Atolls have always fascinated me. Their stunning beauty is amazing, their shapes and sizes, locations. Inhabited ones, uninhabited ones. After having completed many dives at coral reef sites, I've yet to visit an atoll and see one up close above or below the sea. It's definitely on my bucket list!
They are generally formed as ring-shaped (although not always perfect rings as we'll soon see) coral reefs that encircle central lagoons. Predominantly located in tropical oceans, these landforms are essential not only for marine ecosystems but also for human communities in many regions. For centuries, scientists have studied how atolls form, with notable contributions from Charles Darwin, James Dwight Dana, John Murray, and Reginald Daly. Each theorist proposed mechanisms that consider geological, biological, and climatic factors. In this article, we delve into these four theories in detail, comparing their strengths and weaknesses and exploring their modern relevance.
Darwin’s Subsidence Theory
Charles Darwin's Subsidence Theory is the foundation of modern understanding of atoll formation. Darwin developed this theory during the HMS Beagle expedition in the early 19th century, publishing his findings in The Structure and Distribution of Coral Reefs (1842). His observations of tropical islands and reefs in the Pacific and Indian Oceans led him to propose a three-stage process of atoll development:
Fringing Reefs: Coral reefs initially form around volcanic islands, growing close to the shoreline in shallow, warm waters.
Barrier Reefs: Over time, the volcanic island begins to sink or subside due to tectonic activity. Coral growth keeps pace with the subsidence, forming a lagoon between the reef and the island.
Atolls: Eventually, the volcanic island disappears beneath the ocean, leaving a ring-shaped coral reef enclosing a central lagoon.
Darwin argued that the subsidence of volcanic islands is key to this process. Coral reefs thrive in shallow waters, and their ability to grow upward as the island sinks ensures the continuation of the reef structure. He also noted the limitations of coral growth to depths of about 50 metres, emphasising that subsidence must be gradual for atolls to form.
This theory was groundbreaking, as it explained how coral reefs could exist far from landmasses. However, it lacked empirical evidence in Darwin's time, leading to scepticism among some scientists. The confirmation came much later, with 20th-century drilling projects, such as those conducted on Bikini Atoll, which revealed volcanic rock beneath atolls, validating Darwin’s hypothesis.
Dana’s Expansion on Darwin’s Theory
James Dwight Dana, an American geologist and naturalist, supported Darwin's theory but introduced refinements that highlighted the role of environmental factors. In his mid-19th-century work, Dana emphasised the sensitivity of coral growth to ecological conditions such as temperature, light, salinity, and water clarity. His observations included the following:
Coral reefs grow best in warm, shallow, and clear tropical waters, typically between 25°C and 29°C.
The windward (ocean-facing) sides of islands often experience faster coral growth due to nutrient-rich currents and wave activity.
Coral reefs’ distribution is closely linked to the geographic locations of tropical oceans.
Dana's work provided a deeper ecological context for Darwin’s subsidence theory. He argued that the environmental conditions must be ideal for coral growth throughout the subsidence process. Dana’s contributions were particularly valuable in linking coral reef formation to broader climatic and oceanic systems.
While Dana’s refinements did not fundamentally alter Darwin’s theory, they underscored the importance of understanding coral biology and environmental dynamics. His work remains a cornerstone of coral reef ecology and its relationship to geological processes.
Source: https://www.govtgirlsekbalpur.com/Study_Materials/Geography/SEM4_GEOG_CC10_TOPIC9_CORAL_REEFS.pdf
Murray’s Solution Theory
In 1880, John Murray, a Scottish naturalist and participant in the Challenger Expedition, presented an alternative hypothesis to challenge Darwin’s subsidence theory. Murray proposed the Solution Theory, which rejected the idea of subsidence as a necessary condition for atoll formation. Instead, he argued that atolls could form through the accumulation of coral debris and sediment on submarine platforms or banks. His key points were:
Coral reefs grow in shallow water, eventually reaching the ocean surface.
Storms and wave action break coral structures, creating debris that accumulates on submerged platforms.
Erosion or uneven deposition leads to the development of a central lagoon within the reef.
Murray's theory focused on the mechanical processes of reef construction, driven by coral growth and the redistribution of coral fragments. He suggested that reefs could form on pre-existing geological structures without requiring the volcanic island to subside.
However, Murray’s theory faced significant criticism. It struggled to explain the presence of deep volcanic foundations beneath many atolls, as later revealed by drilling projects. Additionally, the theory did not account for the uniform distribution of atolls in tectonically active regions where subsidence is common. While it offered insights into reef-building processes, Murray’s hypothesis lacked the comprehensive explanatory power of Darwin’s subsidence theory.
Daly’s Glacial Control Theory
Reginald Daly, an American geologist, introduced the Glacial Control Theory in 1915, providing a new perspective on atoll formation by incorporating sea-level changes during glacial periods. Daly argued that:
During the last Ice Age, global sea levels were significantly lower (up to 120 metres lower than today) due to the vast amount of water locked in glaciers.
Coral reefs developed along the exposed coastlines of volcanic islands during this period of low sea levels.
As the glaciers melted and sea levels rose, the islands and reefs were submerged. Coral growth kept pace with the rising water, eventually forming atolls around the now-drowned islands.
Daly also suggested that wave erosion during low sea levels created flat-topped volcanic islands, facilitating the later development of lagoons. His theory effectively linked atoll formation to global climatic changes and sea-level fluctuations.
The glacial control theory gained considerable attention because it addressed processes that Darwin had not considered, particularly the impact of glaciation and deglaciation cycles. Modern studies of coral reef growth and sea-level changes have confirmed aspects of Daly’s theory, particularly the role of glacial cycles in shaping reef systems. However, the theory does not replace Darwin’s subsidence model; instead, it complements it by explaining how sea-level changes influence atoll formation.
Comparison of Theories
Each of the four theories offers unique insights into atoll formation, reflecting the evolving understanding of geological and ecological processes over time.
Theory | Proponent | Key Mechanism | Strengths | Weaknesses |
Subsidence Theory | Charles Darwin | Island subsides, coral grows upward | Supported by modern geological evidence; explains volcanic origins | Initial lack of empirical proof |
Ecological Focus Theory | James Dwight Dana | Subsidence with ecological factors | Highlights environmental influences | Builds on Darwin's framework rather than introducing new mechanisms |
Solution Theory | John Murray | Coral debris accumulation | Explains some reef-building processes | Fails to account for deep volcanic bases |
Glacial Control Theory | Reginald Daly | Sea-level changes during glaciation | Links atoll formation to global climatic shifts | Does not explain volcanic subsidence |
Modern Understanding of Atoll Formation
Advances in technology and research have confirmed that atoll formation is a complex process influenced by multiple factors, including subsidence, coral growth, sediment accumulation, and sea-level changes. Drilling projects, such as those at Bikini and Enewetak Atolls, have provided definitive evidence of volcanic foundations beneath atolls, confirming Darwin’s subsidence theory.
However, modern studies also validate aspects of Daly’s glacial control theory, particularly the impact of sea-level fluctuations during glacial and interglacial periods. Additionally, Dana’s focus on environmental factors remains relevant, as climate change and ocean warming now threaten coral reef ecosystems.
The interplay between these theories underscores the importance of integrating geological, ecological, and climatic perspectives to fully understand atoll formation.
Beauties to preserve...
For geographers, the study of atoll formation highlights the interconnectedness of Earth’s systems. As the challenges of climate change and rising sea levels intensify, preserving these unique ecosystems is more critical than ever.
Original books:
Darwin, C. (1842). The Structure and Distribution of Coral Reefs.
Dana, J.D. (1853). Observations on coral reefs and islands.
Murray, J. (1880). Coral formations and their deposition.
Daly, R.A. (1915). The Glacial Control of Coral Reefs.
Other References:
Woodroffe, C.D. (2008). Reef and Atoll Formation: A Review. Annual Review of Marine Science.
Nagel, G. & Guiness, P. (2016). Cambridge Internatinal AS & A Level Geography, 2nd Edition. Hodder Education
Extra resource: https://seos-project.eu/coralreefs/coralreefs-c01-p05.html
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