Plate Tectonics Theory asserts that the Earth is composed of several plates that slide over the mantle. While there are several small plates, nine are vast and major, namely- Pacific, Eurasian, Indo-Australian, Australian, African, Indian, North American, South American, and the Antarctic. The Pacific Plate, extending over approximately 64,000,000 square km, is the largest of all plates. Most of these plates are oceanic plates and are moving in the Northwest direction at the speed of 7 cm per year.
A Brief History
In 1620, Francis Bacon first noticed the “fit” or similar forms of coastlines of Africa and South America.
Over the centuries, many theories were proposed explaining the coastline fit. Philosophers and explorers of the 17th and 18th centuries suggested this to be a biblically catastrophic event or a result of eddy water currents.
However, with advances in sciences in the 19th century, a more focused and unified approach towards the seemingly unrelated geological, palaeontological, geochemical, and geophysical events was considered, and it was eventually established that the coastline fit was a result of “drift.”
In 1915, Alfred Wegner, a German meteorologist serving as an infantry reserve officer during World War 1, proposed the Continental Drift Theory. He opined that the continents floated over the mantle – a denser layer of earth sandwiched between continental crust and central core of the earth.
Wegner predicted that the mantle convection mechanism caused heat rise within the hot mantle creating currents of partially melted rocks that could move continents around Earth’s surface. Like many revolutionary theories of that time, his too was not initially accepted.
While serving as a Commander during World War 2, Harry Hess, a young geologist, made some remarkably interesting observations. His main responsibility as a geologist was to map the ocean floor using echo-sounding techniques. Based on his observations, he proposed a pioneering hypothesis in 1962 that proved crucial in the development of Plate Tectonic theory. Hess proffered that the oceanic crust is considerably younger than continental crust, being merely a few hundred million years old.
The Earth’s total surface area always remains constant. The molten magma upwells from the active mid-oceanic ridges, spreading laterally on both sides of the ridge forming new seafloor. The cycle completes once the ocean floor subducts into the earth at the ocean trenches.
This “recycling” process was later named “Seafloor Spreading.” It results in the movement of the continents as away from the ridges’ new ocean crust development.
His theory addressed a lot of age-old unanswered conundrums such as the movement of continents, sediment accumulations on the seafloor, and others. Hess backed Wegner’s theory of Continental Drift and clarified how once a supercontinent got broken up and eventually led to the formation of seven tectonic plates. Tectonic plates transport the continents that rest on them. Today, Seafloor Spreading stands as the parental hypothesis of Plate Tectonics.
Principle of Plate Tectonics
As depicted in the book “Plate Tectonics,” by Kent C. Condie,
Plate Tectonics is a unifying model that attempts to explain the origin of patterns of deformation in the crust, earthquake distribution, continental drift and mid-oceanic ridge as well as providing a mechanism for the Earth to cool.
Earth’s lithosphere is a rigid surface with an average thickness of 50-100 km and constitutes of large and small plates. The lithosphere rests and slides over a partially molten plastic layer of rocks, termed as the asthenosphere. Interactions between the boundaries of the lithosphere and the asthenosphere lead to the movement of plates.
And these interactions bring about divergence, convergence, and slippage of lithospheric plates. This shapes the terrestrial earth surfaces by processes such as earthquakes, orogeny, and volcanism. The interiors of plates are assumed to remain undeformed.
What drives the Plates?
The mechanism behind the motion of plates is controversial and remains to be understood entirely. However, as of today, the most widely accepted theory is that of mantle convection.
The heat generated in core and mantle is brought to the surface by mantle convection and made available for the plate motions. Now the question arises, in what manner is this thermal energy employed to drive lithospheric plates? Two models were proposed as an answer. a)The Classical or Mantle Drag model, and b)Edge-force Mechanism.
The Classical or Mantle Drag model
This model envisions plate motion as a response to the vicious ‘drag’ exerted on the base of the lithosphere. The lateral movement of the top of mantle convection cells in the asthenosphere causes the ‘drag.’
The convection cells rise beneath oceanic ridges and descend beneath trenches, creating a state of tension at ridges and compression at trenches. Based on this model, the cells would be expected to have a lateral length of about 3000 km, implying a relatively simple cell form. But, this makes it difficult to explain how ‘geometrically simple cells’ are driving plates with irregularly shaped margins?
The classical model failed to account for movements of small plates such as the Caribbean, which can hardly power their convection.
The Classical or Mantle drag model was eventually refuted as being the primary process in driving plate motions. However, Zeigler, in 1963, maintained that mantle drag might have proved to be an essential process in the breakup of Pangea.
Edge-Force Mechanism
This model proposed by Orowan (1969) assumes that the oceanic lithosphere represents the top of the convection system, and plates move in response to forces applied to its edges. He stated that only a small fraction of total mantle energy is made available and is sufficiently enough for driving the plates.
The uplift of ridge crest due to the hot asthenosphere beneath results in ridge push force provides a lateral push to the accreting oceanic lithosphere from the tail end. The downgoing slab’s negative buoyancy at the trench generates the slab-pull force. Phase changes in minerals due to increased pressure act as catalysts in this process.
The edge-force mechanism provides a reasonable explanation for many phenomena than mantle drag model, in particular,
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It is thermodynamically acceptable and effective in heat transportation from the mantle.
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It is consistent with observed patterns of intraplate stress.
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It is consistent with present plate motions.
Consequently, the edge-force mechanism model has been adopted by workers to explain plate motions today.
In recent years, seismic tomography and phase study changes in D” layer has given compelling pieces of evidence for mantle convection theory.
Effects of Plate Tectonics
Wilson Cycle:
Wilson cycle, named after J. Tuzo Wilson, is the cyclical opening and closing of ocean basins caused due to movement of plates. High-quality palaeomagnetic data for the past 600 million years has helped reconstruct the movement of continents over Earth’s history. From these reconstructions, Wilson proposed that an ocean basin has a comparatively short lifespan with several stages.
- The earliest stage, embryo stage, begins with uplift and rupture of continental areas forming rift valleys. This process not only splits the continent into two but also creates new divergent plate boundaries. (e.g., East African Rift system)
The rift valleys eventually evolve into spreading centers and strips of oceanic crust are emplaced on both sides of the rifted continental area. This portion of the oceanic basin losses its buoyancy and sinks below sea level. (e.g., Red Sea opening between NE Africa and Arabia)
- In the mature stage, the ocean basin continues to widen simultaneously with development along the continental shelves with continued production of hot oceanic crust and ridge system. (e.g., Atlantic Ocean)
- Eventually, the expanding system becomes unstable and moves apart from the ridge. The earliest oceanic lithosphere sinks back into the asthenosphere resulting in the formation of Wadati-Benioff Zone along with an oceanic trench subduction system, thus separating descending plate from associated island arcs (e.g., Andean type Volcanism).
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The Subduction Stage is marked by the onset of subduction at the boundary of the ocean.
- The ocean begins to contract once the rate of subduction exceeds the rate of formation of new crust at the constructive boundary. The collision of island arc complexes results in the formation of mountain ranges along the periphery of contracting oceans. This process marks the terminal stage of the cycle.
- The subduction of all the oceanic crust between continental masses marks the end-stage. The continent-continent collision results in the formation of active mountain belts such as The Himalayas and the plate boundary becomes temporarily inactive. However, the collision zones are zones of weakness and possess strong possibilities of becoming active rift zones in the future. And thus, the cycle continues.
From the study of palaeomagnetic reconstructions, one thing is specific that the cycle of supercontinent assembly, breakup, and subsequent reassembly takes about 500 million years to complete. And this is a constantly ongoing process happening even as you read this article.
Climatic changes:
Evidence of past climatic changes are trapped in the continental rocks, and these can be reassembled to study the ancient continental configuration. The rocks bore evidence of activities occurring on the earth’s surface.
It is quite evident from these assumptions that plate tectonics must have played a significant role in Earth’s local and global climates. One of the notable effects was the differences in the thermal properties of the ocean and land. Besides, mountain belts modified the rainfall patterns in regions, e.g., the development of rain shadow regions on the leeward side of mountain belts.
Ocean currents actively control global climates. E.g., NW Europe is warmer as compared to other regions at similar altitudes, mainly due to warm currents coming from Gulf Stream and North Atlantic Drift. Plate tectonics, and hence the geometry of oceans are the main factors governing the ocean currents.
Will Plate Tectonics Reshape Earth’s future?
Plate Tectonics is an active process and will keep reshaping Earth’s terrain for the next 60 million years to come. Recent events such as the formation of Zealandia, a mass of continental crust that submerged after breakup from the parent continental mass New Zealand or the tectonic activities in San Andreas fault, give clear evidence of constant unrest within Earth. In addition to this, many other events have been predicted by geologists in the coming future viz. Separation of Africa from Near East, Australia to eventually join Asia, Separation of a portion of California from North America, among others.
Conclusion
The theory of Plate Tectonics has reached such a level of scientific acceptance today that movement of plates is now being used to infer movement of hot spots concerning Earth’s rotational axis. Plate Tectonics has affected the lives of ordinary people as well. A large population in the world suffers from repercussions of geological hazards from time to time. Experts and the local community involved need to understand the tectonic settings in the region to plan resilience and disaster management activities.
Earth is a dynamic planet. The processes on and within Earth’s surface are constantly changing, and so are the theories explaining them. Though the theory of Plate Tectonics has made immense advancements since the 1960s, it too has its limitations and controversies. I am sure a lot of further research needs to go into it in the years to come until man completely understands Earth and its mechanisms one day.
Image Credits:
Featured image: Plate Tectonics- Mechanism and Effects
Tectonic activity map of the Earth
Pune, India
Trekker, Photographer, Traveller
M.Sc, Geology
It was quite insightful for a geologist too.
Thank you. I am glad you liked it Justin.