It's probably not volcanic this close to the surface.

The only real geological process currently on Mars is aeolian modification.
Fines (micron sized rock particles) are produced by saltation when windblown
particles impact the surface causing fracturing of both at a small scale and lifting
smaller particles into the atmosphere.

Every time you see a global dust storm, a fine layer of dust is lifted into the
atmosphere, mostly from windy areas. It then precipitates more uniformly over
the globe once the turbulence abates. There is actually small fraction of particles
that stays suspended in the atmosphere, helping give that ruddy skyline by Mie
scattering.

Given that these areas were chosen with relatively low windspeed in mind, I don't
think it's a suprise that there is a surface layer of pulverized rock fines here. If I
remember right, the particles probably settle with an electric charge so it could
explain the surface texturing.