Olympus Mons is a shield volcano that towers to an amazing 26 km. That makes it 3 times the height of Mt. Everest. Unlike Everest, Olympus Mons has a very gentle slope. It is up to 550 km at its base. The edge of the volcano’s base is marked by a basal cliff that is 6 km high in some places, but has been eradicated by the overflow of lava in the Martian past.
Olympus Mons is the result of many thousands of basaltic lava flows. The extraordinary size of the volcano has been attributed to the lack of tectonic plate movement on the planet. The lack of movement allows the Martian crust to remain fixed in place over a magma hotspot allowing repeated, large lava flows. Many of these flows have levees along their edges. The cooler, outer margins of the flow solidify, forming the levees and leaving a central trough of molten, flowing lava. In images of the volcano you can see partially collapsed lava tubes seen as chains of pit craters. Broad lava fans formed by lava emerging from intact, subsurface tubes are easily visible as well. Some areas along the volcano’s base show lava flows spilling out into the surrounding plains, forming broad aprons, which are burying the basal escarpment. Crater counts taken by the high resolution images returned by the Mars Express spacecraft in 2004 seem to show that flows on the northwestern flank range in age from 2 million years old to 115 million years old. Since these flows are geologically young, it may indicate that the volcano is still active.
The Olympus Mons caldera complex is made up of at least six overlapping calderas and segments of caldera. Each caldera formed when the roof collapsed following depletion and retreat of the subsurface magma chamber, so each caldera represents a separate eruption. A ‘lake of lava’ seems to have formed the the largest and oldest caldera segment. Using geometric relationships based on caldera dimensions, scientists estimate that the magma chamber associated with this caldera lies about 32 km below the floor of the caldera. Crater size/frequency distributions indicate the calderas range in age from 350 million years ago to about 150 million years ago and may have all formed within 100 million years of each other.
As the largest volcano in the Solar System, Olympus Mons has been extensively studied. Those studies have been helped by the closeness of Mars. Those studies will continue into the future as will the exploration of the entire planet.
Here’s a website all about Olympus Mons, and more information from Exploring Mars.
We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.