This graph illustrates volcanic episodes in martian history, as inferred by G. Neukum et al. using pictures from the High Resolution Stereo Camera on board Mars Express.

The image shows the five major volcanic episodes in the history of the Red Planet.

The X-axis represents time, beginning from 4.5 thousand million years into the past, until the present. Scientific names for the epochs of martian history are given corresponding to the dates on the X-axis.

The Y-axis shows the number of craters per square km that are larger than 1 km in diameter. The scale is logarithmic, which means that when a point that corresponds to lower than 1 on the Y-axis, a crater larger than 1 km in diameter is found over several tens of kilometres and so on.

The five volcanic episodes stretch throughout Martian history, occurring around 3.5 thousand million years ago, 1.5 thousand million years ago, 400-800 million years ago, 200 million years ago and 100 million years ago. Neukum estimates that the dates of the earlier episodes are correct to within 100-200 million years and that the later dates are correct to within 20-30 million years.