Iceland’s Katla volcano sits under the Mýrdalsjökull ice cap like a dragon sleeping under a weighted blanket. Except this dragon breathes fire hot enough to melt kilometers of ice in hours, and when it wakes up—which it does roughly twice a century—the resulting floods carry icebergs the size of three-story buildings across farmland at 200,000 cubic meters per second.
That’s a subglacial volcano.
When Ice Meets Fire and Nobody Wins That Argument
Here’s the thing about putting a volcano under a glacier: physics gets weird. Normal volcanoes do their thing in open air, spewing lava that cools predictably, building cones we recognize from every geology textbook. Subglacial volcanoes erupt under ice sheets sometimes a kilometer thick, and the meltwater doesn’t politely drain away. It pools. It heats. Pressure builds until the water finds an exit, and when it does, you get what Icelanders call a jökulhlaup—a glacial outburst flood that makes regular flooding look like a leaky faucet.
The 1918 eruption of Katla produced a jökulhlaup that expanded Iceland’s southern coastline by five kilometers in a single day. The floodwaters carried so much volcanic debri and ice that they created new land where there’d been ocean. Geologists measured flow rates exceeding those of the Amazon River. For context, the Amazon is the largest river on Earth by volume.
Wait—maybe that’s not even the strangest part.
Subglacial eruptions create these bizarre flat-topped mountains called tuyas. Mount Meager in British Columbia is one. So is Herðubreið in Iceland, which looks like someone sliced the top off a regular mountain with a laser. They form because lava piles up under the ice, constrained by that frozen ceiling, spreading sideways instead of building a classic cone. When the ice finally melts away—maybe ten thousand years later—you’re left with a mountain that appears architecturally impossible, its summit plateau marking the exact thickness of the ancient glacier that shaped it.
The Continent Nobody Knew Was a Volcanic Hotspot
Antarctica has entered the chat. Turns out the frozen continent hides at least 138 volcanoes under its ice sheets, and we only catalogued most of them in 2017 using ice-penetrating radar. Mount Erebus, the southernmost active volcano on Earth, sits partially exposed on Ross Island, perpetually fuming. But its subglacial cousins remain hidden, detectable only through seismic monitoring and gravitational anomalies.
Scientists worry—though “worry” might be understating it—that eruptions under the West Antarctic Ice Sheet could accelerate ice loss. A 2018 study identified a mantle plume beneath Marie Byrd Land generating heat flux of up to 150 milliwatts per square meter, enough to melt significant ice from below. This isn’t theoretical: researchers found evidence of recent eruptions under the ice, including a likely event around 2,000 years ago that nobody witnessed because, well, it happened under two kilometers of ice in the most remote place on the planet.
The ice doesn’t stop the eruptions. It just hides them.
When Volcanoes Explode Harder Because of Ice Physics
Conventional wisdom says water cools lava. Conventional wisdom hasn’t met a subglacial eruption. When magma at 1,200 degrees Celsius contacts ice, the ice doesn’t gradually melt—it flash-vaporizes. Steam explosions fragment the magma into fine ash, and because this happens in a confined space under tremendous pressure, the eruptions become more explosive, not less. The 2010 Eyjafjallajökull eruption in Iceland—the one that grounded 100,000 flights and stranded 10 million passengers—was amplified by exactly this mechanism.
Geologists call these phreatomagmatic eruptions, which is science-speak for “water met magma and things got dramatically violent.” The ash from Eyjafjallajökull was finer and more abrasive than typical volcanic ash because the steam explosions shattered the magma into microscopic glass shards. Jet engines really don’t appreciate inhaling glass powder at 900 kilometers per hour.
British Columbia’s Mount Meager last erupted around 2,400 years ago, producing pyroclastic flows that traveled 30 kilometers down valley. The volcano sits under modern glaciers and remains seismically active. When—not if—it erupts again, the combination of ice melt, ash, and debris could generate lahars affecting communities downstream. Scientists monitor it, but monitoring doesn’t stop eruptions; it just gives people time to leave.
Iceland’s Grímsvötn volcano has erupted roughly 28 times since 1200 CE, making it one of the country’s most active. It sits entirely beneath the Vatnajökull ice cap, Europe’s largest glacier by volume. Each eruption melts billions of liters of ice, and each resulting jökulhlaup rewrites the landscape. The 1996 eruption melted enough ice to fill 400,000 Olympic swimming pools, and the subsequent flood destroyed a 900-meter bridge and severed Iceland’s ring road.
We’ve built civilizaton on a geologically active planet that doesn’t particularly care about our infrastructure.
