FEW talks about the formation of mountains and volcanoes, and I'm curious about the magma. Is the magma contained?

@FlatEarthOrg retweeted a tweet with a picture that has an island looking earth with the magma and all, but with a flat top. Is there an inequality in the distribution of magma to the crust? If so, then the "Southern" part of Flat Earth would still have but little magma, but we have "Northern" pole in the middle.
In other forums here is also "the Dome."

But then the twitter account says or implies that there is no edge.