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In a field of Martian craters lies an unassuming depression that has news sites proclaiming the latest supervolcano discovery. Could this be a new class of volcano?
When the science journal Nature published its recent article on Mars’ newly discovered supervolcanoes, almost every news site covered it. What journalist would miss the opportunity to use the term “supervolcano” in a headline? The coverage ranged from unbridled enthusiasm to conservative analysis. But what makes this finding fascinating isn’t the past power of these Martian volcanoes, but that they are unlike any previously known volcanoes on Mars.
The term supervolcano came into popular usage around ten years ago when a BBC documentary used it to describe the active volcanic area beneath Yellowstone National Park. Scientifically known as a resurgent caldera, a supervolcano on Earth differs from the less super because it releases all of its magma at once – over a 1000 km3 – rather than spewing a tenth or less at a time. When the entire subsurface magma chamber depletes it causes the crust to collapse and creates a massive depression. Now a beautiful place to visit, the most destructive super eruption occurred 74,000 years ago at Mount Toba in Sumatra. Fortunately, the next one isn’t likely to blow for another million years – then again the last one was less than 27,000 years ago.
Unlike Olympus Mons, one of the largest volcanoes in the solar system and almost three times as tall as Mount Everest (~8.8 km compared to ~25 km), to the untrained eye this Martian super caldera looks just like another impact crater. In fact, the lead author, Joseph Michalski, was originally analyzing surface craters in the Arabia Terra with images from the Mars Orbiter Laser Altimeter (MOLA) when he noticed a few had features that more closely resembled volcanic calderas. The most promising candidate was Eden Patera.
A classic impact crater looks like solidified concentric ripples from a pebble dropped in a placid pond. The rings can surround a central peak. Or there can simply be a single depression with an uplifted rim, depending on the size of the colliding object. The circular geometry results from ejected material rather than the form of the colliding object. Eden Patera, had the classic deep depression of an impact crater, but it was missing the signature shape and spray.
According to the researchers, this meant either Eden Patera was an ancient impact crater whose ejecta had eroded away with time, or Eden Patera was an ancient supervolcano whose eruption had caused an enormous crustal collapse. If the features of Eden Patera resulted from an ancient collision it should have had the same diameter to depth ratio as the other ancient craters found across the planet – it did not.  Instead, researchers found that Eden Patera had the same ratio as the young surrounding craters, all of which had preserved surface ejecta. Thus the impact crater hypothesis was eliminated.
A second explanation behind the collapse feature of Eden Patera could be from the presence of ice. In fact, ice shaped several low-lying areas within the Arabia Terra. However, these areas were significantly smaller in size. The sheer volume of the caldera void led researchers to conclude that subsurface ice disappearing over time is unlikely to have caused the massive collapse.
Without another explanation for their uncovered feature, scientists have named this low-lying volcanic super structure the “plains style caldera complex”. A classification that could help identify other previously unrecognized volcanoes and solve a long standing mystery of why the area has the physical and chemical composition of a volcanic region, but does not appear to host any. 
Articles published in the LA times, BBC, The Guardian, and others.