The martian polar caps are among the most dynamic regions on Mars, growing substantially in winter as a significant fraction of the atmosphere freezes out in the form of CO2 ice. Unusual dark spots, fans and blotches form as the south-polar seasonal CO2 ice cap retreats during spring and summer. Small radial channel networks are often associated with the location of spots once the ice disappears. The spots have been proposed to be simply bare, defrosted ground; the formation of the channels has remained uncertain. Here we report infrared and visible observations that show that the spots and fans remain at CO2 ice temperatures well into summer, and must be granular materials that have been brought up to the surface of the ice, requiring a complex suite of processes to get them there. We propose that the seasonal ice cap forms an impermeable, translucent slab of CO2 ice that sublimates from the base, building up high-pressure gas beneath the slab. This gas levitates the ice, which eventually ruptures, producing high-velocity CO2 vents that erupt sand-sized grains in jets to form the spots and erode the channels. These processes are unlike any observed on Earth.