Nigel Harris

Many of Earth’s mountains are formed in orogenic belts aligned along plate margins. Their altitudes (reaching >8,000 m above sea level in the Himalayas) are the result of the balance between tectonic forces causing their uplift and erosive processes causing their destruction. The tectonic forces result, in part, from isostacy which is determined by the plasticity of the asthenosphere, but gravity studies across the Himalayas suggest that the highest parts of the range require an additional force to support their altitude which is provided by the flexural rigidity of the lithosphere. This is well demonstrated by the 2015 earthquake in Nepal where radar images before and after the 7.8 M event record a decrease in altitude of the High Himalayas as a consequence of weakening and rupture of the lithosphere by the earthquake.

Radar images of Venus, a planet of similar size to the Earth, provide evidence of 10,000 m mountain ranges, also similar to Earth. However, venusian mountains do no define the sinuous forms of Earth’s great ranges. This demonstrates that mountains of Himalayan altitudes can form in the absence of plate tectonics. On Venus, persistent compressive forces acting on hot lithosphere has led to significant horizontal shortening by ductile mechanisms to form fold-and-thrust belts within thickened crust around the margins of crustal plateaux, possibly analogous to tectonics on the early Earth. Maximum altitudes are unlikely to be climate limited (surface erosion on Venus is low due to the absence of water in the atmosphere) and so must be limited by rock strength. The similarity in maximum altitudes of mountains on Earth and Venus suggests that rock strength is probably the dominant factor in determining the maximum height of mountains on both planets.

Mountains are also formed from volcanic eruptions. Comparisons are drawn between Earth’s Mauna Kea (10,000 m from it’s ocean floor base) and the martian Olympus Mons (>21,000 m). The staggering dimensions of the latter volcano requires the absence of plate tectonics which allows a hot spot to generate magmatism at the same point on the surface for hundreds of millions of years. However the flexural rigidity of the lithosphere must also be sufficient to support the massive volcanic edifice and this is found to be significantly higher on Mars than on the Earth’s ocean floor, due in part to the greater thickness of the martian crust and the absence of a low-viscosity asthenosphere. The smaller planetary mass of Mars is also a factor in supporting a volcano of such dimensions.

Extra-terrestrial examples of mountain building are also taken from Saturn’s largest moon, Titan and from the dwarf planet Pluto. Mountains ranges on Titan reach altitudes of ~3,300 m, much lower than maximum altitudes on Earth. The mountains are formed in an icy rigid outer layer (surface temperatures are -180ºC) which overlies a water-ammonia sub-surface ocean. On Pluto, with a mass one tenth that of Titan, mountains reach altitudes of ~6,200 m. In both cases the low mechanical strength of the sub-surface layer prevents mountains approaching the much greater altitudes of terrestrial mountain ranges and confirms the paramount importance of material strength in determining the heights that mountains can achieve.