ceoyoyo writes: "The Tharsis volcanoes on Mars show evidence they may have erupted within the last two million years and may still be dormant, not extinct. The three volcanoes also show evidence of erupting in a chain, much like the Hawaiian islands, with the southernmost showing the oldest lava flows and the northernmost the youngest.
On Earth chains of volcanoes are produced when the crust moves over a magma plume in the mantle. On Mars, since there is no tectonic activity, the researchers theorized that the magma plumes themselves move under the fixed crust." Link to Original Source