Bordering the two inhospitable planes of Earth and Fire, it shouldn't be a surprise to any traveler that Magma's an unpleasant place. It's easiest to imagine it as spewing out from the plane of Fire, whose leaping flames die down to a hellish glow and then spread in an evercooling mass to the plane of Earth. The surface constantly churns and shifts as the scabrous, hardening chunks are churned back into the molten flow. Near the plane of Fire, this churning landscape bursts with occasional geysers of cryoplastic flow, lava that splatters nearby. Closer to the plane of Earth float pillars of solid rock, borne away by the searing tide.
To survive on this plane, a blood's got to have the same protections as on the planes of Fire and Smoke. The intense heat will toast anyone and anything to a cinder, if foul gases released by the lava don't choke them first. Like the plane of Fire, Magma has a definite surface with an up and down. A protected being can walk on this, though it constantly roils and shifts under his feet.