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The Perseverance rover landed in Jezero crater, Mars, to investigate ancient lake and river deposits. We report observations of the crater floor, below the crater's sedimentary delta, finding that the floor consists of igneous rocks altered by water. The lowest exposed unit, informally named Séítah, is a coarsely crystalline olivine-rich rock, which accumulated at the base of a magma body. Magnesium-iron carbonates along grain boundaries indicate reactions with carbon dioxide-rich water under water-poor conditions. Overlying Séítah is a unit informally named Máaz, which we interpret as lava flows or the chemical complement to Séítah in a layered igneous body. Voids in these rocks contain sulfates and perchlorates, likely introduced by later near-surface brine evaporation. Core samples of these rocks have been stored aboard Perseverance for potential return to Earth.

Many volcanic arcs display fast seismic shear-wave velocities parallel to the strike of the trench. This pattern of anisotropy is inconsistent with simple models of corner flow in the mantle wedge. Although several models, including slab rollback, oblique subduction, and deformation of water-rich olivine, have been proposed to explain trench-parallel anisotropy, none of these mechanisms are consistent with all observations. Instead, small-scale convection driven by the foundering of dense arc lower crust provides an explanation for the trench-parallel anisotropy, even in settings with orthogonal convergence and no slab rollback.