Tharsis rises and tips the planet over
Three hundred million cubic kilometres of magma built a bulge across a quarter of Mars, raised Olympus Mons some 22 km, and probably tipped the spin axis about 20 degrees.
Tharsis is a welt roughly 5,000 km across and up to 7 km above the datum, covering about a quarter of the martian surface. Three giant shields run along its crest — Arsia, Pavonis and Ascraeus Mons, each 375 to 475 km wide, their summits 14 to 18 km above the datum. Olympus Mons, off the northwestern flank, is larger: a base over 600 km across, an area NASA likens to Arizona, ringed by a scarp up to 6 km high, its summit about 22 km above the datum and over 21 km above the plain at its foot. Quoted heights reach 27 km, on NASA's own pages, because the ground beneath it is not level.
On Earth a plate slides over a fixed hotspot and the magma builds a chain of islands, as at Hawaii; Mars has no plate tectonics, so magma piles into one spot for billions of years. Mars Express stereo imaging dated caldera resurfacing on five big martian volcanoes to the last fifth of the planet's history, with phases as young as two million years.
Phillips and colleagues weighed it in 2001 from Mars Global Surveyor gravity and MOLA topography: about 3 × 10⁸ cubic kilometres of magma. The load rings Tharsis with negative gravity anomalies and a topographic trough and raises an answering high on Arabia Terra. Valley networks nearby drain along slopes that load created, which Phillips read as evidence the trough and high existed by the end of the Noachian.
The team put the volatiles released at the equivalent of a 1.5-bar carbon dioxide atmosphere and a global water layer about 120 metres deep, enough, they argued, for a warmer climate and to cut the valleys — though whether the magnetic field that might have shielded them was still running then is disputed. Volcanoes also emit sulfur, and sulfate aerosols cool rather than warm, so the net effect is unsettled.
A mass that large off the equator is rotationally unstable, and Mars reoriented to bring it onto the equator: true polar wander. Bouley's group reconstructed the pre-Tharsis rotational figure in 2016; the swing usually taken from it is about 20 degrees, poorly constrained in size and date, putting the valley networks on a Mars whose poles sat elsewhere. The reconstruction also makes Tharsis contemporaneous with the valleys rather than earlier, against Phillips.