If you’re one of the intrepid few who dares to descend into the tenebrous halls of El Malpais National Monument’s lava tubes, you’ll most certainly end up wondering to yourself where all the molten rock came from.
The Jemez Lineament
The quick and easy answer is that all that white-hot, silica-rich and slithering lymph welled up from the Earth’s 2,000-foot-thick mantle. But, of course, the real question is why was the scorching material able to break through the crust precisely here? What’s so special about the land around Grants, New Mexico?
According to geologists, the answer to these questions is basically two-fold. First off, this corner of the state is located along a feature known as the Jemez Lineament — a long strip of weak crust stretching hundreds of miles from East-Central Arizona to Northeastern New Mexico, which is evinced by a menagerie of volcanic necks, plugs, cones, welded tuft formations and calderas. Many believe the Jemez Lineament to be extremely old, persisting from the mists precambrian time.
An Active Transition Zone
The second factor geologists cite as important to the creation of lava flows in the area is that El Malpais is also situated on the boundary (or “transition zone”) between the relatively thin and low-floating crust of the Rio Grande Rift and the relatively thick and high-floating crust of the Colorado Plateau.
It’s not so surprising that between these two chunks of the planet there might be a conduit for magma to climb to the surface. In fact, if you look (squint) at the perimeter of the Colorado Plateau, you’ll see a sort of ring of volcanism separating it from neighboring physiographic regions. (The Gila Wilderness to the south and west of El Malpais constitutes another transition zone. I’ll be writing about recent adventures in this amazing place very soon, so keep an eye out.)
Though the underlying architecture of El Malpais is pretty old — over a billion years in the case of the Jemez Lineament and millions of years in the case of the Colorado Plateau/Rio Grande Rift Transition Zone — its superimposed lava flows are quite young.
The flow containing Junction and Xenolith Caves, for instance, issued forth around 100,000 years ago. The youngest flow, which goes by the name McCarty’s, is a mere 3,000 years old! This means there were humans who got to see rivers of pahoehoe where today’s Interstate-40 now runs.
It also means New Mexico is still considered a very active place when it comes to volcanic activity. Probably just a matter of a few centuries before fire and brimstone is visited upon this great state once again.
- Caving In El Malpais National Monument, Part 1: Xenolith Cave (Farther Still)
- Zuni-Bandera Volcanic Field (New Mexico Tech)
- Lava Tube Formation (National Park Service)
(Today’s featured image — a satellite view of the El Malpais lava fields — is courtesy of Google Maps.)