Styles of Phreatomagmatic Activity Adjacent to Volcanic Constructs on Mars

Details Styles of Phreatomagmatic Activity Adjacent to Volcanic Constructs on Mars Styles of Phreatomagmatic Activity Adjacent to Volcanic Constructs on Mars

May 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm...v42a11w&link_type=abstract

American Geophysical Union, Spring Meeting 2001, abstract #V42A-11

Mathematics

Logic

5470 Surface Materials And Properties, 5480 Volcanism (8450), 8414 Eruption Mechanisms, 8450 Planetary Volcanism (5480)

Scientific paper

Early in the analysis of Viking Orbiter data, it was recognized that there were numerous sites on Mars where igneous intrusions may have interacted with ice near the surface. Hrad Vallis (34N, 142E) in Western Elysium Planitia, and Olympica Fossae (25N, 245E) to the southwest of Ceraunius Fossae, were two such candidate areas. New images from the Mars Orbiter Camera show striking differences between these two sites, revealing a wide diversity of depositional and erosional features. We are therefore exploring several potential terrestrial analogs to better constrain models of heat transfer from the igneous intrusion, the style of "eruption" of the water/sediment mixtures, and the hydrologic conditions in the substrate at the time of emplacement. We have found layering at the source of Hrad Vallis, and several nearby impact craters 270 - 530 m diameter that are almost totally mantled, consistent with the deposition of 20 - 30 m of sediment around the source graben. Prominent sub-radial ridges occur within this 8,400 km2 deposit; close to the source, these ridges have a spacing of 100 - 120 m but grade to smaller ridges 60 m apart within 2 km of the source. No "de-watering" features are visible on this unit. In contrast, Olympica Fossae displays no depositional features near the source graben. We interpret these morphologic differences to be due to a higher sediment load of the fluid that reached the surface at Hrad Vallis compared with Olympica Fossae. At neither site are there signs of "weeping" graben walls, indicating that the source of the water was probably at a depth greater than that of the graben (about 60 - 100 m). With due allowance for bulking and for errors of measurement, the volumes of the deposits are comparable to the volumes of their parent source depressions. We envisage that these deposits were created by phreatomagmatic explosions in which heat from a sill-like intrusion melts ice occupying pore space in crustal rocks and boils the resulting water. Calculations show that steam pressures of 1-3 MPa can readily loft the overburden from depths of a few hundred meters and lead to ejecta speeds greater than 100 m/s. Condensation of the water vapor during the explosion process leads to emplacement of a wet deposit, and plausible variations in ice content of the crustal rocks explain the sediment load variations.

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