Physics
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agusm.v11a..04m&link_type=abstract
American Geophysical Union, Spring Meeting 2004, abstract #V11A-04
Physics
5400 Planetology: Solid Surface Planets, 5480 Volcanism (8450), 8100 Tectonophysics, 8125 Evolution Of The Earth, 8130 Heat Generation And Transport
Scientific paper
Io is the most volcanically active body in the solar system. More than 2.5 W m-2 of tidal energy is volcanically brought to its surface. Of the hundreds of volcanic features identified, 70% are caldera-like depressions sunk into the surface. Coexisting with these volcanic features are more than 100 tilted mountain blocks, reaching heights up to ~18 km. Downward advection of the cooled volcanic crustal stack generates 1) tensile elastic overburden stress; 2) potentially strong compressive radial confinement stress; and if volcanism locally or regionally falters, 3) potentially strong compressive thermal stress as the lower crust reheats. The combination of these effects, especially the last two, plausibly explains the origin of Io's spectacular mountains by means of thrust faulting (McKinnon et al. 2001, Geology 29, 103-106; McEwen et al. 2004, in press in \ Jupiter - The Planet, Satellites, and Magnetosphere; Kirchoff and McKinnon 2003, LPSC XXXIV, abs. 2030). The earliest Earth was also quite hot, from accretional and initially high radiogenic heating. Arguably the early Hadean Earth, like Io, dominantly shed its interior heat via vertical magma transport rather than by conduction or lateral plate tectonics. If so, the early Earth plausibly had a cool volcanic crust thick enough to support substantial topography. But what would create the topography? A lack of stable plumes, copious partial melt at shallow levels, and possibly, a lack of water in the earliest (pre-plate tectonics) mantle could have resulted in muted surface topography so that little or no emergent land existed beneath the early global ocean (<3-km deep). Regardless, mountain formation by thrust faulting may have occurred by the mechanisms above. The greater gravity and size (radius of curvature) of the Earth compared with Io would have increased the elastic overburden stress and decreased the radial confinement stress so that they approximately canceled, leaving the dominant role for compressive thermal stresses (as long as the advective heat flow was high). Greater gravity also implies the Earth's crust was stronger than Io's at similar depths (Byerlee's rule), but pore pressure (due to downward advection of H2O in the volcanics) and "semibrittle" failure would have limited crustal strength. Mountain heights would have been lower than on Io today, but gravitational scaling from Io implies maximum heights exceeding 3 km - emergent land subject to erosion - and possibly the Earth's first source of substantial sediment. Eventual mountain collapse, as seen on Io, volcanic burial, and downward advection would have brought water, crust and wet sediment into the partially molten upper mantle, where wet remelting would have resulted in more silica-rich magma, and possibly, embryonic continental crust.
Kirchoff Michelle
McKinnon William B.
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