate begin to approach their melting temperatures at a depth of about 100 miles (160 kilometres), and partial melting begins.
• The most active example of continental convergent plate boundaries is the Himalayan Mountain Range.
• Around 55 million years ago, India and Asia collided, forming the Himalayas, the world's largest mountain range.
• The Indian and Eurasian plates are colliding right now.

Transform Plate Boundaries

• Two plates slide past each other in this type of interaction, and no new landforms are created or destroyed; only the present landform is deformed.
• Transform faults are planes of separation in the oceans that are often perpendicular to the mid-oceanic ridges.
• The best example of a transcurrent edge on continents is the San Andreas Fault (Silicon Valley sits dangerously close to the faultline) on the western coast of the United States.

Evidence that Support Plate Tectonic Theory

Paleomagnetism

• The most important evidence is paleomagnetic rocks.
• The orientation of iron grains in earlier rocks is oriented to the existence of the South Pole, which was originally located somewhere between present-day Africa and Antarctica (polar wandering).

Older rocks form the continents while younger rocks are present on the ocean floor

• Rocks dating back up to 3.5 billion years have been discovered on continents, whereas the oldest rock discovered on the ocean floor is only 75 million years old (western part of Pacific floor).
• As we get closer to the ridges, we see even younger rocks.
• This indicates that the seafloor is spreading effectively along oceanic ridges, which are also plated borders.
• See floor spreading is virtually similar to plate tectonics except that it exclusively investigates the interaction between oceanic plates.

Gravitational anomalies

• The gravitational constant 'g' is weaker in trenches where subduction has occurred (convergent edge). This suggests that there has been a material loss.
• Gravity measurements surrounding the Indonesian islands, for example, have revealed that the oceanic trench bordering Indonesia is connected with huge gravity anomalies.

Earthquakes and Volcanoes

• The fact that all plate boundary zones are prone to earthquakes and volcanic eruptions support plate tectonics theory.

Significance of Plate Tectonics

• Plate tectonics is responsible for almost all important landforms.
• With magmatic eruptions, new minerals are thrown up from the core.
• Near the plate borders, economically valuable minerals such as copper and uranium can be discovered.
• The shape of landmasses in the future can be projected based on current knowledge of crustal plate movement.
• If current trends continue, North and South America, for example, will split. The east coast of Africa will be separated by a sliver of land. Australia will become more Asia-centric.

Conclusion

Plate tectonics is a grand unifying geoscience theory that explains how continents move. Most significant characteristics on Earth's surface, such as mountain construction, development of new lithosphere, consumption of old lithosphere, and mid-ocean ridges, are caused by earthquakes and volcanism.

FAQs

MCQs

1. Which layer of the Earth is divided into tectonic plates?

A. Core
B. Mantle
C. Lithosphere
D. Asthenosphere

Answer: (C) See the Explanation

2. At which type of plate boundary does sea-floor spreading occur?

A. Divergent boundary
B. Convergent boundary
C. Transform boundary
D. None of the above

Answer: (A) See the Explanation

3. What is the primary force driving plate movements?

A. Gravity
B. Convection currents
C. Magnetic fields
D. Wind

Answer: (B) See the Explanation

4. Which geological feature is formed at convergent plate boundaries involving two continental plates?

A. Rift valleys
B. Ocean trenches
C. Mountain ranges
D. Mid-ocean ridges

Answer: (C) See the Explanation

5. Which plate boundary is associated with earthquake activity along fault lines?

A. Convergent boundary
B. Divergent boundary
C. Transform boundary
D. All of the above

Answer: (C) See the Explanation

GS Mains Questions and Model Answers

Q1: Explain the mechanism of plate tectonics and how it has reshaped the Earth's surface over time.

Answer: The Plate Tectonics Theory describes the movement of Earth's lithosphere, which is divided into several tectonic plates. These plates float on the semi-fluid asthenosphere and move due to convection currents, ridge push, and slab pull. Over millions of years, these movements have led to the formation of major geological features such as mountain ranges, volcanic islands, and ocean basins. For example, the Himalayas were formed by the collision of the Indian and Eurasian plates. Plate tectonics also explain phenomena like earthquakes and volcanic eruptions, which occur at plate boundaries.

Q2: Discuss the significance of sea-floor spreading in the development of the Plate Tectonics Theory.

Answer: Sea-floor spreading, observed at mid-ocean ridges, provided critical evidence for the Plate Tectonics Theory. It showed that new oceanic crust is created as tectonic plates move apart, proving that Earth's surface is constantly being reshaped. The discovery of symmetrical patterns of magnetic striping on either side of mid-ocean ridges further supported this theory. Sea-floor spreading also validated earlier concepts like continental drift, proposed by Alfred Wegener, by demonstrating how continents could move over geological time.

Q3: Analyze the impact of plate tectonics on the distribution of earthquakes and volcanic activity around the world.

Answer: The movement of tectonic plates at their boundaries is the primary cause of earthquakes and volcanic activity. At convergent boundaries, where plates collide, subduction zones often generate intense seismic and volcanic activity, such as along the Pacific Ring of Fire. Transform boundaries, like the San Andreas Fault, are also major sources of earthquakes as plates slide past each other. Divergent boundaries, such as the Mid-Atlantic Ridge, produce volcanic activity as magma rises to create new oceanic crust. Thus, plate tectonics play a crucial role in shaping the planet’s seismic and volcanic landscape.

Previous Year Questions from Theory of Plate Tectonics

1. UPSC CSE Prelims 2018

Question: At which type of plate boundary is new oceanic crust formed?
A. Convergent boundary
B. Divergent boundary
C. Transform boundary
D. None of the above

Answer: B

Explanation: New oceanic crust is formed at divergent boundaries where plates move apart, allowing magma to rise and solidify, creating mid-ocean ridges.

2. UPSC CSE Mains 2020 (GS Paper 1)

Question: Discuss the role of plate tectonics in the formation of mountain ranges and ocean basins.

Explanation: Plate tectonics play a critical role in the formation of mountain ranges and ocean basins. At convergent boundaries, continental plates collide, forming towering mountain ranges like the Himalayas. In contrast, divergent boundaries create ocean basins through the process of sea-floor spreading at mid-ocean ridges. These dynamic processes demonstrate how Earth's surface is continually reshaped by the movement of tectonic plates.