Where underground? I thought the Earth was a flat disc, to speak of an "underground" implies the Earth is cylindrical, think about it!
Yes yes, you've found a fatal flaw in the theory; the Earth cannot be an idealized two-dimensional plane, with no thickness whatsoever. Man, so foolish of us.
If convection current theory isn't going to be derided here, then how come there isn't a constant upwelling of lava at the "centre" of the Earth pushing the continents to the outer edge of Earth?
How come there would be such an upwelling? There's upwelling in other parts of the world... ridges and such, and, I suppose, volcanoes.
In fact how can there be lava in a flat Earth yet ice sitting on top of it?
How can there be lava in a round Earth yet ice sitting on top of it?
I though hot air rises?
You thought correctly.
Fortunately, lava is heavier than air.
It would melt the ice wouldn't it?
Well when the hot air rises, probably the direction it will go is "up". If it's still hot by the time it gets to... whatever ice you think there is up above us, yes, I suppose it would melt it. Then I guess it would rain. Or something.