The Medusae Fossae Formation, found on Mars’ equator, is a large geologic unit whose origin has remained mysterious. It appears to be composed of volcanic materials, but has no obvious volcanic source. New research by Peter Mouginis-Mark at the University of Hawaii and James Zimbelman at the Smithsonian Institution suggests an innovative formation mechanism: the materials that form the Medusae Fossae Formation could have been produced at the base of Olympus Mons, from which they were eroded and floated across an early Martian sea to its present location. This phenomenon is known to occur on Earth when submarine volcanism produces large patches of floating pumice in the ocean. On Mars, the lower units of Olympus Mons would have erupted into ice-filled sediment, producing a lightweight rock known as hyaloclastite. Subsequent landslides would expose this rock to a shallow sea, where its low density would allow it to float. Wind would carry the rock until it encountered a coastline, where it would deposit and eventually erode to form the modern Medusae Fossae Formation. This mechanism could also explain the appearance of the Olympus Mons basal escarpment, which is “missing” material that might otherwise be expected to be there. READ MORE