Post-drift Studies

The Big Idea

Wegener's continental drift theory was built almost entirely on evidence gathered from the continents — fossils, rock types, and glacial deposits like tillite. But the theory had a fatal weakness: it could not explain what force actually moved the continents. That gap in understanding is exactly what the post-drift studies tried to fill. After World War II, new tools and techniques allowed scientists to look beneath the oceans for the first time, and what they found transformed the entire debate.

Convectional Current Theory

In the 1930s, Arthur Holmes proposed that convection currents operating in the mantle could provide the missing mechanism for continental drift. These currents, he argued, are generated by radioactive elements within the Earth, which produce heat and create thermal differences in the mantle. Where the mantle is hotter, material rises; where it is cooler, material sinks. This sets up a continuous circulation system throughout the entire mantle.

Holmes argued that such a system of currents exists across the whole mantle portion, not just in isolated pockets. The rising currents would push the crust apart, carrying the continents along with them, while the sinking currents would pull material back down. This was a direct attempt to answer the question of force — the very issue on which contemporary scientists had rejected Wegener's theory. Holmes gave them a physically plausible engine for continental movement.

Mapping of the Ocean Floor

The most dramatic new information came from detailed research into ocean configuration. Before this period, the ocean floor was assumed to be a vast, featureless plain. The post-war expeditions revealed something entirely different: the ocean floor is full of relief. Submerged mountain ranges and deep trenches exist, and these features are mostly located closer to the continent margins.

Three findings from this mapping stood out. First, the mid-oceanic ridges were found to be the most volcanically active zones on the planet. Second, dating of rocks from the oceanic crust showed that they are much younger than the continental areas — the ocean floor is geologically young compared to the ancient continental cores. Third, and most striking, rocks on either side of the crest of an oceanic ridge, at equal distances from the crest, showed remarkable similarities in both their constituents and their age. The pattern was symmetrical, as if the seafloor had been spreading outward from the ridge.

Ocean Floor Configuration

The ocean floor can be divided into three major divisions based on depth and relief forms: continental margins, deep-sea basins, and mid-ocean ridges. (The full details of ocean floor relief are covered in Chapter 12.)

Abyssal Plains. These are extensive plains lying between the continental margins and the mid-oceanic ridges. They are the areas where continental sediments that move beyond the margins get deposited. In other words, as sediments are carried off the continents and across the margins, they settle here, building up thick layers on the deep ocean floor.

Mid-Oceanic Ridges. These form an interconnected chain of mountain systems within the ocean. Though submerged under oceanic waters, this is the longest mountain chain on the surface of the Earth. The ridge is characterised by three features along its entire length: a central rift system at the crest, a fractionated plateau, and a flank zone. The rift system at the crest is a zone of intense volcanic activity — these are the mid-oceanic volcanoes you were introduced to in the previous chapter.

Distribution of Earthquakes and Volcanoes

When you plot the locations of earthquakes and volcanoes on a map, a clear pattern emerges. A line of dots runs through the central parts of the Atlantic Ocean, almost parallel to the coastlines on either side. This line extends into the Indian Ocean, where it bifurcates a little south of the Indian subcontinent — one branch moves into East Africa, and the other meets a similar line running from Myanmar to New Guinea. This line of dots coincides exactly with the mid-oceanic ridges. …