A volcano erupts and everything nearby disappears under rock, lava, and ash.So here’s the obvious question that occurs to almost everyone at some point. Why not just plug it? Fill the opening with concrete and stop the whole thing before it starts.The idea sounds childish until you look at the numbers, at which point it becomes genuinely interesting, and then genuinely alarming.Concrete is the most used material on the planet, with around 18 billion metric tonnes going into construction every year. It’s in nearly everything around you.It also lasts. The Colosseum was built using concrete nearly 2,000 years ago and it’s still standing.The recipe is unglamorous, just gravel, water, and a bit of cement, and the material actually strengthens over time. The toughest varieties can withstand the pressure at Challenger Deep, the deepest point in any ocean.So the real question is whether it survives a volcano, or whether the whole thing turns into a very deadly version of dropping Mentos into Coca-Cola.Start with heat, since that’s the intuitive objection. Pour concrete into a volcanic vent and surely it just melts?Actually, no. Concrete melts at around 1,500 degrees Celsius, while even the hottest orange lava tops out near 871 degrees.The concrete would hold. Which is exactly the problem.Volcanoes erupt because pressure from the magma below builds until the ground above it gives way. The bigger the pressure difference between the magma and the surface, the more violent the eruption.Cap it with concrete and you haven’t removed any pressure. You’ve just guaranteed more of it accumulates before something finally breaks.And when it breaks, it goes sideways.If magma can’t get through the plug, it forces its way out through the flanks instead. That’s called a Vulcanian eruption, and there’s a well documented example of what it looks like.In 1980, Mount St. Helens built up pressure until one side of the mountain bulged outward into a dome. The dome eventually failed, and the volcano erupted laterally, throwing material at 350 meters per second.Dozens of people died.A concrete plug wouldn’t just produce that outcome. It would add its own contribution, because the plug itself would burst apart in the blast.Concrete dust is not something you want in the air after a disaster. Prolonged exposure causes fatal lung disease and cancer. Which suggests the material isn’t useless here, just aimed at completely the wrong target. Lava flow from a natural eruption is usually slow and reasonably predictable. That makes it something you can steer rather than something you have to stop.Mount Etna proved the point in 1983, when lava threatened three Italian towns and crews built barriers from rock and ash to redirect it. The first barrier stood 18 meters high and was overrun fairly quickly. The second one held, keeping the flow from moving further west.So concrete does have a job in all this. Build walls and barriers, protect the towns, and let the volcano finish doing what it was always going to do.Just don’t try to cork it.