The theory of continental drift, proposed by German meteorologist Alfred Wegener in the early 20th century, was a radical departure from the established scientific consensus of fixed continents. Wegener suggested that the Earth’s landmasses were not stationary but had slowly moved across the planet’s surface over geologic time. His idea centered on the existence of a single supercontinent, which he named Pangaea, meaning “all earth,” that began to break apart approximately 200 million years ago. This hypothesis challenged the prevailing belief that the continents and ocean basins were permanent features.
Wegener’s Hypothesis and Supporting Evidence
Wegener’s hypothesis was built upon four distinct lines of evidence that collectively pointed toward a unified landmass:
- Geographical fit: The coastlines of South America and Africa appeared to lock together like pieces of a jigsaw puzzle. This fit was most precise when using the edge of the continental shelf.
- Fossil evidence: Identical ancient plant and animal remains were found on continents now separated by vast oceans. For instance, fossils of the freshwater reptile Mesosaurus were discovered in both South America and Africa.
- Matching geology: Similar geological structures and rock types were found across the Atlantic. Mountain ranges, such as the Appalachians, aligned with geologically similar mountains in Greenland, Great Britain, and Norway.
- Paleoclimatic evidence: Glacial striations, or scratch marks left by massive ice sheets, were found in tropical regions of South America, Africa, India, and Australia, suggesting these landmasses were once clustered near the South Pole.
The Critical Flaws in Wegener’s Proposed Mechanism
Wegener’s theory faced widespread rejection because he failed to propose a scientifically plausible mechanism to drive the continents. Established geophysicists demanded a demonstrable force powerful enough to move enormous landmasses, and Wegener’s suggestions were insufficient. He hypothesized that continental movement was caused by a combination of two forces: Polflucht (the pole-fleeing force related to the Earth’s rotation) and tidal attraction from the Moon and Sun.
Physicists quickly demonstrated that these forces were orders of magnitude too weak to overcome the friction and inertia required to push continents. The prevailing view was that the Earth’s crust was composed of a lighter continental rock resting upon a denser oceanic floor. Wegener’s model required the continents to “plow” through this solid, denser oceanic crust, an idea that was widely considered physically impossible.
The scientific community insisted that without a credible mechanical explanation, the hypothesis was mere speculation, regardless of the circumstantial evidence. They preferred alternative explanations for the evidence, such as sunken land bridges to account for fossil distribution or a contracting Earth to explain mountain formation. This lack of a driving force meant the continental drift hypothesis was generally discarded by the 1930s.
The Paradigm Shift: Plate Tectonics
The core idea of continental mobility was ultimately vindicated decades later by post-World War II scientific discoveries. Advances in technology, such as sonar, allowed scientists to map the deep ocean floor, revealing an unexpected geography, including a massive, continuous system of underwater mountain ranges known as mid-ocean ridges.
Further research led to the concept of seafloor spreading, proposed by geologist Harry Hess, suggesting that new oceanic crust was continuously generated at these ridges and moved outward. Evidence came from paleomagnetism, the study of Earth’s ancient magnetic field, which revealed symmetrical “magnetic striping” on either side of the mid-ocean ridges. These stripes recorded the periodic reversals of the planet’s magnetic field, confirming that the seafloor was moving.
These discoveries provided the missing mechanical link that Wegener lacked, leading to the unified theory of Plate Tectonics. This modern theory explained that the Earth’s rigid outer layer, the lithosphere, is broken into large plates that move atop the softer, underlying mantle, driven by heat-induced mantle convection currents. The recognition that entire plates moved as coherent units finally resolved the scientific objections and confirmed Wegener’s original vision.