Reading — Test 38

Reading passage

The Theory of Continental Drift Long before satellite imagery could confirm the shapes of the continents, observers noticed something curious about the world map: the eastern coastline of South America appeared to fit against the western coastline of Africa like two pieces of a puzzle. This observation, first noted informally by cartographers as early as the sixteenth century, remained a geographic curiosity for centuries. It was not until 1912 that the German meteorologist and geophysicist Alfred Wegener transformed this casual observation into a comprehensive scientific hypothesis. Wegener proposed that all the continents had once been joined together in a single supercontinent, which he called Pangaea, meaning "all lands" in Greek. According to his theory, Pangaea began to break apart roughly 200 million years ago, and the resulting fragments slowly drifted to their present positions, a process he termed continental drift. Wegener did not rely on coastline matching alone; he assembled evidence from multiple scientific disciplines to strengthen his case. He pointed to the distribution of fossilized plants and animals, noting that identical species of the ancient reptile Mesosaurus had been found in rock formations on both the eastern coast of South America and the western coast of Africa, despite the fact that this small freshwater creature could not plausibly have swum across an entire ocean. Similarly, fossils of the fern Glossopteris appeared across South America, Africa, India, Australia, and Antarctica, landmasses now separated by thousands of kilometers of ocean. Wegener also examined rock strata, observing that mountain ranges and distinctive layers of sedimentary rock on separated continents lined up when the landmasses were arranged as a single unit. Glacial striations, the scratches left by ancient ice sheets, provided further support: identical patterns appeared on continents now located in vastly different climate zones, suggesting they had once been positioned near a common polar region. Despite the breadth of this evidence, Wegener's hypothesis was met with considerable skepticism, and in some quarters outright hostility, from the geological establishment of his time. The central objection was mechanical rather than evidential: Wegener could not adequately explain what force might be powerful enough to move entire continents across the ocean floor. He speculated that tidal forces and the Earth's rotation might supply the necessary energy, but physicists quickly demonstrated that these forces were far too weak to shift landmasses of such immense size and mass. Without a credible mechanism, most geologists dismissed continental drift as an intriguing but fundamentally flawed idea, and Wegener, who died in 1930 during an expedition to Greenland, did not live to see his theory vindicated. Vindication arrived gradually, beginning in the 1950s and 1960s, through research that Wegener himself could never have anticipated. Studies of the ocean floor revealed the existence of massive underwater mountain chains, known as mid-ocean ridges, where new oceanic crust was continuously being formed through volcanic activity. Geophysicists also discovered that this newly formed rock preserved a record of the Earth's periodically reversing magnetic field, creating symmetrical bands of alternating magnetic orientation on either side of the ridges. This pattern, known as seafloor spreading, provided exactly the mechanism that had been missing: molten rock rising at the ridges pushed the ocean floor outward in both directions, carrying the continents along with it like passengers on a conveyor belt. These findings were soon incorporated into the broader framework of plate tectonics, which describes the Earth's rigid outer shell as a mosaic of interlocking plates that move gradually atop a more pliable layer beneath them. The eventual acceptance of continental drift, reframed within plate tectonic theory, stands as an instructive episode in the history of science. Wegener's core insight, that the continents had once been joined and had since moved apart, proved essentially correct, even though his proposed mechanism was wrong. His case illustrates that a hypothesis can be substantially right in its conclusions while being incomplete in its explanations, and that scientific consensus often requires not just a compelling pattern of evidence but also a plausible physical process to account for it. Only when both elements were finally in place did continental drift move from a marginalized speculation to a cornerstone of modern earth science.