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Tohil Mons: The Ionian Mountain that Defies its Very Existence

Dramatic shadows sweep across a mountainous landscape on Jupiter's moon Io revealing a mysterious peak named Tohil Mons - a mountain that shouldn't exist.

The constant eruptions of Io's many volcanoes should prevent the formation of mountains formed by up thrusting rock, yet there it stands. Tohil Mons towers about 18,000 feet (5.4 kilometers) above the surface.

Tohil Mons is considered to be one of the most geologically complex mountains in our Solar System. Despite its volcanic origins, there are several theories about the geologic nature of Tohil Mons. It shows further evidence of forming from other mountains through tectonic and erosional (collapse) processes, in part due to viscous silicic (silica-rich volcanic rock or magma) volcanic activity.1

Computer generated image showing ridge of Tohil Mons.
A vertical enhanced graphic of Tohil Mons.

Io's volcanic activity does not generally produce conical mountains -- usually their lava run out into thin long flows. 2 There are very few examples where the lava is thick enough to form positive relief features. Mountains of Io are generally explained as tectonic features. Why?

The answer lies in the erratic nature of volcanoes. It is this nature that explains how mountains on Io originated.

Black and white image of Tohil Mons taken from orbit.
Galileo orbiter mosaic of Tohil Mons.

Io's lava makes it to the surface and normally radiates its heat into space (Io has no atmosphere to speak of). But volcanoes are not stable land forms - if volcanism falters in one region, the surrounding crust begins to heat. This causes the crust to expand, generating compressive thermal stress in the crust, which in turn forces the crust apart, forming faults and mountains.3

This helps explain why concentrations of mountains are seen on Io that are separated from concentrations of volcanoes. Researchers have proposed that similar events may have occurred on Europa, and the early Earth.

Major questions remain about how Io's mountains form and how they are related to Io's ubiquitous volcanoes. Although Io is extremely active volcanically, few of its mountains appear to be volcanoes. However, two volcanic craters do lie directly to the northeast of Tohil's peak, a smaller dark-floored one and a larger one at the very edge of the mosaic. Furthermore, the shape of the pit directly east of the peak suggests a volcanic origin.


1 H. Hargitai and D. Karátson: Silicic Volcanism on Io? Evidence fromTohil Mons and Other Possible Volcanic Cones. Lunar and Planetary Science XXXIV (2003) #1544.

2 H. Hargitai and D. Karátson: Silicic Volcanism on Io? Evidence from Tohil Mons and Other Possible Volcanic Cones. Lunar and Planetary Science XXXIV (2003) #1544.

3 R. R. Wilson and P. M. Schenk: From Tohil to Inachus: An Ionian Topography Progress report. Lunar and Planetary Science XXXIII (2002) #1995.


M.H.Carr et al: Volcanic features of Io. Nature Vol 280 (1979) pp 729-733.

D.H. Grinspoon: Venus Revealed. Addison-Wesely Publishing Co. (1997) p221.

Last Updated: 22 February 2011

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Last Updated: 22 Feb 2011