This radial valley system on a martian volcano is found at Tyrrhena Patera (22°S, 253°W), which also appears to be one of the oldest highland volcanos on the basis of the number of large superposed impact craters. The formation of this volcano may have been characterized by phreatomagmatic eruptions similar to those seen at the coast on the Big Island (slide #15), except at a much larger scale. At Tyrrhena Patera, multiple layers of material (probably ash?) can be seen exposed in the walls of the valleys, which in some places may be 200-300 meters deep. The grid on the image is 50 kilometers. The island of Oahu, Hawaii, has been superimposed in red for scale comparison.
Viking Orbiter 87A14; rectified image produced by M. Robinson.