Theories of Origin of Coral Reefs | UPSC

• Various theories have been proposed to explain how coral reefs formed. These theories take into account the fluctuation of sea level during the Pleistocene period and the stability of the land on which corals grow. The land could either be sinking (subsiding), staying put (stationary), or rising (emerging) — and each of these conditions would produce a different type of reef.
• Out of the three main types of reefs, the fringing reef is the simplest to explain. Corals in the past attached themselves to suitable underwater structures within about 30 fathoms (roughly 50 metres) of depth. They grew upward until they hit the low tide level — after that, upward growth stopped because coral polyps cannot survive long exposure to air. But the outward growth towards the sea continued.
• Waves eroded some of this reef material and deposited it on the reef surface, building it up further. So a fringing reef is basically corals growing outward from a stable shoreline.
• The origin of the other two types — barrier reefs and atolls — is much harder to explain. That is why different scientists came up with different theories.

All the theories of reef formation can be broadly put into two groups:

1. Subsidence theories — reef formation happens because the land is sinking
2. Non-subsidence theories — reef formation can happen without the land sinking

Darwin’s Subsidence Theory

• This theory was put forward by Charles Darwin in 1837 and modified in 1842. He came up with it during his famous voyage on HMS Beagle, when he realised that coral polyps can only grow in shallow waters.
• Darwin’s idea was straightforward. First, coral polyps gather on a suitable underwater platform and grow upward toward the low water level. At this stage, you have a fringing reef — corals growing along the shore of an island.
• But then — and this is the key part — Darwin assumed that the seafloor and the island itself started sinking (subsiding). As the island sank, the living corals found themselves in deeper water. To survive, they had to keep growing upward and outward to stay in the shallow zone where sunlight reaches.
• So Darwin proposed that fringing reef → barrier reef → atoll are three stages in the life of a single reef. As the island keeps sinking:

1. First, a shallow lagoon forms between the reef and the shore as the corals grow outward while the island sinks — this creates a fringing reef with a lagoon.
2. More subsidence → the lagoon gets wider and deeper → you now have a barrier reef separated from the island by a proper lagoon.
3. Even more subsidence → the island sinks completely below the water → all you are left with is a ring of coral (the atoll) enclosing a lagoon. The original island is gone.

• Despite the island continuing to sink, Darwin said the lagoon stays shallow because sediment from the subsiding land keeps getting deposited on the lagoon floor. That is why lagoons are flat and shallow.
• The theory is simple and elegant. But it has one big implication — it means barrier reefs and atolls can only exist in areas where the land is subsiding. And the huge thickness of coral material in some reefs can only be explained if the land has been sinking for a very long time while corals kept growing upward to keep pace.

Evidence in Support of the Theory:

• There is solid evidence of subsidence in many coral areas. Submerged valleys have been found in the east of Indonesia and along the coastal areas of Queensland (Australia). If the land had not been subsiding, the sediment produced by reef erosion would have filled up the lagoons and killed the corals long ago.
• Eroded material gets continuously deposited at the bottom of the sinking lagoon. That is why the lagoons remain shallow even after millions of years.
• During an experimental boring done to a depth of 340 metres in the Funafuti atoll, dead corals were found at that depth. Now, corals normally cannot grow below 100 metres because there is not enough sunlight. So finding dead corals at 340 metres means those corals originally grew in shallow water and then got pushed down to that depth by subsidence.
• Also, these deep corals showed signs of dolomitisation — a chemical process that only happens in shallow water. This confirms that the corals were once near the surface and later sank to greater depths.

Evidence against the Subsidence Theory:

• Scientists like Agassiz and Semper pointed out that corals have developed in places where there is no evidence of any subsidence at all. The island of Timor is one such area.
• Lagoons that are 40-45 metres deep and many kilometres wide cannot be easily explained by subsidence alone.
• Why would subsidence be uniform only in tropical and sub-tropical areas and not in other parts of the world? Darwin could not explain this.
• Kuenen described some areas where fringing reefs and barrier reefs are found close to each other. If subsidence had been a continuous process, this would not be possible — the fringing reef should have already evolved into a barrier reef.
• If we accept that all coral islands in the Pacific are products of subsidence, we would have to assume that a vast landmass once existed in the Pacific Ocean and then sank completely. There is no geological evidence for such a massive land area ever existing there.

Murray’s Stand Still theory

• John Murray was completely against the idea that coral reefs formed because of subsidence. His basic argument — you do not need the land to sink for reefs to form.
• According to Murray, any submarine platform could be brought to a suitable height for corals by one of two processes — either by erosion (waves wearing down a high platform to the right depth) or by deposition (sediment building up a low platform to the right depth). Once the platform reaches within 30 fathoms of the surface, corals start growing on it.
• The corals first form a fringing reef on this platform. Since the outer edges of the reef get more food and oxygen (from ocean currents and waves), they grow faster than the inner part. This uneven growth turns the fringing reef into a barrier reef over time.
• For atolls, Murray said that when corals grow outward in all directions from the top of a submarine platform, the shape naturally becomes a ring. The centre of the ring becomes the lagoon. Inside the lagoon, dead corals dissolve in the seawater, making the lagoon deeper. On the outer edge, living corals keep growing and building up the reef.
• Murray also had an answer for why corals are found below 30 fathoms. Above that depth, you have living reef. Below it, you have coral debris (broken pieces of dead coral) that gets cemented together by seawater. So the reef can extend deeper than the living coral zone.

Criticisms of Murray’s Theory:

• The existence of flat submarine platforms everywhere at exactly the right depth is doubtful. Where did all these platforms come from?
• Murray said lagoons form because seawater dissolves the dead coral in the centre. But seawater is not a good solvent — it cannot dissolve coral limestone quickly enough to form deep lagoons.
• Coral reefs are found well below the 30-fathom depth, which Murray’s theory cannot fully explain without invoking coral debris.
• Murray assumed that both erosion and deposition are active at 30 fathoms depth at the same time and place. This does not seem logical — usually one process dominates over the other.

Daly’s Glacial Control Theory

• Reginald Daly, while studying the coral reefs of Hawaii, was struck by two things. First, the reefs were very narrow. Second, there were clear marks of past glaciation on the islands. This made him think that there must be a close relationship between reef growth and temperature changes.
• According to Daly’s hypothesis, during the last glacial period (Ice Age), massive ice sheets formed across the continents. This locked up huge amounts of water as ice, which pulled water out of the oceans. The sea level dropped by about 125-150 metres.
• The corals that existed before the Ice Age now faced two problems at once — the temperature fell (too cold for corals to survive) and the sea level dropped (exposing the reefs to air). Both of these killed the existing coral reefs.
• With the corals dead and exposed, sea erosion went to work on the old reef structures. Waves planed them down into flat platforms at the new, lower sea level.
• When the Ice Age ended, temperatures rose and the ice sheets melted. Water returned to the oceans and the sea level started rising again. Now that conditions were favorable again, corals started recolonising these eroded flat platforms.
• As sea level rose, the coral colonies grew upward to keep pace. But here is the key point — corals grew more along the edges (circumference) of the platforms because the outer edges had better access to food, nutrients, and ocean currents than the inner areas. This is why reefs took the shape of the platform edges.
• A long coral reef growing on the continental shelf along the eastern Australian coast — the Great Barrier Reef — fits this model. Atolls developed on top of submerged plateau tops that were flattened during the Ice Age.
• According to Daly, after the Ice Age, the platforms were not disturbed by any tectonic forces. The earth’s crust stayed stationary — no subsidence needed. This is the biggest difference from Darwin’s theory.

Evidence in Support of Daly’s Hypothesis:

• The experimental borings done on the Funafuti atoll support Daly’s idea. If all platforms were cut down to the same sea level by marine erosion during the Ice Age, then the depth of these platforms and the depth of lagoons within barrier reefs and atolls should be roughly equal. And studies show that lagoon depths are roughly equal at different places.
• The biggest merit of Daly’s theory is that it does not require any subsidence of the earth’s crust — unlike Darwin’s theory. This is simpler and avoids the awkward question of why subsidence would be so uniform across the tropics.
• Sea waves and currents could have easily cut down volcanic islands and converted them into low, flat platforms during the glacial period when sea levels were lower.

Evidence against Daly’s Hypothesis:

• Some platforms are so large that marine erosion alone could not have created them. The Nazareth Platform is one example — it is 350 km long, 100 km wide, and about 600 metres high everywhere. No amount of wave action could plane down such a massive structure.
• Daly could not explain why coral colonies exist at depths of 100 metres or more. He had to admit that local subsidence must have occurred in some deeper areas — which undercuts his own argument against subsidence.
• Daly calculated that the sea level fell by about 80 metres during the Ice Age. But more accurate measurements — based on the angle of walls in submerged V-shaped valleys — suggest the actual fall was more than 80 metres. His calculation was off.
• Daly said that the temperature drop during the Ice Age killed the corals. But there is no direct evidence of widespread coral death caused specifically by the Ice Age temperature drop in tropical waters.

From all of this, it appears that the theories of Darwin and Daly are not contradictory but complementary. Both together explain different aspects of the coral reef problem. Darwin covers the subsidence angle, Daly covers the sea-level change angle. Both are needed for the full picture.

Davis’ Application of Physiography to the Problem of Origin of Coral Reefs:

• William Morris Davis gave his explanation in 1928 in order to revive and re-establish the old idea of submergence as applied to the coral reef problem. He tried to provide concrete physiographic evidence to explain problems that earlier theories had left unsolved.
• First, Davis reasserted the validity of subsidence. He stressed that the indented and embayed coastlines found in coral seas are clear evidence that the land has submerged. Coastlines don’t become indented and embayed unless the land sinks and the sea floods into the valleys.
• According to Davis, the flatness of lagoon floors does not show the true original bottom — it is only due to the deposition of debris (broken coral, sand, sediment) that has accumulated over time. Similarly, the shallowness of the lagoon is also a result of the land subsiding while sediment keeps filling in the lagoon.
• Davis also factored in changing sea levels. He argued that when sea level drops on an already-subsiding island, it would create cliffs and spurs along the coast. But most of these cliffs would be protected by the reef growing along the shore, so they would not be visible. And even if cliffs did form, further subsidence would drown them below the water surface.
• So Davis’s theory is basically the old subsidence idea of Darwin, but with a fresh application of physiographic evidence. It is also more comprehensive because it includes both sea-level changes (like Daly’s theory) and tectonic changes of the landmass (like Darwin’s theory).
• However, one thing Davis could not explain — the assumed equal depth of lagoons everywhere. He said the flat floor and shallow depth are due to sedimentation, which is fair. But this does not prove that the original bottom of the lagoon (hidden beneath the sediment) was at the same depth everywhere. It could well be at different depths in different places.

Modern Perspective on Coral Reef Origin

Today, scientists no longer treat these theories as competing against each other. Instead, they are seen as complementary pieces of the same puzzle.

• Modern research accepts that coral reef formation is driven by a combination of tectonic subsidence (Darwin), sea-level changes during glacial cycles (Daly), and biological growth patterns on stable platforms (Murray). No single theory explains everything on its own.
• Studies have shown that Pleistocene glacial cycles over the last 500,000 years played a crucial, cyclical role in shaping modern atoll structures — the reefs died during glacial periods when sea levels dropped, and regrew during interglacial periods when sea levels rose again.
• Advances in satellite imagery, ocean floor mapping, and genomic techniques are allowing scientists to study reef history with much more precision. Recent research (2025) using genomic sequencing has revealed hidden species diversity in well-known corals like the “table coral” (Acropora), showing that reef biodiversity is even greater than previously thought.
• A major question today is whether modern reefs can keep pace with current rates of sea-level rise driven by climate change. Past reefs adapted to slow, natural sea-level changes over thousands of years. But the current rate of rise is much faster, and ocean temperatures are also increasing — which causes coral bleaching. Whether reefs can grow fast enough to survive this combination of threats is an active area of research.

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