Darwin’s classic explanation of atoll formation supposedly solves the great mystery of atolls – they’re made up of the eoskeletons of shallow-water corals. If the atolls are built up on submerged seamounts from deep below, how can they even get started if the corals require shallow water in which to build and breed? And why are the stolls roughly circular, with a central lagoon?
His explanation was that the coral reefs get started around a seamount so high that it’s an island, and the reefs are building just offshore in the shallows off the island. As the island sinks, the corals continue to build on the skeletons of previous corals, and so build up a thicker layer. as the island continues to sink, the coral keeps building, thicker and thicker, higher and higher. Finally, the island disappears beneath the waves, and you’re left with a classic atoll – a coral ring with a lagoon in the middle. Coral can’t fill it in, because the center’s still deep.
I’ve read this explanation in countless books. It seems to make sense
But…
1.) Why the hell does the original island subside? Nobody ever explains why the island sinks. In fact, island sinking seems to be an odd and bizarre thing. I don’t know of any other islands, in non-atoll-forming areas of bthe world that sink. When peaks disappear elsewhere in the world, it’s due to erosion. But that doesn’t seem to be the case here.
2.) Why doesn’t the coral fill in the lagoon when the island has sunsided to low-sea level? It’s not as if the island is disappearing rapidly. In order for this explantion to hold, the subsidence must be slow enough to allow the coral to form stable layers. Are conditions inside the lagoon really that different than on the seaward side?
3.) Whatever this weird subsidence process is, it’s happened many more times than once. There are many atolls in the south Pacific.
I know I’m not supposed to speculate in General Questions, but wouldn’t the receding of glaciers after the last ice age cause the proper slow rise in sea levels to effect the subsidence of the seamount relative to the average sea level?
I think the ice age/sea level explanation does not give enough time for this process to work.
These islands were once volcanoes. Once they became dormant, and stopped growing (by lava being expelled), they start the process of subsidence. Essentially, the volcano is so massive, it will, over time, sink into the earths crust. Subsidence also happens as the fluid magma leaves the subsurface reservoir.
The ex-volcano actually does sink slowly beneath the waves.
The caldera often collapses when the magma leaves the chamber - I’m not sure about this, but I think when the volcano sinks low enough, the caldera will fill with water fairly rapidly, and the water in the middle will be too deep for corals to grow.
Darwin’s observations relate mainly to the Pacific. My understanding is that the reason the islands sink is usually due to plate tectonic movements. As the Pacific plate moves further and further away from the mid-ocean spreading center, the plate cools and thus becomes denser and lower, carrying the island with it. This is why atolls are prevalent in the South Pacific but not elsewhere.
Also, seafloor formation at the mid-oceanic gradually ridges pushes some islands deeper.
(Seamounts and guyots are what you have instead of atolls when you’re not in a tropical clime, which is the only place coral grows and can form atolls.)
Euphonius and Colibri’s explanations are different and incompatible. Colibri’s would explain why so many atolls are in the same place. But I’ve never heard that tectonics plates can rise or fall as a unit. I was under the impression that they floated at pretty much the same level, with creation and subduction at the edges making or destroying the plates. Do you have any kind of cite for the plate sinking?
As for Euphonius’ reply, I re-iterate that I’ve never heard of a volcano sinking. The volcanos on dry land don’t seem to do it, and the volcanic islandsd I’ve heard of – like the Hawaii chain – seem to weather away, but I haven’t ever heard that they were subsiding. Do YOU have any sort of cite for this?
I’m not being snarky. As I said, I’ve never heard any explanation for the samount’s subsidence, but I’ve never heard of any other sort of island or mountain subsiding by any means other than erosion.
Toadspittle’s cite tends to confirm my own explanation. Perhaps I was not clear. It’s not that the plate sinks, it’s that as it moves from the spreading center it becomes denser and hence less thick. Because it is less thick, its surface is now relatively deeper below the surface of the ocean. The midpoint of the plate is still essentially at the same level, it’s just that the top and bottom are closer together.
Here’s an image of the bathymetry of the Pacific Ocean. Note how the shallowest areas (yellow) are along the mid-ocean ridge. As you move away from the ridge itself, the ocean becomes deeper (shading to dark blue in the oldest parts of the basin). This indicates that the surface of the plate becomes deeper the farther it gets from the ridge.
Conditions for coral growth are actually better on the seaward side of the reef rather on the protected side. From this article:
Because the coral grows much faster on the outer side of the atoll, it outstrips the growth in the inner side of the lagoon as the island subsides, producing the ring.
Try this, then: Part A: Volcanoes, like every other upthrust feature of the Earth’s crust, erode over time. As long as a volcano is active, such erosion is at least compensated for by vulcanism, either intrusive or extrusive. When, however, (a) the hot spot producing the vulcanism dies away, or (b) the volcano moves off the hot spot owing to plate tectonics, then it is no longer in a state of anabolism or equilibrium with regard to the geologic forces acting on it, but rather one of catabolism, with erosion dominant. Over time, erosion wears the volcano away, both above and below the surface. Compare Hawaii, where the Big Island includes the two tallest (though not highest) mountains on Earth, Maui includes substantial if extinct mountains, and as one moves northwest, each island features less mountainous, smoother terrain, with Niihau scarcely showing mountain remnants, Nihoa a mere islet, and French Frigate Shoals hardly broaching the surface.
Part B: Many active volcanoes occur on mid-oceanic ridges and on island arcs. Owing to seafloor spreading from that very upthrust activity, over time volcanoes move off the ridge, along with the seafloor around them, and this is akin to sliding slowly down a slope to the abyssal floor. The Atlantic is especially instructive in this regard, with Iceland and the Azores prominent above-sea-level extrusions on the ridge itself, and scarcely anything broaching the surface of the ocean which has moved off the ridge.
Both elements, plus the relative density of the crust at varying points, contribute to the gradual submersion of extinct volcanoes in the ocean.
I will conclude by reflecting on the uninhabited lagoon and coralline fringing islets that were named for the lead singer of Herman’s Hermits back in the 1960s: Nobody lives there, Noone Atoll.
