What governs the maximum height of mountains on Earth? Could a mountain as high as Olympus Mons on Mars (21.9 kilometres, more than double the height of Everest) be possible on our planet?
Hillary Shaw
Newport, Shropshire, UK
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Any mountain on a near-spherical object causes an excess load on the surface, as gravity is trying to pull it down to ground level. Rock isn鈥檛 quite solid, either. Given enough time, heat and stress, it will flow like very stiff molasses.
So, on Earth, Mars or elsewhere, a mountain can only get so high before the crust below ceases to support its weight, which is determined by gravity and the density of the rock the mountain is made of. The crust will sag and flow, and the mountain will recede. The stiffness of the rock also determines how much the mountain resists flowing into a flatter hill. Try making a steep mound from mud, for example.
Earth鈥檚 tallest mountain is technically Mauna Kea in Hawaii, at about 10 kilometres from base to peak. Because Mars鈥檚 surface gravity is just 38 per cent as strong as Earth鈥檚, Olympus Mons is only 2.7 times as heavy as Mauna Kea, despite being far taller and wider than it. The increased width also spreads the mountain鈥檚 weight.
On smaller objects like Pluto or Ceres, mountains could be bigger relative to the object鈥檚 diameter. That is, until we get down to asteroids, which are effectively all 鈥渕ountain鈥�, as they lack enough gravity to pull them into a sphere.
At the other extreme, the highest mountain on a neutron star, where surface gravity is 200 billion times more than on Earth, is probably 1 millimetre high. That is around a fifteen-millionth of its radius, compared with Mauna Kea, which is 1/640th of Earth鈥檚 radius, or Olympus Mons, which is 1/170th of Mars鈥檚 radius. But don鈥檛 try to hike up a mountain on a neutron star: gravity would smear you into a 1-atom thick layer many kilometres wide as soon as you landed there. That鈥檚 if the neutron star鈥檚 magnetic field hadn鈥檛 shredded your atoms anyway.
Mark Wareing
Ashbourne, Derbyshire, UK
Experiments comparing the height of small piles of dry sand or rocks can demonstrate that maximum height depends on the size of the base of the 鈥渕ountain鈥� and the material from which it is made. The gradual growth of a large, rocky mountain 鈥� whether by volcanic action or tectonic plate movement 鈥� will eventually be exceeded by its partial collapse under gravity. On Earth, this happens at about 10 km above sea level. Height then declines over time by the action of the elements or tectonic plate movement.
Undersea mountains are supported by water and can reach greater heights.
Jonathan Wilkins
Deganwy, Conwy, UK
Mountains don鈥檛 simply drop into existence 鈥� they are formed, and their formation is a dynamic process driven by forces inside Earth and counteracted by gravity and weathering processes. It has taken the collision of the Indian and Eurasian tectonic plates to create Everest and the Himalayas, and the height of those mountains is still increasing slowly.
There is thickening of the continental crust at the edges of the colliding plates, which float above the denser mantle rock, and the buoyancy of the lighter continental rocks keeps the collision zone elevated.
Simultaneously, unstable rocks on the surface fall under the influence of gravity and experience chemical attack from the atmosphere and biological agents. These processes of weathering will carry away particles of minerals and ions in solution, and will reduce the mountains鈥� height.
Volcanoes, like Olympus Mons and Mauna Kea, were built as lava erupted and cooled to form solid rock. The growth of a volcano is a balance between the rate of arrival of magma from the mantle and the forces of erosion and weathering, but another critical matter is the pressure that must be exerted on the magma to cause its ascent to the top of the volcano.
As the pressure increases with vertical growth, the structure of the volcano may be insufficiently strong to contain the magma, and it will burst out as a flank or fissure eruption, or part of the edifice will collapse along fractures to form a landslide. Finally, it is likely that a large volcanic edifice may start to sink if the strength of the crust is insufficient to support its weight.
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