A color image of the Tyrrhena Patera Region of Mars; north toward top. The scene shows a central circular depression surrounded by circular fractures and highly dissected horizontal sheets. A patera (Latin for shallow dish or saucer) is a volcano of broad areal extent with little vertical relief.This image is a composite of Viking medium-resolution images in black and white and low-resolution images in color. The image extends from latitude 17 degrees S. to 25 degrees S. and from longitude 250 degrees to 260 degrees; Mercator projection.Tyrrhena Patera has a 12-km-diameter caldera at its center surrounded by a 45-km-diameter fracture ring. Around the fracture ring, the terrain is highly eroded forming ragged outward-facing cliffs, as though successive flat-lying layers had been eroded back. Cut into the sequence are several flat-floored channels that extend outward as far as 200 km from the center of the volcano. The structure may be composed of highly erodible ash layers and the channels may be fluvial, with the release of water being triggered by volcanic activity (Carr, 1981, The surface of Mars, Yale Univ. Press, New Haven, 232 p.).