The rock that makes up the West Mesa escarpment is vesicular basalt. The basalt flow originated from fissures marked by five volcanic spatter cones that can be seen along the western horizon of Albuquerque. Located within the monument boundary, these cones are considered part of a sacred landscape by many Puebloan people today. The last volcanic eruption occurred approximately 150,000 years ago. Although the area is still geologically active, geologists consider these spatter cones to be extinct.
The basalt that makes up the boulders on which the petroglyphs are carved was originally a light gray color. Over time the surface of the rock was coated by a thin black or dark brown layer of oxidation scientists call "desert varnish." When the surface of the boulder is pecked or abraded, the lighter rock underneath is exposed, displaying a stunning light gray-on-black contrast.