How to date a volcano

Without rock samples to analyze in a laboratory, how can you tell the age of a Martian volcano? A team led by Stuart Robbins (University of Colorado) used high-resolution images to count craters in the calderas (summit collapse pits) of Mars’ 20 largest volcanos. The more craters a surface shows, the more time has passed since it was last resurfaced by eruptions or some other means. Writing in Icarus for February 2011, they found that Mars has made volcanos throughout most of its history. Roughly 3.5 billion years ago, eruptions changed from an explosive eruption style to one that created more widely reaching lava flows. Apollinaris Patera was the first major volcano to cease activity, about 3.5 billion years ago. The last stage of caldera activity in Tharsis (on Olympus Mons) was about 150 million years ago.