Lycus Sulci
Shortly after the discovery of the giant shield volcano Olympus Mons on the Red Planet by the U.S. space probe Mariner 9 in 1971, planetary scientists had noticed that the volcano was surrounded by an aureole of very characteristic terrain. It extends over hundreds of kilometers into the surrounding area. The aureole tells the story of the catastrophic collapse of the lower flanks of Olympus Mons that occurred in its distant past.
Planetary scientists suspect that the landslides started several 100 million years ago, when large amounts of lava flowed down Olympus Mons. The flows probably deposited on bedrock that contained large amounts of water in the form of ice. The volcanic heat melted this ice, so that the rim areas of the volcano became unstable, broke off and partly slid away. During the collapse, huge rockfalls and landslides dropped from the lower slopes of the shield volcano over hundreds of kilometers of the surrounding volcanic plains. The characteristic crumpled appearance likely resulted during the landslide as material slid away from the volcano and became compressed or pulled apart as it travelled across the surface. Over time, wind erosion of weaker material between the peaks accentuated this effect. The hills and ridges are also covered by fine wind-blown dust. Single aureole landslides can be some hundred meters thick. Due to the superposition of several slides, thickness of more than 2000 m can be reached. This indicates, that multiple collapse events took place. These phases of activity can also be seen in the HRSC observation (see annotated image). Here, the deposit showing a large fracture is overlying the older landslide deposit which ends in the smooth plains.
Even today, Olympus Mons displays a high basal scarp, some of which rise up to seven kilometers above their surroundings (see overview map). Comparable Landslides, but on a much smaller scale, are also known from terrestrial volcanoes. For example, the volcanic islands of Hawaii and the Canary Islands are surrounded by such deposits of large rockfalls. Thus, the great similarity of the Martian volcanoes with their terrestrial counterparts becomes obvious here, if one disregards the dimensions of the Martian volcanos. Olympus Mons, with a height of 21.9 kilometers and a diameter of 600 kilometers, is the largest and highest mountain in the solar system. Another example: The 8.2 km Yelwa crater visible at the right (northern) part of the HRSC image is located incredible 1050 km northwest of the Olympus Mons caldera.