LJ wrote:
What would happen if a thick layer of lava was deposited over deep deposits of CO2?

Alex writes:
Though I'm no CO2 expert it would seem to first order to depend on what one defines as "deep deposits of CO2." As Lanagan et al. note, shallow deposits of CO2 are implausible at these latitudes. They further note that, on the assumption that the only source for thermal energy for rootless eruptions comes from the lava flow itself, the general rule of thumb is that the volatile (in this case assumed to be water or ice) must lie at a depth not greater than one half the thickness of the lava flow for water vapor pressure to exceed the lithostatic pressure for phreatic explosions on Mars.

See also
Allen, C. C., Volcano-Ice Interactions on the Earth and Mars, in Advances in Planetary Geology, NASA TM-81979, 161-264, 1980.

Alex R. Blackwell
University of Hawaii