Black Mountain is a cinder cone--or what remains of a cinder cone now that much of it has been mined to provide decorative rock for landscaping. I'd read about Black Mountain but had never visited or seen it before. Although it's a pretty large feature, it doesn't stand out prominently on the horizon, in part because the miners have knocked down the top of the mountain.
As you get close to Black Mountain you get a better impression of its size, and a vivid impression of how much of it is already gone.
This view shows the road that's been dug out of the mountain's main mass.
Some great geological features still remain, though. This volcanic dike--the frozen evidence of an intrusion of magma into a crack, stands above the desert like a prehistoric sentinel.
The tour's second stop was Aden Crater, a shield volcano. Visitors reach the crater over a very difficult road, where your--necessarily 4x4--vehicle crawls directly over rocks. From outside the crater, you can see the wall of lava surrounding the crater.
From the area where we parked our cars, we zigzagged up the crater's wall.
After the crater's wall were formed, basically as a large bowl, the center of the bowl filled up with lava. Here's a view from just inside the crater, looking north over the crater's rim.
Some of the lava in the crater's bowl has been taken over by vegetation such as ocotillo.
This is the pit from which the lava flowed to fill the crater.
The "slide" is an interesting feature: A colder, more solid chunk of lava skidded down a hotter, still plastic section of lava, leaving these traces in what is now solid rock.
Another interesting feature, near the source of the lava, is a fumarole, a deep hole through which volcanic gasses vented. This narrow hole goes down over 100 feet. I didn't go down there and measure it personally, though. Remains of a sloth, some 11,000 years old, were found at the bottom by spelunkers.
Looking southeast, with El Paso's Franklin Mountains on the horizon, you can see these collapsed lava tubes.The third volcanic feature we visited on the tour was Kilbourne's Hole. I'd been to the rim a couple of times before but had never gone into the crater before. The crater was formed much differently than Aden Crater. Kilbourne's Hole is the result of a single, almost incomprehensibly powerful steam explosion. This area had been an inland sea, eventually forming deep layers of sedimentary rock. Volcanic activity then covered the sedimentary rock with a layer of lava, which formed a kind of seal. When water made its way down to the hot magma miles below the surface, this seal meant that the steam had no where to go. Eventually the pressure built up to the point where the whole thing exploded, creating a crater about two miles across, strewing the rock from the explosion across the landscape. The result was one of the largest maar craters in the world.
Some of the material from the explosion, and then erosion products, partly filled in the crater. It's still a couple of hundred feet deep, though. At places on the crater's rim the various layers are quite clear. The lowest layers are the sedimentary materials of the old sea. Above the sedimentary layer lies the lava layer, not that thick but distinct. And about the lava lie the layers of material from the explosion and subsequent depositions.
In other places along the crater wall the layers are less distinct as thousands of years of erosion have weathered the contours.
This erosion has brought some of the explosion materials from the crater's rim back into the crater's bowl. Sandy washes inside the crater testify to the ongoing processes of weathering and erosion.
In the original explosion, minerals from the hot magma that heated the steam were ejected onto the surrounding landscape. With the erosion of the tops of the crater walls, some of these minerals have made their way back into the crater. The most spectacular mineral to be found in Kilbourne's Hole is olivine, fragments of which can be found lying on the surface in and around the washes.
Olivine, when found in larger crystals, is called peridot. I collected a few samples of olivine before returning to the crater's rim and then driving back to El Paso.For more complete information on these volcanic features, see
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