The volcanoes are located near the middle of the Rio Grande Rift Valley. A rift valley is a zone of weakness and thinning in the Earth’s crust. As the crust is pulled apart, large blocks of land drop down forming the valley. Thin cracks open deep into the Earth releasing molten lava while blocks on one or both sides of the valley rise. The Sandia Mountains, just east of Albuquerque, formed by uplift along a major fault that marks the eastern edge of the valley. The Rio Grande Rift Valley extends from southern Colorado south to El Paso, Texas. It is one of only few active rifts. Others include the East African Rift, the Rhine Graben in Germany, and the Lake Baikal Rift in Russia.