Welcome to Earth – a mass of molten rock hurtling through space at 100,000 km/hour. The land we call home is a fragmented eggshell of plates that crack and collide on top of currents of magma. Yet most of us don’t pay attention to the geophysical reality until a violent jolt reminds us: Oh yeah, we live on a ball of fire.

The Icelandic volcano, which grounded flights in 2010, provided one such reminder, but this was a trifling inconvenience compared to the cataclysmic consequences of a supervolcano erupting. Topping the Volcanic Explosivity Index at level 8, supervolcanoes are exceptionally violent, ejecting volumes of ash, magma and rock tens to hundreds of times larger than what mankind has ever experienced. Worryingly, there is a supervolcano that is alive and well in Yellowstone National Park in the United States. Since 2004, the pool of magma below Yellowstone has been swelling, pushing up the land by 25 cm and triggering 900 small earthquakes in 2008/9, although that activity has since slowed down.

What if the Yellowstone supervolcano explodes? Expect total devastation. The last Yellowstone eruption, roughly 640,000 years ago, was 1,000 times more explosive than the 1980 eruption of Mount St Helens in the US state of Washington. Whole ecosystems would be destroyed, not to mention most of North America’s crops and livestock.

Some human lives would be saved through successful evacuation, but the evacuation area would have to span the continent. Global food prices would skyrocket. In the days following a Yellowstone eruption, expect entire sections of the US and Canada blanketed in ash; ash that smothers the soil, causes buildings and infrastructure to collapse, clogs engines and machinery, and creates an epidemic of respiratory disease. Consider the fact that, when Indonesia’s much smaller Tambora volcano erupted in 1815, it coated parts of the Indian subcontinent, thousands of kilometres away, in three-metre-deep ash deposits and caused a “year without summer” across the whole northern hemisphere, leading to famine and riots.

If Yellowstone erupted, global temperatures could drop 3-5°C and stay low for years. A prolonged volcanic winter, similar to the concept of a nuclear winter, could set in, devastating harvests and ecosystems the world over. As over 7 billion people begin to fight for survival, security and the rule of law would in all likelihood break down, and the human condition could once again become nasty, brutish and short.

Could this really happen? Yes. Scientists calculate that supervolcanoes erupt every 50,000 years, with the last known event at Lake Toba in Indonesia occurring 74,000 years ago. Such big figures quell some concerns, but bear in mind that there is also a 10% chance of a Tambora-scale event occurring in the next 40 years. To put this into perspective, the devastating eruption at Tambora Mountain in 1815 may not even have counted as a supervolcano, but its blast killed 71,000 people and was 16 times more explosive than the largest hydrogen bomb ever tested. Its ash plume stretched 43 km into the stratosphere, compared with a measly 9 km for Iceland’s volcano. If something on this scale exploded, it would make the eruption of Eyjafjallajokull in 2010 look like a child playing with firecrackers.

This series of “What if” blog posts is inspired by the findings of the Global Risks 2012 report. The scenarios they depict are far-fetched, but ultimately plausible. Two more posts – What if antibiotics stopped working? and What if there was no more space in space? will follow later today.

Pictured: Iceland’s Eyjafjallajökull volcano erupting in 2010. A supervolcano would be far more explosive (Thomson Reuters).