Is this really correct? You'd think with modern imaging it would be fairly easy to spot a 100m high magma formation the size of california. How does magma even flow with such volume, it sounds like it would create an island that everyone would know the name of.
In an explosive eruption, the magma doesn't just flow downhill and solidify in place like it does in Hawaii. It gets blown to smithereens by expanding gases and the resulting fine particles are scattered all over the world. All that's left is a hole in the ground, which will be quickly filled in if it's below sea level.
