The conductive heat flow through ice is given by icethickness
Where lambda (=2 W/(m K)) is the thermal conductivity of ice, deltaT is the temperature difference between top and bottom of ice layer, and deltaz is the thickness of the layer.
Imagine again snowball Earth conditions, under which the whole world ocean is frozen over. The only heat flux from the ocean to the ice would originate from geothermal heat. We know that the current geothermal heat flux, F, is about 0.09 W/m2. At a given latitude the temperature at the top of the ice is -30oC, and the water below the ice has a temperature of -2oC (the freezing point of seawater). What would be the thickness of the ice?