Every now and then, the Earth experiences a so-called “supereruption,” wherein a caldera (cauldron) volcano containing a vast magma chamber bursts and releases around 1,000 cubic kilometers (240 cubic miles) of magma into the environment. Supereruptions have devastating effects, causing enormous tsunamis, searing the land around them with immense pyroclastic flows, and often plunging the Earth into a volcanic winter, causing significant global temperature drops. Although there is not much we can do about these gargantuan beasts erupting, scientists have now found a way to track how fast their magma chambers fill up before they explode. Bad news: it’s incredibly fast, taking no more than 500 years. The new study is published in Geology.

Magma chambers, including the ones hiding beneath supervolcanoes, are traditionally thought of as being permanently molten, but recent work shows that they likely spend most of their life as crystal “mushes” that activate when a new, gas-filled magma source intrudes into the chamber, re-heating and re-melting it. The average time taken for these magma chambers to pool, become molten and be able to erupt (sometimes referred to as becoming “active”) has been hotly debated in the scientific community, although estimates have ranged from hundreds to millions of years.

A new team of geoscientists has identified what they call a “geospeedometer” allowing them to identify the “activation time” of these huge magma chambers. Tiny quartz crystals grow within the magma as it pools inside chambers; as these crystallize out of the magma as it cools, they trap small inclusions of melted rock. These are initially round, but over time, when the crystal is adrift in hot magma, these diffuse and change shape, eventually taking on the form of the polygonal crystal they’re residing in. This transformation process is, however, interrupted if the magma erupts and rapidly cools.

Using a technique called 3D X-Ray tomography, the researchers could measure the shape of the inclusions with incredible precision, allowing them to work out how long the magma had been pooling for. The team then used this technique to look at the quartz melt inclusions within the magma of four supereruptions that took place between 760,000 and 27,000 years ago. Troublingly, they found that the time between the formation of active magma and the eventual supereruption was no longer than 500 years.

This has some worrying implications. Yellowstone caldera is perhaps the most infamous supervolcano: sitting in Wyoming, America, it covers 240 square kilometers (93 square miles) of land, meaning it can only be seen properly from low-Earth orbit. Its three most recent supereruptions were 2.1 million, 1.3 million, and 640,000 years ago, meaning that it erupts catastrophically every 650,000 years or so.

Although there is a large margin of error on this, this could mean that the next supereruption from Yellowstone is due in the next 10,000 years. However, the ground there is rising by an average of 7.6 centimeters (3 inches) each year, indicating that magma is still entering the subterranean chamber.