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This question came to me after reading H.P. Lovecraft's "Dagon"- the protagonist has fallen asleep drifting on the ocean in a small boat. He wakes up one morning to discover he's been grounded on what appears to be a vast region of mire that used to be the ocean floor, which had very suddenly risen to the surface:

Through some unprecedented volcanic upheaval, a portion of the ocean floor must have been thrown to the surface, exposing regions which for innumerable millions of years had lain hidden under unfathomable watery depths. So great was the extent of the new land which had risen beneath me, that I could not detect the faintest noise of the surging ocean, strain my ears as I might. Nor were there any sea-fowl to prey upon the dead things. [...] The region was putrid with the carcasses of decaying fish, and of other less describable things which I saw protruding from the nasty mud of the unending plain.

This seems to indicate that some enormous geological activity of vague nature resulted in part of the sea floor itself being thrust up to the surface in what can only be assumed to be the sudden formation of a large new island. This activity is implied to have happened without anyone knowing about it (until now).

I know very little about oceanic geology, and the plausibility of this whole thing confuses me. Is it possible for the ocean floor itself to be pushed up above the water? Is it possible for this, or any other process of land formation, to occur so fast as to leave marine life stranded on the now-surface? Lastly, is it possible for such geologic activity to occur unnoticed by humans?

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    $\begingroup$ Not enough for an answer on it's own, but to supplement the others.. In the 2004 Indian Ocean earthquake "1,600 km (1,000 mi) of fault surface slipped (or ruptured) about 15 m (50 ft)". It was the 3rd largest ever recorded and it only raised the surface by a fraction of ocean depth. It would be hard not to notice what is described in your passage, particularly when considering "unprecedented volcanic" activity. $\endgroup$
    – coblr
    May 15 at 18:49
  • $\begingroup$ This might be more on-topic on worldbuilding.se $\endgroup$
    – dalearn
    May 15 at 18:52
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    $\begingroup$ In H.P. Lovecraft's works such events happen due to supernatural beings, not naturally. In no way is it implied in the books that such events are due to normal natural geologic activity. $\endgroup$
    – vsz
    May 16 at 6:47

4 Answers 4

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Generally the Earth's geology moves very slowly, ... very slowly. When people use the term geological time scales they mean a very long period of time, usually in the millions of years. In volcanic regions with very shallow water, volcanic activity might raise a small fragment of the sea bed above the water. Other than that, such movements don't happen.

Firstly, the forces required to do would be immense and explosive. It would also have the potential to create a tsunami by putting so much energy into to water. Secondly, the fragment of the sea bed raised in such a manner would need to be supported by rock from underneath. It would need a foundation to rest upon.

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Very implausible.

If your sailor is actually in the middle of the ocean, there are several kilometres of water underneath. Nothing that we know of can uplift kilometres of rock overnight, at least something that will not destroy the Earth.

Alternatively, your hero might be in an area where the seafloor is rather shallow, potentially several metres. These aren't necessarily rare. Some parts of the Torres Strait or the North Sea are rather shallow. An earthquake might cause some parts of it to rise several metres, just enough for it to poke above the water. However, these waters should be shallow enough for the sailor to know that it's shallow, and not "in the middle of the ocean".

A third option is a volcanic eruption, as these are known to create land. However, these events are rather violent and your hero is unlikely to survive this.

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Putting a piece of oceanic crust on land is an existing geological process called obduction. However, it is not a sudden process, but rather a very slow one.

Now, there is one process that can lift the ground quite rapidly: bradyseism. It occurs at volcanic calderas, the best documented example being the bradyseismic activity of the Campi Flegrei caldera near Naples. The mechanisms of bradyseism are still debated (there are various models trying to explain the phenomenon), but it results in alternating phases of ground uplift and subsidence.

In the twentieth century, there has been two episodes of uplift in Campi Flegrei (Orsi et al., 1999): one in 1969$-$1972 (1.74 m of vertical displacement), another one in 1982$-$1984 (1.79 m of vertical displacement). Both episodes showed an average rate of about 1.4 mm/day. In terms of geological timescales, this is super fast. But not quite as "sudden" as what you are looking for... But wait, there's more!

In the past, there has been more intense uplift episodes in the Campi Flegrei caldera, although for those we don't have ground displacement measurements, only historical chronicles. Guidoboni & Ciuccarelli (2011) mention at least three episodes where major uplift caused new land to emerge from the sea:

  • In an edict dated from October 6, 1503, the town of Pozzuoli was granted an area of "dried-up sea" (marem desiccatum).
  • In a similar edict from May 23, 1511, the town of Pozzuoli was granted another area of "dried-up sea".

The extent of the uplift was such that a quantity of land emerged on which human activities could take place. The area, previously occupied by the sea, was requested by the inhabitants of Pozzuoli for the building of new houses and buildings, because their villages had mostly collapsed or become uninhabitable due to the repeated earthquakes of previous years.

  • The last, and most known episode, occurred on September 28, 1538, hours before the Monte Nuovo eruption:

Uplift from the seabed in the area between Lake Averno and Monte Barbaro (ca. 2.5 km). The sources report that this uplift produced a considerable quantity of dead fish in a kind of shoal. It is very likely that this involved a rapid movement, although the sudden death of the fish which were brought to the surface could also be due to other causes (such as a rapid variation of temperature, or infusion of gas?). This uplift was not described in the sources as such, but rather as a drying up of the sea, and was interpreted by naturalists at the time as the result of a rapid absorption of sea water by the seabed.

There you go, dead fish and all, just like in Lovecraft!

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The rock on the ocean floor is not likely to well up to the surface anytime soon. But methane can.

Methane, formed from biological processes or decay of organic matter at sea bottoms, forms water-ice clathrates, or hydrates, in which methane under pressure combines with water to produce a solid material stable under ocean-floor conditions -- if the pressure is maintained. These hydrates may accumulate methane gas under this pressure, forming gas-hydrate pingoes (the name is carried over from land-based pingoes, but this is a different phenomenon). The pingoes sometimes explode ("Ping!") when some phenomenon, such as a drop in sea level, decreases the pressure and thus breaks the hydrate down. Such explosions release great volumes of greenhouse-producing methane gas and may have enough blasting force to form craters on the ocean-bottom rock below.

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