Some lava eruptions occurred where ice lay close to the surface. The magma melted the ice, releasing clouds of steam and possibly floods of water. In other areas, enormous mounds and hills of fine-grained material may have formed when magma mixed explosively with groundwater.
A field of volcanic ring and cone structures in the Tartarus Montes region may have formed when lava flowed over wet or icy ground, causing steam to explode through the lava.
Mars Reconnaissance Orbiter, HiRISE
NASA/JPL/University of Arizona image
The Medusae Fossae Formation is a collection of hills and long mounds that cover large areas near the Tharsis volcanoes. These deposits probably consist of ash and small rocks that rained from the sky during huge explosive eruptions. They have since been sculpted by the Martian winds.
NASA/JPL/University of Arizona image
Radar reveals the thickness and possible physical properties of the Medusae Fossae Formation. The upper image shows radar sounding data that provide a cross-sectional view to a depth of 600 meters (1,970 feet). The lower image shows the orbiter's path across the surface (blue areas are lower, red areas higher). The arrows show where the radar signal penetrates volcanic ash deposits and reflects from the plains surface below.
Mars Reconnaissance Orbiter, SHARAD
Smithsonian/NASA/JPL-Caltech/ASI/University of Rome/SWRI image
