The Tallest Mountains

This week’s image shows part of the Rheasilvia basin. This large impact crater covers much of the southern hemisphere of 4 Vesta, one of the largest bodies in the solar system. This is one of the largest craters in the solar system, and is one of the most prominent features on Vesta. The large positive relief feature in the centre of the image is the craters central peak. Central peaks are often found in very large impact craters, where the centre of the crater rebounds to form an “uplift feature”. Large craters can exhibit a variety of different uplift features, ranging from single peaks to complex structures such as rings of peaks around the centre of the impact basin. More information about types of crater can be found at this link.

The central uplift of Rheasilvia isn’t that complicated, having only a single peal. However it is quite significant nonetheless. At 22 km it is the tallest mountain in the solar system. This may be surprising, as for a long time this position had been held by the 21.9 km tall Olympus Mons, the largest mountain on Mars. It wasn’t until the Dawn probe arrived at Vesta in 2011 that the height of Rheasilvia’s central peak became apparent.

Despite being of similar height these mountains are very different, because they formed in dramatically different manners. As discussed above Rheasilvia formed as a result of rebound from the impact that created the impact basin. Olympus Mons is a massive shield volcano, and formed gradually as a result of numerous eruptions spewing lava onto the surface of Mars. 

Olympus Mons

Olympus Mons formed through the same process as the tallest mountains on Earth, Mauna Loa and Mauna Kea on the large island of Hawai’i. These shield volcanoes are 10.2 km tall, but the majority of their height is under water. Even so they are far smaller than their martian counterpart, and one of the main reasons for this is plate tectonics. 

 On Earth the tectonic plates are constantly moving, rearranging the positions of islands and continents as they gradually shift across the surface of the Earth. One of the most striking expressions of this movement in the landscape can be seen in strings of islands like those which make up Hawai’i. The Big Island on which Mauana Kea and Mauana Loa are found is just the southernmost of an island chain. All of these islands formed through volcanism, but not all of those volcanoes remain active in the present day. These are just the tallest of a vast chain of underwater mountains called the Emperor Seamounts which extend across the northern Pacific Ocean. 

These volcanoes formed above a “hotspot” in the Earth’s mantle, where lots of very hot material wells up from deep within the planets interior, causing extensive volcanism at the surface. Over time the pacific plate has moved position, shifting the older mountains away from the centre of the hotspot, and bringing a new area of seafloor to the point where volcanic activity is strongest. Over time a chain of volcanic mountains forms, as new volcanoes grow over the hotspot, before being moved away by continental drift.

Olympus Mons formed in much the same way, except that there does not appear to be any plate tectonics on Mars. This means that the surface of the planet isn’t moving relative to the hotspots in Mar’s mantle. Olympus Mons stayed above the hotspot far longer than any of the mountains in the Hawaii-Emperor seamount chain. It had more time to grow, and so is vastly larger than its terrestrial counterparts. 

Image Credits:

“Shaded-relief topographic map of Vesta southern hemisphere showing two large impact basins. GSA Panel Presentation - Paul Schenk 2." Publicdomain image

3d view of Olympus Mons: courtesy of NASA, who also have some more information about Olympus Mons. 

Map of the Hawaiian Islands based on NOAA data via Google Earth