The Sandia Mountains are a fault block range, on the eastern edge of the Rio Grande Rift Valley. The Sandias were uplifted in the last 10 million years as part of the formation of the Rio Grande Rift. They form the eastern boundary of the Albuquerque Basin.
The core of the range consists of Sandia granite, approximately 1.5 billion years old (there is also some metamorphic rock of age 1.7 billion years). This is topped by a relatively thin layer (approximately 300 feet/100 meters) of sedimentary rock (mostly limestone, and some sandstone) of Pennsylvanian age (circa 300 million years ago). Potassium-feldspar (K-spar) crystals embedded within the Sandia granite give the mountains their distinct pink color.
The Sandias are part of a single larger geologic unit, the Sandia-Manzano Mountains. The other part consists of the Manzano Mountains which lie to the south of the Sandias. The two ranges are separated by Tijeras Canyon, which leads to a historically important pass; the canyon is traversed by Interstate 40, following the route of historic U.S. Route 66.