Plate Tectonics Theory

Theory of Plate Tectonics states that the earth’s outer crust i.e. 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.

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 100km thick. These plates are floating over the semi-molten asthenosphere.

3 types of Plate boundaries

1. divergent Plate boundary

• A divergent boundary occurs when two tectonic plates move away from each other.
• Along these boundaries, lava spews from long fissures and geysers spurt superheated water.
• Frequent earthquakes strike along the rift. Beneath the rift, magma—molten rock—rises from the mantle.
• It oozes up into the gap and hardens into solid rock, forming new crust on the torn edges of the plates.
• Magma from the mantle solidifies into basalt, a dark, dense rock that underlies the ocean floor.
• Thus at divergent boundaries, oceanic crust, made of basalt, is created.

Seafloor Spreading

• Seafloor spreading is the process of magma welling up in the rift as the old crust pulls itself in opposite directions.
• Cold seawater cools the magma, creating a new crust.
• The upward movement and eventual cooling of this magma has created high ridges on the ocean floor over millions of years.
• The East Pacific Rise is a site of major seafloor spreading in the Ring of Fire.
  • It is located on the divergent boundary of the Pacific Plate, the Cocos Plate (west of Central America), the Nazca Plate (west of South America), the North-American Plate and the Antarctic Plate.

Rift Valleys

• A rift valley is a lowland region that forms where Earth’s tectonic plates move apart, or rift.
• Rift valleys are found both on land and at the bottom of the ocean and are created by the process of seafloor spreading.
• Rift valleys differ from river valleys and glacial valleys in that they are created by tectonic activity and not the process of erosion.
• The Great Rift Valley System which stretches from the Middle East in the north to Mozambique in the south is a geologically active area.
  • It features volcanoes, hot springs, geysers, and frequent earthquakes.

2. Convergent Plate boundary 

• When two plates come together, it is known as a convergent boundary.
• The impact of the two colliding plates buckles the edge of one or both plates up into a rugged mountain range, and sometimes bends the other down into a deep seafloor trench.
• A chain of volcanoes often forms parallel to the boundary, to the mountain range, and to the trench.
• Powerful earthquakes shake a wide area on both sides of the boundary.
• If one of the colliding plates is topped with oceanic crust, it is forced down into the mantle where it begins to melt.
• Magma rises into and through the other plate, solidifying into new crust. Magma formed from melting plates solidifies into granite, a light colored, low-density rock that makes up the continents.
• Thus at convergent boundaries, continental crust, made of granite, is created, and oceanic crust is destroyed.
• Examples

• The Washington-Oregon coastline of the United States is an example of oceanic-continental convergent plate boundary.
  • • Here, the Juan de Fuca oceanic plate is subducted beneath the North American continental plate.
  • An example of the oceanic-oceanic convergence is the Mariana Trench, the deepest point on Earth.
    • The mighty Pacific plate subducts beneath the smaller, less-dense Philippine plate.
  • The Himalayan Mountain Range is the best active example of continental convergent plate boundaries.
    • India and Asia crashed about 55 million years ago, slowly giving rise to the Himalayas, the highest mountain system on Earth.
    • Here, the Indian and Eurasian plates are currently in collision.

3. Transform plate boundary

• Two plates sliding past each other forms a transform plate boundary.
• Natural or human-made structures that cross a transform boundary are offset—split into pieces and carried in opposite directions.
• Rocks that line the boundary are pulverized as the plates grind along, creating a linear fault valley or undersea canyon.
• As the plates alternately jam and jump against each other, earthquakes rattle through a wide boundary zone.
• In contrast to convergent and divergent boundaries, no magma is formed.
• Thus, crust is cracked and broken at transform margins, but is not created or destroyed.
• The San Andreas Fault in California is an example of a transform boundary, where the Pacific Plate moves northward past the North American Plate.
• It is one of the most active faults on the Ring of Fire.

Latest findings made in understanding Plate Tectonics

• Axial seamount = It refers to a live recording of volcano mountain. The volcano rising from Juan de fuca ridge demonstrates it. It supports the divergent movement.
• After 2012 Sumatra Indonesia earthquake in Indian ocean the Indo Australian plate broken into many plate. It was mainly due to slipping of plate in interpolated and hence the activation of Barren volcano happened.
• Zealandia:-It’s a new continent. It broke from Antarctica 100 million years and from Australia 80 million yrs ago. Its formation supports movement of plates.
• Heat from the base of the mantle contributes significantly to the strength of the flow of heat in the mantle and to the resultant plate tectonics. Buoyancy is created by heat rising up from deep within the Earth’s core.

Major tectonic plates

1. Antarctica and the surrounding oceanic plate
2. North American plate
3. South American plate
4. Pacific plate
5. India-Australia-New Zealand plate
6. Africa with the eastern Atlantic floor plate
7. Eurasia and the adjacent oceanic plate

Minor tectonic plates

1. Cocos plate: Between Central America and Pacific plate
2. Nazca plate: Between South America and Pacific plate
3. Arabian plate: Mostly the Saudi Arabian landmass
4. Philippine plate: Between the Asiatic and Pacific plate
5. Caroline plate: Between the Philippine and Indian plate (North of New Guinea)
6. Fuji plate: North-east of Australia
7. Turkish plate
8. Aegean plate (Mediterranean region)
9. Caribbean plate
10. Juan de Fuca plate (between Pacific and North American plates)
11. Iranian plate.

• There are many more minor plates other than the ones mentioned above.
• Most of these minor plates were formed due to stress created by converging major plates.
• Example: the Mediterranean Sea is divided into numerous minor plates due to the compressive force exerted by Eurasian and African plates.

Evidences that supports the Plate Tectonic Theory

Palaeomagnetism

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.