Science Puzzle
Can You Read the Volcano’s Warning Signs?
A volcano that has been quiet for decades starts behaving strangely. Instruments record swarms of tiny earthquakes, the mountainside begins to swell outward by a few centimetres, and far more sulphur dioxide gas than usual is drifting from the summit.
What do these three signs together tell the scientists?
The Answer
Magma is on the move, forcing its way upward, and the three signs are three different fingerprints of the same event.
The earthquakes come from rock cracking apart as molten rock shoulders through it, thousands of tiny fractures rather than one big rupture. The bulge is simply the mountain being inflated from within, like a balloon under the skin, which sensitive tiltmeters and satellites can measure to the millimetre. And the gas is the giveaway: sulphur dioxide is dissolved in magma under pressure and comes out of solution as the magma nears the surface, exactly like the fizz escaping an opened bottle. A sharp rise in sulphur dioxide means magma is now shallow.
No single sign proves anything. Volcanoes rumble and steam without erupting all the time. It is the three arriving together, and accelerating, that convinces volcanologists to order an evacuation. Reading them correctly at Mount Pinatubo in 1991 saved tens of thousands of lives.
The principle: Converging lines of evidence. Seismicity, ground deformation and gas emission each track rising magma, and together they give warning that no single measurement could.