Texas Bureau of Economic Geology Down to Earth at Tuff Canyon, Big Bend National Park, Texas

There has been a lot to take in during the tour of Tuff Canyon today, but before reflecting on all that you have seen, take time to consider the canyon in its wider setting. For example, where did the pyroclastic flows and surges come from? Well, the same sequence of rock layers seen from the east observation platform, for instance, is found in a canyon 1,600 feet east-southeast of Tuff Canyon (fig. 23). There, basalt lava is overlain by surge and debris-flow deposits, and these in turn are covered by pumice-rich pyroclastic-flow deposits, all containing abundant blocks of the underlying basalt. The pyroclastic-flow deposits are capped by more surge and debris-flow deposits and then, unlike those in Tuff Canyon, by welded rhyolite spatter (fig. 24), and finally by a thick rhyolite lava flow. Most rhyolite lava flows have high viscosity and therefore don't travel very far from their volcanic vents. In this case, a vent had to have been nearby. There is no marked trail to this vent area, so a visit is not recommended. Among other unpleasant surprises if you were to try it, the flat surface of the rhyolite lava flow ends suddenly at cliffs that drop vertically into a deep pool of water below.

Figure 23. Topographic map (from U.S. Geological Survey, Cerro Castellan 7.5'-quadrangle map), indicating vent area east-southeast of Tuff Canyon and positions of Tuff Canyon and Cerro Castellan. (click on image for a PDF version)

Figure 24. Welded rhyolite spatter in vent area of figure 23. Dark, flattened blobs are pumice fragments from which most of the vesicles were squeezed out; light-colored rock fragments were cooler and more rigid, so they were not flattened. Pen for scale.

But volcanic vent areas do not have to be tall mountains. Some very energetic eruptions leave no accumulation of lava or pyroclastic material near the vents, which are just holes in the centers of shallow basins. Small accumulations of spatter can form around vents, while the pyroclastic material moves farther outward and spreads more thinly. In this part of the park, rhyolite magma broke through the surface at several vents (perhaps all the rhyolite eruptions happened at the same time or perhaps there were tens of thousands of years between them; we just don't know!) and expelled pyroclastic material before highly viscous lava came out to build domes and flows. In an idealized rhyolite vent (fig. 25), the sequence of deposits from the bottom upward is (1) surge deposits and unwelded pyroclastic flows, then (2) welded pyroclastic flows, (3) welded spatter, and, finally, (4) lava. This progression is caused by decreasing gas content of the magma; in effect, it is a case of foamy magma going flat like a carbonated drink as bubbles burst and gas escapes. As explosive behavior wanes, the amount of remaining gas decreases and the proportion of rock fragments does too. The latter part of such a sequence is most easily seen close up at Burro Mesa Pouroff (fig. 1). Although pyroclastic-flow deposits at Tuff Canyon are not welded, some are at the pouroff. Those at Tuff Canyon possibly travelled farther from their vent and cooled below the welding temperature before they stopped moving, or perhaps the eruption clouds were too cool, when they came out, to permit welding. Anyway, a visit to Burro Mesa Pouroff is worthwhile. From Ross Maxwell Scenic Drive, a well-marked road to the pouroff branches off about 9 miles northeast of Tuff Canyon.

Figure 25. Idealized block diagram of rhyolite vent. Scale approximate.

Stacks of rock layers similar to that at Burro Mesa Pouroff can also be seen on Goat Mountain and Cerro Castellan from Ross Maxwell Scenic Drive. Other, less obvious vents occur at Kit Mountain and Trap Mountain (also visible from the road), and there are at least two others along the length of Burro Mesa, but these require hiking on the west slope of the mesa. These locations are shown in figure 1. Several other probable volcanic vents for the rhyolite are needed to explain its distribution but remain unidentified. In general, welded spatter and rhyolite lava do not travel far from vents, but in a few places (one is 3.1 miles north of the Tuff Canyon parking area on Ross Maxwell Scenic Drive), rhyolite lava lies directly on clay, without any pyroclastic deposits in between.

Shortly after the eruptions ended, this part of the park probably looked a lot like the chain of craters and lava domes you can see today south of Mono Lake, California (fig. 26). Here, pumice forms low rings around the volcanic vents, some of which are plugged by lava domes that formed when rhyolite lava oozed out but could not move far.

Figure 26. Air view of rhyolite lava domes, craters, and pyroclastic deposits south of Mono Lake, California. These volcanic features, about 600 years old, probably resemble what the vent areas in Big Bend National Park looked like about 29 million years ago. Photo by Roland von Huene, U.S. Geological Survey.

The walls of Tuff Canyon record some violent days in this region's past, although the last volcanic eruption in any part of Texas occurred about 17 million years ago. In Big Bend country in the geologically foreseeable future, we expect no new volcanoes to be born, nor old ones to wake up, for perhaps the next million years at least. Weathering, erosion, and deposition will nevertheless continue to move rocks and change the landscape, some of which we have explored today.

A postscript: some people are worried that volcanic eruptions and earthquakes are happening more and more often. Is the Earth falling apart? There is absolutely no evidence to support this concern. Eruptions and earthquakes are being reported more often, but this is because there are more people in more places around the planet and these people have more technology to help them send their news to other places.