Because Mars currently lacks plate tectonics, volcanoes there do not show the same global pattern as on Earth. Martian volcanoes are more analogous to terrestrial mid-plate volcanoes, such as those in the Hawaiian Islands, which are thought to have formed over a stationary mantle plume. read more
There are many volcanoes on Mars. So many, that the planet is broken down into volcanic provinces for easier reference. Quite a few of them are very large because the planet has not had tectonic plate action for billions of years, so a single hotspot could flow unabated for millenia. read more