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P. Schenk, R. Wilson, H. Hargitai (LPI)
A global catalog of mountains on Io has been completed. Locations, dimensions, heights, morphologic type, and associated structures are cataloged. 105 mountains have been identified that are not due to erosion of layered plains. The largest is 570 km long, the highest (Boosaule Montes) is 16±2 km high. There are 5 major morphologic types: mesa, plateau, ridge, peak, and massif, each of which may or may not be related to different modes of formation. Although volcanic calderas often abut mountains, mountain and volcano spacing indicates this is strictly related to crowded surface conditions and not directly to any causal relationships. Ridge, peak, and massif type mountains often resemble tilted blocks, consistent with models of crustal thrust faulting and block uplift. Whether all mountains start as thrust blocks is unclear but observations of landslides at roughly 10% of mountains (and of other mass-wasting landforms in high-resolution images) suggests that some mountains may indeed evolve from one morphologic type to another. No dependence on latitude was observed, but mountains are most concentrated near 70 and 270 longitude. The essentially random pattern superposed on the antipodal concentration is consistent with recent models involving horizontal compression deep in the crust, whether due to global burial of the surface by volcanism or large scale variations in heat flow at the base of the crust, modulated by global-scale asymmetries, such as variations in the dissipation or migration of tidal heat influx.
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