Learn · Hazards · 7 minute read
Volcanoes
Runny magma pours. Sticky magma explodes. The difference is gas, and it decides who gets hurt.
Some volcanoes pour out lava you can walk away from. Others explode with no warning anyone can see from outside, and throw ash into the stratosphere. The difference is mostly in the magma: how sticky it is, and how much gas it holds.
Runny and sticky
Magma is molten rock with gas dissolved in it, mostly water vapour, carbon dioxide and sulfur gases. As it rises, the pressure drops and the gas comes out of solution as bubbles, just as a bottle of fizzy drink bubbles when you open it.
What happens next depends on silica, the main ingredient of most rocks. Basalt magma is about half silica. It is the hottest and runniest common magma, so bubbles escape easily and the lava flows. Hawaii and Iceland are made of it. Rhyolite magma is more than two-thirds silica. It is cooler and very sticky, so bubbles cannot get out. Pressure builds until the magma shatters into ash and pumice and blasts out of the vent.
Volcanoes above subduction zones, where one plate sinks under another, tend to have stickier, gassier magma. That is why the volcanoes of the Pacific Ring of Fire, such as Mount St Helens and Pinatubo, are the dangerous explosive kind.
How big was it?
Volcanologists rank explosive eruptions on the Volcanic Explosivity Index, or VEI, from 0 to 8. It mostly measures how much material came out. From VEI 2 upwards, each step is ten times more than the one below.
Mount St Helens in 1980 was a VEI 5. The eruption began at 8:32 in the morning of 18 May with an earthquake that set off the largest landslide ever recorded, about 2.5 cubic kilometres of rock. Pinatubo in 1991 was a VEI 6. It put nearly 20 million tonnes of sulfur dioxide into the stratosphere, and the haze it made cooled the whole planet by about 0.5 °C for the next two years.
Tambora in 1815 was a VEI 7, the largest eruption in recorded history. Tens of thousands of people died, most of them from the famine and disease that followed. The next year, 1816, Europe and North America had crops fail in the cold and wet. It became known as the year without a summer.
Volcanoes in Britain
Britain has no active volcanoes, but it has plenty of old ones. Arthur's Seat, the hill in the middle of Edinburgh, is the remains of a volcano that started erupting about 342 million years ago, when Scotland lay near the equator. Ice sheets later scraped it down to the hard core you can climb today.