I've read an answer explaining how it could be done but I was wondering what anyone thought about creating island chains the size of continents etc. using magma under the ocean. What would be the method of creating large scale magma plumes or volcanoes and using them to create massive island chains.
- Exclude the ocean. First you need to make a dam and exclude the water. This will relieve a tremendous amount of overlying pressure on the magma. Continued pressure on remaining magma outside the exclosure will squeeze the lava up into the exclosure like toothpaste. An exclosure is a thing, isn't it? This is the volcano nozzle as proposed by the inimitable bungston.
There may be an optimal size for these exclosures. It might be tough to reuse the same one over and over because they will be full of magma. You might need to add magma column by adjacent column.
- Magma vermiculite. Basaltic magma is heavy as has been observed. Lighter materials or floats should be included so the mass does not sink back down. This might be as simple as pumping air down to the bottom of the magma before it has solidified, to make bubbles in the magma. Or water, since you have a lot handy. Pipes for this purpose should be placed before the magma fills the exclosure.
Bubbly, pumice-like lava will also facilitate filling to the top, as the weight of the magma will be decreased by adding bubbles. The ideal pumice would weigh less than the water it displaced. The whole thing will not float away on completion because you will add magma to take it higher than sea level, and this topmost mass only displaces air.
Using basaltic magma for creating islands is going to give a problem: because of its density it will, over time, sink under the ocean floor, like it is happening on all the atolls scattered in the Pacific Oceans, unless you keep pumping new magma to keep raising the island for at least the same sunken amount.
It seems therefore very difficult to make something the size of a continent with this method.
Silicate rich lava is instead lighter and more suited to stay afloat on the denser basaltic rock, but that is formed when continental crust sinks underground at some converging plate border, so it's kind of hard to find in the middle of the ocean: you need at least one continental plate (silicate rich) to be involved in the process, which is precisely what happens with Indonesia.
If you want to pursue this, I think you have to settle for something the size of Hawaii islands, which goes on shrinking over time.
The Hawaiian islands were formed by volcanic activity initiated at an undersea magma source called the Hawaiʻi hotspot. The process is continuing to build islands; the tectonic plate beneath much of the Pacific Ocean continually moves northwest and the hot spot remains stationary, slowly creating new volcanoes. Because of the hotspot's location, all currently active land volcanoes are located on the southern half of Hawaiʻi Island. The newest volcano, Lōʻihi Seamount, is located south of the coast of Hawaiʻi Island.
You can't make a permanent continent out of oceanic (basalt) rocks, they don't have enough light metals, it'll just sink back into the sea once it stops being erupted and gets cold. The elements that make continental crust light enough to stay buoyant on top of the mantle have been depleted from the mantle, that's why there is a division. Calcium, Lithium, Aluminium, and Sodium are vital to the formation of continental rock and they're all tied up in the current continents.
Having said that the processes in question are slow something the size of The Big Island takes millions of years to be eroded and sink to something the size of Niʻihau and the bigger the landmass the slower it cools and erodes. So you could have a temporary continent made of basaltic rock pumped up from the magmatic systems beneath the seabed crust that lasts long enough that no-one living on it is any the wiser.
To do this technologically you need drills and pumps (and/or stupid amounts of explosives) that will survive the temperatures and pressures involved, a blatant disregard for the local environment, and a willingness to risk global catastrophe:
You need to cut deeply enough into the crust to create a breach that magma can rise through, once such a breach exists it will pour out lava flows until isostatic pressure equilibrium is restored. You then need pumps, or lots of explosives to keep blasting away the newly set top rock, because oceanic crust is almost exclusively already at equilibrium and any hole in the crust will simply freeze over so you need to hold the hole open and continually remove the top pressure.
All life around and most life down current from your artificial vent is going to die; buried under the flood of lava and/or poisoned by the chemical soup created by the lava reacting with saltwater on the seabed.
The carbonic and sulfurous gases that are produced by the artificial vent system you'd need in order to create a continent on some kind of human timescale have the potential to upset the global climate and kill everyone. It doesn't matter if those gases get into the atmosphere or dissolve into the ocean they'll still do immense damage to the whole planet.
There is also the potential issue of disruptions to oceanic circulation due to changes in basin configuration.