Fig. 4. Double ridge formation mechanism (not to scale).
Gray areas are ice, white/light gray shows pore space, and blue areas are liquid water. Red arrows show the direction of forces acting on the sill and the direction of water flow if relevant. a Stage 1—sill emplacement. In Greenland, this likely occurs through concentrated drainage of surface meltwater41. On Europa, sills might form by direct injection from the subsurface ocean27,29,43, from melting over rising diapirs32,44, or through shear heating26,30. b Stage 2—the sill fractures due to internal overpressure or other near-surface stresses and water fills the central conduit. c Stage 3—the sill is divided by refreezing within the central conduit or fracture network. d Stage 4—a double ridge develops as overpressurized water within the refreezing sill is forced out along the planes of weakness flanking the refrozen central conduit, producing the symmetric surface doming.