But look at the picture, the lava is trapped.

I think we can still work with the spirit of the solution. The main issue is that the lava will break the concrete. The solution is pretty simple. If we take some buckets of water we can pour it into the volcano creating a layer of obsidian. Obsidian is much harder and will easily contain the magma.

What you mean? Cement stronk!

Not to mention the average normal size of an opening you’d have to plug to begin with. Even if it didn’t just drain down the hole or dissolve the moment it got to lava, it would be a ludicrous amount of concrete just to make a layer a few feet thick. Even if you did manage to make a plug a hundred or more feet deep and it didn’t melt or move, an eruption would likely just blow the mountain apart from around it.

Gonna need to see some credentials buddy. If I don't see volcanoologist, I'm leaving.

Or as the guy at bike shop put it "no you can't put duct tape on your pierced tyre. If a nail got through rubber, its probably gonna also get through tape."