According to new research reported in the journal Earth and Planetary Science Letters, the 10-mile- (16 km) high Martian shield volcano called Arsia Mons produced one new lava flow at its summit every 1 to 3 million years during the final peak of activity; the last volcanic activity there ceased about 50 million years ago — around the time of Earth’s Cretaceous-Paleogene extinction. This digital-image mosaic of Mars’ Tharsis plateau shows...