Volcanoes are many things: spectacular, terrifying, gargantuan, and enigmatic. They free us from ice ages, destroy civilizations and build new islands from the sea as we watch. They captivate both scientists and non-scientists alike, so whenever a new one is found, it’s always a cause for celebration.

These volcanoes aren’t on Earth this time around, however. They can be found on a peculiar southern region of our faded crimson neighbor, Mars. As reported by NASA’s Jet Propulsion Laboratory (JPL), the Mars Reconnaissance Orbiter (MRO) spotted some unusually textured surfaces in Sisyphi Montes, an area packed with flat-topped mountains.

The characteristic signatures revealed the presence of a collection of minerals – zeolites, sulfates, and iron oxides – which can only collectively form from one type of volcanic eruption, one that occurred through a layer of ice. As ice is no longer present in the region, this means that the MRO has uncovered evidence of an ancient subglacial eruption, probably from one of those suspicious, volcano-like mountains in the region.

“Rocks tell stories. Studying the rocks can show how the volcano formed or how it was changed over time,” Sheridan Ackiss, a graduate student at Purdue University, West Lafayette, Indiana, and one of the researchers who discovered the long-lost volcanic eruption, said in a statement.

^{Curious mineral patches have been identified in a region of Mars long-suspected of having volcanic activity. NASA/JPL-Caltech/JHUAPL/ASU}

Volcanic activity on Mars is thought to be long-dead, but the epic volcanic monuments left behind within the geology of the Red Planet betray to us its much more violent past. A recent study revealed how one epic volcanic effusion billions of years ago actually tipped the entire planet over by 20°. Olympus Mons once squeezed out floods of lava across the Martian landscape; if it grew any larger, it would be so heavy that it would sink into the crust.

The mineral patches found in the Sisyphi Montes region, which extends from 55 to 75° south, can only be formed when lava mixes somewhat explosively with ice. Today, the ice-capped south pole is around 1,600 kilometers (1,000 miles) away from this region, which means that ice sheets were once present in this region.

We have these types of subglacial volcanoes on Earth too: The Eyjafjallajökull eruption in Iceland in 2010 is a good example of this, where the meeting of hot magma and cold ice generated prolonged, huge ash-filled plumes of steam and lava blebs.

These volcanoes either erupt under or onto a volume of ice. Whereas the latter situation tends to be calm and uneventful, the former is almost always more explosive. Ice caps above magma chambers increase the pressure difference between the magma and the outside world, and a higher pressure gradient always means a more explosive eruption.

 

 

^{What happens when lava erupts onto ice. Science Channel via YouTube}

Also, the searing heat of the magma encountering the far colder ice initiates something called a molten fuel coolant reaction (MFCI); the greater the temperature difference, the more energetically the heat from the magma is transferred to the ice, and the more explosive the eruption will ultimately be.

Curiously, magma just erupting onto ice never produces an explosion. Volcanologists think that the ice or water needs to be wrapped up inside the magma, where it will rapidly heat and expand into a gas under additional pressure, before it explodes. This mechanism is likely behind the production of these mineral patches just discovered on Mars.