Huge sea level rise in a specific area [duplicate]

Question

I have a continent and I want the sea level to rise temporarily, for several hundred years. The area that needs to be covered by water is about 3 million square kilometers.

I was thinking about a big earthquake, triggering sinking of a tectonic plate. The side of the plate near the sea sinks and the other side stays mostly unchanged. This will be experienced as leaning of the ground. The molten rocks that were below the plate must go somewhere so that will probably trigger volcano eruptions and mountain formations.

The sea must cover whole mountains near the sea (hundreds of meters). After several hundred years the process has to reverse (the sea level has to drop).

Is anything like that possible in such a short time range? Are there any alternative explanations for such a phenomenon?

Answer 1

Only way to get this to happen is, as pointed out in this A continent lost underwater? for the area in question to be in a below sea level basin that floods, like the Mediterranean & Black sea flood events. Otherwise, you could have a gradual worldwide rise/fall from ice caps melting & refreezing, but even with humans currently doing their best to assist the process, it'll take centuries.

In brief, continents are continents for a reason. They're masses of lighter granitic rock floating on the denser basalt of the crust, so they don't really go up & down much, and the changes that do happen are on plate tectonic timescales.

Answer 2

If rubber science goes? I doubt the numbers actually match for the scenario you describe...

Eastern part of the continent has a huge pocket of magma under it. The pressure of the magma has lifted the ground by a hundred meters. Most mountains of the area are in fact extinct volcanoes created previously by the pocket.

The pocket erupts. The range of volcanoes on the coast almost explodes. Huge amounts of ash and toxic gases erupt all over the area. The eruptions last for decades. Entire cities suffocate and then get permanently stored under meters of ash and lava. The ash and sulphur compounds released trigger a small ice age lasting more than a century, the resulting famines and population movements keep people on other continents too busy to worry about refugees.

With all the magma that was removed and blasted to the sky the magma pocket collapses. The ground that it was pushing up follows. Huge areas are flooded, which would cause local residents some concern except most of them already left because of all the eruptions or otherwise permanently ceased to care. The subsidence is largest near the highly fertile mountainous area created by previous volcanism that most of the large cities used to be around.

After the eruptions cease, what is left is a huge shallow gulf dotted with volcanic islands.

With all the mass lost in the eruptions, the eastern side of the continent is now much lighter and tries to rise up. Friction from neighbouring plates prevents this. Until a huge series of earthquakes loosens the fault lines and the eastern part of the continent rises up from beneath the waves. Which is kind of unfortunate for all the fishermen living on those island and the old coastline...

Answer 3

Let's start with a planet with a smaller total water coverage. Maybe 50/50 water over land. More important than total coverage percentages, let's say that the water is divided in several independent ocean size seas. Each sea is completely landlocked.

When the ground beneath any one of these seas rises, the water is displaced into adjacent lands. The effects are not distributed as a small rise in water levels around the world. Instead, 3 million square acres of adjacent low plains suddenly submerges to become the new home of that sea.

It's not very deep, but its impossible to tred water all the time so people either leave or die.

Three hundred years later, the survivors have build towns and fishing villages along the new shoreline. Legends tell of the fertile fields which now lay beneath the waves. Maps to the now lost gold and silver mines are found by archeologists, proving that the legends are true. A plan is proposed to recover the lost lands.

Fifty years later, a man made channel opens, freeing the trapped waters to pour down into the adjacent nation's farmlands. At first the neighboring farmers are thankful for the help in irrigating their crops. Then as the water continues to flow, they get worried, then angry, then scared. Over the passing decades which follow, the lost lands are recovered as the neighboring nation begins telling new legends of their own.