Question

Passage:
During the nineteenth and early twentieth centuries, the geological mechanisms responsible for the formation of atolls—ring-shaped coral reefs enclosing a lagoon—were the subject of intense scientific debate. Charles Darwin’s subsidence theory, formulated during his voyage on the *Beagle* in the 1830s, long dominated the discourse. Darwin posited that coral reefs initially form as fringing reefs around a volcanic island. As the island gradually subsides into the ocean due to tectonic forces, the coral continues to grow upward at a pace matching the subsidence, eventually forming a barrier reef and, once the island is completely submerged, an atoll. This elegant model relied heavily on the assumption of widespread, slow tectonic subsidence across the Pacific basin, a premise that, while theoretically sound, initially lacked empirical verification.

In the late nineteenth century, oceanographer John Murray challenged Darwin’s model. Drawing on data from the *Challenger* expedition, Murray proposed that atolls could form on stationary submarine plateaus or volcanic peaks that had been eroded by wave action or built up by the accumulation of pelagic sediment. According to Murray, once these submarine platforms reached the photic zone—the shallow depth where sunlight penetrates—corals would colonize them. The characteristic ring shape, he argued, resulted from the fact that corals on the outer edge of the reef grew more vigorously due to greater access to nutrient-rich ocean currents, while the inner corals died and dissolved, forming the central lagoon. Thus, Murray’s model entirely circumvented the need for tectonic subsidence.

A third major hypothesis, the glacial control theory, was advanced by Reginald Daly in the early twentieth century. Daly recognized that the Pleistocene ice ages had caused massive fluctuations in global sea levels. He argued that during glacial maximums, when sea levels were drastically lowered, pre-existing volcanic islands were subjected to intense wave erosion, which truncated their summits and created flat, shallow platforms. When the glaciers subsequently melted and sea levels rose, corals recolonized these platforms, growing upward to keep pace with the rising waters. Like Murray, Daly provided an alternative to Darwinian subsidence, but he attributed the critical changes in water depth to eustatic sea-level fluctuations rather than tectonic movement.

It was not until the mid-twentieth century that deep drilling on Enewetak Atoll finally provided conclusive evidence. The drilling revealed over a thousand meters of shallow-water coral limestone resting directly upon a volcanic basalt foundation. Because reef-building corals cannot survive at such extreme depths, the presence of shallow-water coral fossils far below the modern sea level demonstrated unequivocally that the volcanic foundation must have sunk over millions of years. This finding resoundingly validated the core mechanism of Darwin’s subsidence theory, although modern geologists recognize that Daly’s glacial sea-level fluctuations also played a significant role in shaping the modern surface morphology of atolls.

According to the passage, John Murray's theory posits that the central lagoon of an atoll is formed primarily by which of the following?