Thursday, January 28, 2016

Dreams of Summer: Going Underground on New Mexico's Continental Divide

When one thinks of the Continental Divide, one might imagine high peaks of the Rocky Mountains, piercing the sky with glacially carved ridges. It's not always quite that way. As we made our way last summer across the flat plateau lands east of Petrified Forest National Park, we passed the Zuni Pueblo, and reached a forest of Ponderosa pines.
The flat highway crossed a barely perceptible rise, and we almost missed a sign that said we were crossing the Continental Divide. Somehow, we had driven to an elevation of more than 8,000 feet and barely noticed. We crossed the divide and entered El Malpais National Monument.
It was late in the day and we were on a tight schedule, so we didn't have time to explore this strange and wonderful landscape as much as we might have wished, but we couldn't pass up the chance to go underground for a while.
El Malpais (Spanish: "Bad Lands") lies at the edge of the Colorado Plateau where it drops off into the Rio Grande Rift. The rift valley a vast fault trough where the continent started to split apart starting 30 million years ago. Rifting is never a gentle process, and the tearing of the crust allows magmas to form and erupt. Most of El Malpais National Monument is a geologically recent series of basalt flows, the youngest ranging in age from 2,500 to 17,600 years ago.
Basaltic lavas are more fluid than silica-rich lavas like andesite or rhyolite. They can flow for miles before cooling, but the flow is aided by the formation of a crust on the surface. The crust acts as insulation, keeping the lava hot and fluid. At times, especially near the end of the eruption sequence, the lava drains from under the crust, leaving behind a system of linear caves called lava tubes. That's what we were visiting that evening. There are undoubtedly many lava tubes in the park, but four are specifically open for exploration.

Junction Cave is several hundred feet long and is easily accessed at the end of a short trail off the main highway. If you intend to visit, be sure to get a free permit from the visitor center. The cave is bat habitat, and the park is extremely worried (with good reason) that the white-nose fungus might attack the bats. The fungus has decimated bat populations back east, and has been moving west over the last few years.

Lava tubes are not "decorated" in the manner of limestone caverns, as solution (the origin of stalactites and the like) is not a prominent process in the volcanic rocks. On the other hand, there are occasionally flow structures and drip structures ("lavacicles") that can be seen. Some of the caves may even preserve ice masses long after winter has ended (there is a commercial ice cave at the park boundary).

New Mexico is famous for caves, Carlsbad Caverns being one of the premier caves of the world. But in some little forgotten corners there are interesting caves of a completely different kind.

1 comment:

Lyle said...

The east side of El Malpais is also interesting with the most recent lava flow being next to the road and also flowing down the drainage along I-40 from the NM 117 exit, for several miles. NM 117 runs right along the eastern edge of the flow and there are several pull out areas to see features of this most recent flow. As your pictures imply the western flows are older than this most eastern flow. There is also a place off 117 called lava falls which shows lava going over a cliff. Further you also see nice views of the sandstone bluffs on the edge of the valley that the flows are in. This last eruption appears to have occurred between 700 and 1540 so it may be newer than Sunset Crater. Native American Acoma legends report the flow destroying crops. Clearly this lava was far more fluid than that at sunset crater.