Volcanology of Mars through the crust to produce volcanic mountains. However, the largest volcano on the planet, Olympus Mons, is thought to have formed when the plates were not moving. Olympus Mons may have formed just after the plate motion stopped. The mare-like plains on Mars are roughly 3 to 3.5 billion years old. The giant shield volcanoes are younger, formed between 1 and 2 billion years ago. Olympus Mons may be "as young as 200 million years." Norman H. Sleep, professor of geophysics at Stanford University, described how the three volcanoes that form a line along the Tharsis Ridge may be extinct
Planet hydrostatic equilibrium (to be rounded under its own gravity), but not enough to sustain core fusion like a star. By definition, all planets are "planetary-mass objects", but the purpose of this term is to refer to objects that do not conform to typical expectations for a planet. These include dwarf planets, which are rounded by their own gravity but not massive enough to clear their own orbit, the larger moons, and free-floating planemos, which may have been ejected from a system (rogue planets) or formed through cloud-collapse rather than accretion (sometimes called sub-brown dwarfs). A dwarf planet is a planetary-mass