Olympus Mons may once have been an island
An ancient landslide at the giant volcano may have been carried away by an encircling ocean
“This all adds support to the idea that ancient Mars once hosted a great ocean in its northern hemisphere basin”
Olympus Mons on Mars is the tallest volcano in the entire solar system. Its summit towers 25km above its base, which is around 620km in diameter. But it’s not just the sheer size that is notable about this mountain. Unlike any other Martian volcano, the slope of Olympus Mons doesn’t smoothly merge with the surrounding ground, but is cut off around the circumference with a steep cliff face. The geological term for this is a ‘basal scarp’. It’s clearest along the northern and western flanks of the mountain, but has been partially or completely obscured around the rest of the volcano’s perimeter by later lava flows.
Spreading out from the edge of the steep basal scarp are 10 or so huge oval-shaped, rough deposits known as aureoles. Various theories have been proposed since the 1970s as to what created these aureoles, from lava flows to volcanic deposits emplaced beneath glaciers. But the consensus today is that their shape is best explained by catastrophic landslides – rubble that spilled out across the surrounding landscape from landslides off the flank of the mountain around 3.5 billion years ago.
Fabio Vittorio De Blasio in the Department of Earth and Environmental Sciences at the University of Milan has been studying the basal scarp and surrounding aureole deposits at Olympus Mons. By using the very accurate topological maps that we now have of the Martian surface, De Blasio was able to calculate the total amount of material making up the aureole deposits. From this he could reconstruct what the original outline of Olympus Mons might have looked like, before its flanks collapsed in these colossal avalanches of rock.
He estimates that the initial shape of Olympus Mons would have been 200km wider in the western and north-western directions before the collapse of the aureole landslides. What’s really strange about the northern and western aureole deposits, though, is just how far they have spread across the surface – over 700km from the scarp. That’s an extremely long way for rocky boulders to roll along the ground. What De Blasio argues is that, in fact, these particular landslides fell into an early ocean on Mars and the debris was carried further along the seafloor in the powerful turbidity current created in the water, just as happens in Earth’s deep-sea with underwater landslides. Indeed, when De Blasio looked in detail at a ridge located at the end of the northern aureole he found a sedimentation pattern that looks very much like a tsunami deposit.
This all adds further support to the idea that ancient Mars once hosted a great ocean in its northern hemisphere basin. And for me, the striking mental image is of a youthful Olympus Mons, not towering over dusty plains today, but as a volcanic island, its gentle flanks forming a shoreline lapped by waves. De Blasio points out that the volcanoes on Earth with a profile most similar to that of Olympus Mons are those of the Canary Islands and Hawaii. These volcanoes have experienced landslides into the ocean, and have steep cliffs like the basal scarp of Olympus Mons. Not only that, but the surrounding seafloor is also covered with farspreading deposits similar to the Martian aureoles.