"Along the length of the Rift (and particularly the western boundary) there have been extensive volcanic episodes."
New Mexico is bisected by a 600-mile-long valley which runs from southern Colorado into northern Mexico. This is a "rift valley" formed by two parts of the earth's crust pulling away from each other. Because the rift is underground, we don't really know the width of the rift itself but the basins or valley floors vary from a few miles across to nearly 100 miles in the southern part of the state. The Rio Grande Rift began to form about 30 million years ago and continues to expand between 0.5 - 2.0 mm per year.
These dry facts, however, don't tell the whole tale. Along the length of the Rift (and particularly the western boundary) there have been extensive volcanic episodes. The lava flows of Malpais Monument, the volcanoes on Albuquerque's West Mesa, the slot canyon at Kasha-Katuwe Tent Rocks and the canyons in the Jemez Mountains all owe their existence to the Rift.
So does the Rio Grande Gorge near Taos and White Rock Canyon just west of Santa Fe. Although the Rift Valley was well established fifteen million years ago, it wasn't until much more recently--about one million years ago--that the Rio Grande found it's current path and cut deeply through the black, basaltic lava we all admire on the walls of those canyons.
When we drive visitors to White Rock Overlook or to hang their heads over the railing at the Taos Gorge Bridge, they are seeing a huge, beautiful sweep of New Mexico geologic history.