As far as we can tell, there is no longer any volcanic activity on Mars, but there are plenty of tell-tale signs showing that it once happened. One recent study, for example, revealed that Mars was tipped over by as much as 20° by a profuse period of volcanism that forced much of its deeper mantle up onto the crust.

Now, the intrepid Curiosity rover has managed to detect the presence of tridymite within the Gale Crater, hinting at the presence of powerful, explosive volcanism on the ancient Martian surface. Writing in the Proceedings of the National Academy of Science, the research team led by both NASA and the California Institute of Technology imply that the ancient history of Mars needs to be partly rewritten thanks to this discovery.

“It's really nifty, but we were shocked,” Richard Morris, the NASA scientist who led the study, told CBC News. “Something is going on on Mars that we don't fully appreciate.”

Ancient volcanism on Mars is generally considered to have been mostly effusive, calm, and prolonged, which tends to form shield volcanoes, like Olympus Mons. Thanks to Mars’ low atmospheric pressure, it’s likely that any fire fountaining out of one would have reached considerable heights compared to their terrestrial equivalents, but this doesn’t count as explosive volcanism.

For volcanism to be defined as “explosive,” the magma must be thick, gloopy (“viscous”), and full of gas. These two traits, generally speaking, will produce greater depressurization events when the eruptions occur, and will thereby produce more violent and sometimes cataclysmic eruptions. More viscous magma is typically associated with stratovolcanoes on Earth, such as Mount Fuji or Mount St. Helens.

In addition, plate tectonics are required to produce volcanic eruptions that are conventionally explosive; without this mechanism, viscous, silica-rich magma – the type that may feature minerals like tridymite – cannot form, and many geologists still consider Mars to have never experienced it.

^{Image in text: A common form of tridymite, from the Eifel region of Germany. Fred Kruijen/Wikimedia Commons; CC BY-SA 3.0 nl}

So not only is the presence of tridymite a sure sign that true explosive volcanism has occurred on Mars in the past, but it also suggests that plate tectonics were in fact operating here. However, without any source vent or volcano yet located, researchers are unable to say where it’s come from. The mineral samples found within the Gale Crater were likely carried there by ancient water flows, so it’s likely that the source volcano is a long way away from the site.

This isn’t the first time evidence for explosive volcanic activity has been found on Mars. Just this April, a team working with the Mars Reconnaissance Orbiter (MRO) discovered patches of iron oxides and sulfates on the surface that indicated that there was once explosive magma-ice interactions on the Red Planet.

Although not fully understood, contact between magma and water or ice can sometimes cause massive explosions, but only if they mix in a turbulent fashion. They do not require viscous, silica-rich magma to occur though; as seen at Eyjafjallajokull in Iceland in 2010, very runny, high-temperature basaltic lava emerging from a fissure was mixed with overlying glacial ice, which generated enormous, sustained ash columns.

^{Gale Crater, where Curiosity is currently exploring. NASA/JPL-Caltech/ESA/DLR/FU Berlin/MSSS}

So in the case of Mars’ magma-ice explosions, no silica-rich magma was required – any magma works here. As for Curiosity’s tridymite, though, geological wisdom suggests that not only is a tall stratovolcano required, but so is a semblance of plate tectonics.

This brings this study to a remarkable conclusion: Either the ancient geological activity of Mars has been severely underestimated, or this particular mineral can be formed in another, as-of-yet unknown way. Let’s hope it’s the former, because if it is, then this little robot has just made a groundbreaking discovery.