Convectional Current Theory, Meaning, Significance, Diagram

In the 1930s, Arthur Holmes discussed the possibility of convection currents in the mantle portion, generated by radioactive elements, causing thermal differences. The convective currents depend on two factors: first the thickness of the crust near the equator and the poles, and second being the uneven distribution of radioactive elements in the crust. 

Arthur Holmes proposed that a system of such currents exists throughout the entire mantle. This was an attempt to explain the force issue, which led contemporary scientists to reject the continental drift theory.

What is a Convection Current?

Convection currents are driven by heat; they occur in the air, ocean, and mantle due to temperature differences often caused by varying proximity to a heat source. 

• This temperature difference directly affects the density of the material, enabling the occurrence of convection currents in any non-solid substance. 
• Some convection currents, like those in the atmosphere, are relatively quick cycles, whereas others, such as those in the mantle, are very slow. 
• Governed by the principle that warm air rises and cool air sinks, convection currents are responsible for phenomena such as plate tectonics, thunderstorms, the formation of desert and tropical regions, and even the Earth's magnetic field.

Convection Current Theory

The convection current theory explains the movement of the Earth's tectonic plates through the action of convection currents in the mantle. These plates, which compose the Earth's crust, are in constant motion, moving approximately 2–3 cm per year. This movement has led to a significant alteration of the continents' distribution over millions of years.

Origin and Driving Force of Convective Currents

The convective currents within the Earth's mantle are mainly driven by temperature gradients. This differential heating is attributed to the radioactive decay of elements within the mantle, which acts as a continuous source of heat, overcoming concerns about the sufficiency of heat to drive these currents.

Movement and Heat Transfer of the Currents

There are mainly two currents associated with movement and heat transfer:

• Divergent convective currents: These currents arise when the mantle's heat causes the crust to stretch and eventually rupture, forming new crustal blocks that drift apart, creating ocean basins. This process directly involves the horizontal flow of thermal energy.
• Convergent Convection Currents: Conversely, when these currents move towards each other under continental masses, they generate compressive forces that lead to the formation of mountains and geosynclines.

Categorization of Currents

The theory categorises convective currents into rising and falling columns, addressing the metamorphism and density issues by implying that the density changes in geological materials (e.g., the transformation of amphibolites into denser eclogites) are integral to the sinking or rising within these columns. This process is essential for the recycling of crustal materials and the formation of new geological structures.

Distribution of Currents

This theory states that convective currents do not form uniformly across the mantle but originate at specific centres where conditions favour their development. This selective origination is important for explaining the various geological phenomena observed on Earth, such as the formation of mid-ocean ridges and continental drift

Cyclic Nature and Dynamic Processes

The cyclic nature of the convective processes, with periods of activity followed by dormancy, reflects a dynamic Earth system where convective currents wax and wane. This cyclic pattern facilitates the continuous reshaping of the Earth's surface, including the sedimentation in geosynclines and the orogenic (mountain-building) processes, thereby incorporating concerns about the accommodation of materials and the density-driven sinking of geological formations.

Convection Current Theory Significance

The convection current theory is crucial for various reasons, including providing a thorough explanation for the movement of the Earth's tectonic plates and the geological phenomena related to this movement.

• Plate tectonics: The theory explains the mechanism behind the movement of tectonic plates. By understanding convection currents, scientists can explain how and why continents drift, mountains form, and earthquakes occur.
• Predicting geological events: The knowledge of convection currents helps in predicting geological events such as volcanic eruptions and earthquakes. By studying the patterns of these currents, scientists can identify potential areas of risk.
• Explaining the formation of geological features: The theory aids in explaining the formation of various geological features on the Earth's surface, including mountain ranges, ocean trenches, and mid-ocean ridges.
• Study of the Earth's Interior: It provides insights into the processes occurring in the Earth's mantle and core, areas that are otherwise inaccessible for direct study.
• Climate and weather patterns: Convection currents in the mantle can influence the distribution of heat on the Earth's surface, indirectly affecting climate and weather patterns.

Convectional Current Theory Criticisms

Despite its novel methodology, the convectional current theory received much criticism for its fundamental assumptions, the source and sufficiency of heat to drive these currents, and the mechanics of the geological processes it sought to describe.

• Dependence on Unknown Factors: The theory is considered innovative but relies heavily on factors that were largely unknown at the time. The phenomena of rising and falling heat columns, which are central to the theory, were considered doubtful, making the theory's explanation of natural phenomena questionable.
• Questionable Heat Source: The theory assumes that heat generated by radioactive elements in the mantle is sufficient to drive convective currents. 
  • However, doubts were raised regarding the availability of enough heat from radioactive elements for this purpose. 
  • Additionally, the loss of heat through conduction as rising currents pass their heat into the crust could weaken the currents, further questioning the theory's viability.
• Doubts on Horizontal Flow: The horizontal flow of thermal convective currents beneath the continental and oceanic crusts was also questioned due to the speculated lack of sufficient heat. 
  • Without enough heat, the horizontal convergent movement of convective currents and the formation of falling columns (and thus mountains) would not be possible.
• Metamorphism and Density Issues: According to the concept, certain rocks convert into heavier varieties, which sink deeper into the earth. 
  • However, there is still debate over whether these rocks will get heavy enough to sink as expected. 
  • If they do not settle properly, they may obstruct the natural fluxes within the Earth that help move its outer layer, producing interruptions in the process.
• Selective Origination of Currents: The theory states that convective currents originate at a few centres under the crust, raising questions about why they don't originate everywhere. 
  • If currents originated at many centres, the crust would break into several blocks, causing widespread volcanic eruptions, which contradicts observations. 

Convection Current Theory UPSC PYQs

Question 1: What do you understand by the theory of continental drift? Discuss the prominent evidence in its support. (UPSC Mains 2013)

Convection Current Theory FAQ

Q1. What are convection currents and how do they occur?

Ans. Convection currents are fluid movements caused by temperature differences, often due to proximity to a heat source. In the Earth's mantle, they are driven by heat from radioactive decay, creating temperature gradients that move the mantle material.

Q2. How do convection currents contribute to plate tectonics?

Ans. Convection currents in the Earth's mantle drive the movement of tectonic plates by causing the crust to stretch, rupture, and form new crustal blocks that drift apart or converge, leading to geological phenomena such as continental drift, mountain formation, and the creation of ocean basins.

Q3. What is the significance of the Convectional Current Theory?

Ans. The Convectional Current Theory is crucial for understanding tectonic plate movement and related geological phenomena. It explains continental drift, mountain formation, and earthquakes, aiding in predicting geological events and studying the Earth's interior.