At first blush, an eruption in an ocean basin is a lot easier:
- Thinner crust.
- Lower lifting forces -- You don't have to push it up as far.
- Buoyancy of water further decreases required force by about 40% (figgering basalt at s.g. 2.5
I grew up on the western edge of the central Washington flood basalts. And flood is the right descriptor. Very flat layers 10 to 80 feet thick.
I would expect some response similar to the way the crust rises and sinks at the end/beginning of an ice age. Since basalt and granite are similar in density, (basalt slightly heavier) you'd get something very similar to our existing igneous provinces except larger in scale. I don't think you could get an Olympus Mons here on Earth.
That said, consider the following scenario:
A large asteroid that has a relatively high content of radio isotopes hits the planet in an overtaking orbit, and does so at a relatively slow velocity. (Ok, this is improbable. Solar engineering gone wrong?)
The asteroid hits, and much of the matter sinks deep into the mantle, where it provides a continuous heat source creating a permanent plume.
The plume creates a chain of flood basalts as continental drift caries the tectonic plate over it.
This is pretty much the case of Yellowstone. I think the plume under Yellowstone at an earlier hotter stage was responsible for the flows that created the scablands of central Washington, as well as the Idaho batholith.
To get a larger province without global extinction, you need more time. You need to introduce CO2, SOx, and other aersols at a rate that is survivable.
In general it's not absolute values that concern you, but rate of change.
It's possible that you can create a world where the normal level of CO2 is several percent. People start having trouble at about 2% and serious difficulties starting at about 4%, but I expect solutions to this would evolve fairly quickly.
This introduces some notions for conflict: The heat balance is strongly driven by the amount of CO2 in the air, but also the particulates. Climate would fluctuate wildly, making for mass migrations both of animals and people. Evolving on such a planet the urge to seek greener pastures would be genetically encoded.