About 200,000 years ago a series of volcanic eruptions began to create this slope of eroded volcanic boulders. The first two fissure eruptions yielded extensive amounts of liquid lava which flowed naturally down-slope, east towards the Rio Grande valley. The following four eruptions produced thicker lava flows that eventually blocked the volcanic cones, called the Three Sisters. This 17 mile volcanic escarpment is known today as Albuquerque's West Mesa.
The volcanic rock produced by the eruptions is basalt. Sometimes basalt has many little holes in it called vesicles. Vesicular basalt is a product of gases escaping from the lava before it cools and hardens. The exposed surface of basalt darkens in color due to precipitation, extreme temperatures, airborne particles, and action by micro-organisms which cause the metal and minerals to oxidize or rust. This thin layer of oxidation is called desert varnish.