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Hypothetical question: If I have a fairly large island (say, the size of Vancouver Island) and pop it out of existence entirely (to, about five or so meters below the sea level), I'd assume there's be a tsunami of sorts. How large would such a tsunami be, and what factors would influence it? For example, would it make a difference to have the island pop out of existence in the deep ocean, or a shallower ocean section?

The island would be removed within a matter of moments/instantly. I was wondering about this after watching a documentation about how tsunamis were triggered after the ocean refilled the crater of the asteroid that supposedly killed off the dinosaurs.

EDIT: The island would be mostly made of sedimentary rock. There'd be some sand on the beaches, of course. I think England or Ireland would be a good match

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    $\begingroup$ The biggest factor would be what is the island made of. Is there a lot of loose soil or sand on it? Is it mostly rock? Does it involve a lot of coral reef? $\endgroup$
    – Mary
    Jun 27 '20 at 21:15
  • $\begingroup$ The island would be mostly made of sedimentary rock. There'd be some sand on the beaches, of course. I think England or Ireland would be a good match. $\endgroup$
    – BeeMKay
    Jun 28 '20 at 1:21
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For each square metre dropping 5m there would be 1 cubic metre of sea water falling 4m to fill the gap, another falling 3, another 2 and another 1m, to fill the gap, equivalent to 1000kg of water dropping 10 metres. Potential Energy = mass x height x g so that’s 10001010 = 100,000 Joules for one square metre.

The area of Vancouver Island is 31 billion square metres so the total energy involved is roughly 3.1 x 10^15 Joules That’s roughly a magnitude 7 earthquake or 0.5 Mega tons of TNT. Not to be sneezed at, but not earth shattering either. In addition the energy released would not be released suddenly as in an Earthquake where there is a sudden movement of the ground. In this case the water would just flood into the centre of where the island used to be.

A 5 metre wall of water would rapidly degenerate into a huge cataract moving at perhaps 10 m/s. As Vancouver island is 100km wide at its widest it would take well over an hour to flood completely giving a lot of time for some of that energy to dissipate. However the waters would still meet roughly in the middle, be pushed up into the air by water flooding in behind and then flood back in the opposite direction. But there again at only 5m mean depth it would not be possible to build a huge deep water ocean wave so a lot more energy would be dissipated as the wave crashed in on itself.

That said I wouldn’t want to be in a waterfront property in Vancouver. Chances are there would still be very substantial flooding.

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The relevant research area is fluid mechanics - Software exists that would be capable of doing such a simulation. The only factor that affects the dynamics substantially is geometry - what kind of seafloor does this new 5m-deep depression in the ocean have to travel over? Is the bottom surface perfectly smooth, or has the seafloor become magically much deeper?

In terms of a rough estimate of the violence involved, assuming a missing cylinder of water of depth 5m below sea level, area 31000 km$^2$ displaces 155 cubic kilometers of water. The gravitational potential energy released as it fills from the (much larger) ocean totals 3.7 Petajoules of energy, equivalent to over 100 Hiroshima bombs, comparable with a magnitude 7.4 earthquake. That sounds pretty bad, but actually it's relatively feeble on the tsunami scale - the 2004 tsunami was 200 times bigger than this. So it'll wreak havoc locally, but will not cause global effects anywhere near as bad. An earthquake of comparable magnitude struck Mexico 5 days ago.

To get a detailed prediction of the radius of destruction this would cause, you'd meed a more sophisticated analysis that takes local geography into account. Tsunamis dissipate faster as they become taller in shallow water, and those sorts of effects are hard to eyeball.

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    $\begingroup$ Beat me to it ! $\endgroup$
    – Slarty
    Jun 28 '20 at 15:37

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