Naked Science Forum
General Science => Question of the Week => Topic started by: evan_au on 03/06/2013 12:19:50
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Eugenie Podolsky wrote in with: I am wondering just how high a mountain on Earth could be?
The Olympus Mons volcano on Mars is about 20km high.
But could a peak on Earth reach this height; and if not, why?
How do mountains form, and what is to stop them reaching even further into the sky?
What do you think?
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On Earth, the height of a mountain is a race between:
- The force pushing up the mountain ("orogeny (http://en.wikipedia.org/wiki/Orogeny)" or "epeirogenesis (http://en.wikipedia.org/wiki/Epeirogenic_movement)"),
- And the forces crushing it down, which include:
- gravity, which tends to make Earth shaped more like a sphere
- erosion wearing it down
- and the balance of flotation ("isostasy (http://en.wikipedia.org/wiki/Isostasy#Isostatic_effects_of_plate_tectonics)") which submerges the base of the mountain into the Earth's mantle (where it melts away).
Gravity is lower on Mars (3.7 m/s2 vs Earth's 9.8 m/s2), which allows a volcano to grow much higher on Mars.
Mars is currently too cold, and the atmosphere is too thin to carry rain, which is a major source of erosion on Earth, although it does have winds strong enough to whip up duststorms of fine dust.
The highest individual mountains on Earth are the Hawaiian volcanoes. The tallest rise only 4.1km above sea level, but that is 9.1km above the surrounding Pacific sea floor. This mass of rock is "floating" on the rock of the seaflooor; like an iceberg, there is a lot more that lies unseen below the surface - below the seafloor, in this case.
Earth has active tectonic plates (http://en.wikipedia.org/wiki/Tectonic_plates).
- The ongoing collision of India into Asia has is pushing up the Himalayan mountain chain. Mount Everest rises above the mound of the Himalayan plateau.
- The ocean floor subducting under a continent has pushed up mountain chains like the Andes.
- The mountain rocks are split apart by melting & freezing water, eroded down and washed out to sea.
- This motion of plates also means that any large volcanic accumulations from a hot mantle plume (like Hawaii) move to different spots over time; this has left a dotted line of relic volcanoes in the Hawaii chain which sink down into the mantle, and are worn down by erosion of rain and the sea; they may eventually be recycled into the mantle at a plate boundary.
Mars is thought to have no active plate tectonics, so the hot spot under Olympus Mons has generated lava in a single spot for an extended period. This massive accumulation of mass has produced a series of fault lines that extend half way around the planet (http://en.wikipedia.org/wiki/Geology_of_Mars#Tharsis_and_Elysium_volcanic_provinces).
See: http://en.wikipedia.org/wiki/Volcanism_on_Mars#Differences_in_volcanic_styles_between_Earth_and_Mars
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At the other end of the scale, astronomers have been able to estimate the height of mountains on Pulsars - at a height of millionths of a meter (much less than the width of a human hair)!
The regular radio pulses from the ashes of these very compact, rapidly rotating stars allows astronomers to measure their rotation rate very accurately. When the mountains get too "high", they collapse under the immense gravitational field, emitting a huge burst of gamma rays and changing the radio pulse rate by a measurable amount.
See: http://en.wikipedia.org/wiki/Starquake_(astrophysics)#Starquake
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Interesting question.
Apparently, according to this list, (http://en.wikipedia.org/wiki/List_of_tallest_mountains_in_the_Solar_System) Mars ranks number two, with Vesta holding the record for the tallest peak in the solar system.
You may be right that there is something about the size of the planet. As we know, gravity forces planets to be spherical, so the smaller moons and asteroids around our solar system are not spherical, but our moon, and the major planets are spherical. This may limit the size of the mountains on larger planets.
All of the planets in our solar system that are larger than Earth are considered to be Gas Giants. This means that they somewhat loose the distinction between solid, liquid, and gas, so it may be difficult to determine what is a mountain peak on Jupiter, or our Sun. Thus, I might also question the conclusion of anything resembling a sub-millimeter mountain on a pulsar.
Anyway, on Earth, many volcanoes go through a building phase and an explosive phase. So, for example Mt. St. Helens (http://en.wikipedia.org/wiki/Mount_St._Helens) lost about 1300 feet during its last eruption, and Mt Mazama (http://en.wikipedia.org/wiki/Mount_Mazama) is estimated to have lost about 4000 feet in its last eruption. It is quite possible that the probability of an explosive eruption increases with the height of the mountain, and the distance of the magma chamber from the peak.
It is possible that the Hawaii volcanoes could get very high, but the crust slowly drifts with respect to the hot spot, creating a chain of islands instead.
A lack of drifting of the crust may be the major difference between Mt. Olympus Mons & Hawaii.
Nonetheless, some inter-plate volcanoes such as Yellowstone can be very explosive.
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Since a rotating planet or larger body adopts an ellipsoidal shape (http://en.wikipedia.org/wiki/Reference_ellipsoid#Ellipsoid_parameters). So the height of any mountain should be measured relative to this imaginary surface.
The maximum height of a mountain depends on its weight, its shape, its mechanical strength and the quality of its foundations. Weight in turn depends on gravity.
It may be significant that Olympus Mons and the Hawaii volcanoes are both shield volcanoes, which are wide compared to their height, spreading the weight over a large area.
It is thought that both volcanoes are made of basalt, although the low atmospheric pressure on Mars may have made the lava from Olympus blast out like a bottle of carbonated drink with the lid removed. It is hard to imagine that the the water geysers on Saturn's moon Enceladus (http://en.wikipedia.org/wiki/Tiger_Stripes_(Enceladus)#Cryovolcanism) would have the mechanical strength to build up significant mountains. Sulphur plumes originally detected on Jupiter's moon Io (https://en.wikipedia.org/wiki/Io_(moon)#Mountains) would not have the strength to build large mountains (but they have subsequently been discovered to also contain a lot of silicate minerals).
Earth has a plastic mantle, which means that mountains will sink into it. Mars appears to have a more solid interior (lower mass & relatively larger surface area=faster cooling), which forms a more solid foundation for a large mountain.
I am not familiar with the phase diagram of Neutronium, but some "cooler" Pulsars (https://en.wikipedia.org/wiki/Neutron_star#Structure) (like less than a million degrees) are thought to support a solid crust - perhaps a crystalline matrix of iron nuclei, compacted by a gravitational field billions of times stronger than on Earth, floating on a sea of superfluid neutronium?