Since the 1970s, planetary geologists have known that volcanic features cover large swaths of Mars. Early Mariner 9 images revealed massive shield volcanoes and lava plains on a scale unlike anything on Earth. Olympus Mons, the tallest volcano in the solar system, stands nearly three times higher than Mount Everest. Alba Mons, the planet’s widest volcano, spans a distance comparable to the length of the continental United States.
Both Olympus Mons and Alba Mons were primarily built by basaltic effusive eruptions—relatively calm outpourings of “runny” lavas that spread across the surface in sheets. This is thought to be the most common type of volcanism on Mars, accounting for the vast majority of its volcanic landforms. However, a small portion was produced by explosive volcanism of the sort that forms volcanic cones, pyroclastic flows, and ashfalls.
The dearth of explosive volcanic features on Mars has long puzzled geologists. With an average atmospheric pressure 160 times lower than Earth’s and only a third of the gravity, explosive eruptions should theoretically occur more easily on the Red Planet, said Petr Brož, a planetary geologist with the Czech Academy of Sciences. That rarity is part of what makes features like the volcanic cones (shown above) found in Mars’ Ulysses Colles region so compelling to planetary geologists.
“They appear to be scoria cones—a clear sign of explosive volcanism,” Brož added. “They were the first identified in the Tharsis region in the 2010s, and they helped paint a broader and more complete picture of Martian volcanism.”
The CTX (Context Camera) on NASA’s Mars Reconnaissance Orbiter captured this image (second image above) of Ulysses Colles above on May 7, 2014. Ulysses Colles is located at the southern edge of Ulysses Fossae, a group of troughs within the Tharsis volcanic region.
The OLI (Operational Land Imager) on Landsat 8 captured an image with similar cones in the San Francisco Volcanic Field (SFVF) in northern Arizona on June 19, 2025 (top). Planetary geologists consider the cones …