Geography · Ch 10 — Distribution of Oceans and Continents
Convectional Current Theory
Convectional Current Theory
The map is a set of five small world maps, each drawn in an oval, Mollweide-style projection with a grid of latitude and longitude lines. Each map shows the same ten numbered landmasses, but arranged differently. The numbers are consistent across all five panels: 1 is Africa, 2 is South America, 3 is Antarctica, 4 is Australia, 5 is India, 6 is China, 7 is North America, 8 is Europe, and 9 and 10 are two separate blocks of Siberia. The five panels are labelled with their ages: 540 million years ago, 420 million years ago, 300 million years ago, 120 million years ago, and the present day.
Reading the panels in order, you watch the continents move. At 540 million years ago, the landmasses are scattered, with no obvious single cluster. By 420 million years ago, some of them have begun to drift toward one another. At 300 million years ago, the arrangement is strikingly different: most of the numbered blocks have come together into a single, compact supercontinent — the classic Pangaea configuration, with Africa near the centre, South America and North America on one side, and India, Australia and Antarctica tucked together on the other. At 120 million years ago, that supercontinent has split apart, and the modern shapes are beginning to separate into the Atlantic and Indian Ocean basins. In the final panel, the present, the continents sit in their familiar positions, with India having moved northward to collide with Asia.
What the map teaches is the core visual evidence for continental drift: the same ten landmasses, recognisable by their outlines, rearrange themselves over hundreds of millions of years. The numbered labels let you track a single block — say, India — from a position near the southern continents at 300 million years ago to its present location, jammed against China and Siberia. The map does not show arrows or forces; it simply presents the positions at five instants, and the eye fills in the motion between them. …
The continental drift theory had a fatal weakness: it could explain that continents moved, but not why or how. Critics rightly asked what force could possibly push massive continental blocks across the Earth's surface. The Convectional Current Theory was an attempt to supply that missing mechanism.
Arthur Holmes, in the 1930s, proposed that convection currents operate in the mantle — the layer beneath the crust. These currents, he argued, are generated by radioactive elements present within the Earth. The decay of these elements produces heat, creating thermal differences in the mantle. Hotter material becomes less dense and rises; cooler material sinks. This sets up a continuous circulation — a convection current — throughout the entire mantle.
Holmes argued that there exists a system of such currents covering the whole mantle portion, not just isolated pockets. The significance of this idea was that it provided a physical force capable of moving continents. The rising limbs of these currents could push the crust apart, while the sinking limbs could drag material downward. This gave the drifting continents an engine.
The theory was a direct response to the objection that had led contemporary scientists to discard the continental drift theory — the lack of a credible force. By grounding continental movement in the physics of heat and convection, Holmes offered a plausible driving mechanism where none had existed before.
The Convectional Current Theory is not a rival to continental drift — it is the missing force that makes drift physically possible. The heat source is radioactive decay; the medium is the mantle; the result is a system of currents that can move continents.
Key points to remember:
- Proposed by: Arthur Holmes, in the 1930s. …