This Viking Orbiter image shows Uranius Tholus, one of the smaller volcanos in the Tharsis region of Mars. It is only 60 kilometers across and 3 kilometers higher than the surrounding plains. In comparison with Olympus Mons (slide #11), the greater number of impact craters near Uranius Tholus implies that it is substantially older than Olympus Mons. One such crater in the top center of the image has been flooded by lava from the surrounding plains. Because this crater must have formed after the volcano but before the plains, the plains must be younger than the volcano. (This is an example of using superposition relationships to determine the relative age of a series of features by determining which features lie on top of other features.) This area is believed to be more than 3 billion years old.
Viking Orbiter image 516A23