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

It's good to have a new perspective here on this platform. From here you can look to the right, back at the place you just came from, to the left at the third platform, and down to the canyon floor. Looking eastward (back upstream, to your right) you can easily see the dark basalt lava on the farthest part of the canyon floor that is still visible (fig. 2). Look at the contrast between the gentle slopes of the upper canyon walls and the vertical lower walls. The walls of the upper canyon are silt and gravel; the walls of the lower canyon are pyroclastic material. Also note the layers in the pyroclastic material; these are easily seen in the canyon wall opposite the observation platform (fig. 6) and to your left (downstream). The lower wall, a smooth gray rock, has near its top an irregular layer containing large pieces of dark lava in a light-colored matrix. This layer is a debris-flow deposit, formed when a torrent of rocks, mud, and water flowed rapidly over the pyroclastic deposit and was then buried by more pyroclastic material. Debris flows commonly accompany explosive volcanic eruptions because abundant loose material lies on the surface of a volcano, waiting to be washed downslope. Also, heavy rainfall can be concentrated near the volcanic vent because water vapor is a large part of the gas emitted by a volcanic eruption. Water droplets condense on ash particles in the eruption cloud and fall as dirty rain, which gathers in channels and picks up loose material to become thick but fast-moving mud. Debris flows leave their record as poorly sorted mixtures of large and small rock fragments that look like concrete. In fact, when the debris flows were moving they must have behaved like concrete being poured; they were thick, heavy, and could knock over trees and erode older rocks and soil as they ran down gullies.

Figure 6. Wall of Tuff Canyon opposite central observation platform. A coarse debris-flow deposit about 3 feet thick fills a channel in the lower part of the photo.

Near the top of the canyon wall is a more colorful stack of layers. As with any kind of deposit, this layering tells the story of how conditions changed during deposition. In this case, the layers were dumped from pyroclastic surges, hot clouds of particles and gas that swept over the ground surface rapidly in a turbulent (tumbling) flow. In contrast, the thick layer of more uniform gray rock below was deposited by pyroclastic flows, hot clouds of particles and gas that were denser than surges (because of a higher proportion of particles to gas) and that moved in a laminar (sliding) flow (figs. 7, 8).

Figure 7. Sketches showing difference in how pyroclastic flows and surges travel. Pyroclastic flows and surges are hot clouds of gas, rock particles, and liquid lava droplets. Pyroclastic flows, denser than surges, travel closer to the ground and move with a laminar flow (shown by smooth, parallel arrows). Surges move with a turbulent flow (shown by irregular arrows).

Figure 8. Pyroclastic-surge and -flow deposits. (a) Surge deposit, Latera, Italy. Pen in center for scale. (b) Surge deposit, Pulvermaar, west Eifel volcanic field, Germany. Exposed face is about 10 feet high. (c) Pyroclastic-flow deposit containing pumice lumps, Castel' Ottieri, Italy; pen (for scale) rests on a rock fragment. (d) Alternating air-fall and thinner pyroclastic-surge deposits overlain by a thicker pyroclastic-flow deposit, all from A.D. 79 eruption of Vesuvius, at Oplontis, Italy. Holes in lower part of photo are molds of tree trunks knocked down and carried by surges.

Pyroclastic flows and surges result in dramatic, scenic vistas, but they are also extremely destructive. They are hot; they carry rock fragments at high speed; and they can burn, kill, and bury any plants or animals in their way. Pyroclastic-flow and -surge deposits from Mount Vesuvius, for example, buried Pompeii and Herculaneum in A.D. 79. Today the Roman buildings there are still fairly well preserved because they were rapidly filled with pyroclastic material. If these cities had been overrun by lava flows, walls would have been pushed over, wood and wall paintings would have been burnt, and everything would have been buried under hard rock, making excavation much more difficult and expensive. On the other hand, advancing lava flows would have moved so slowly that people would have had plenty of time to escape, and thousands of lives would not have been lost to pyroclastic flows and surges on that long-ago day in Italy.

Worldwide, among the most destructive volcanic eruptions of the last 100 years have been those involving pyroclastic surges and flows at Mont Pelee, Martinique, West Indies, in 1902; Mount Saint Helens, Washington, northwestern U.S., in 1980; El Chichon, Chiapas, Mexico, in 1982; and the Soufriere Hills volcano on the island of Montserrat, West Indies, starting in 1995. In early June 1991, two friends of mine were killed by a pyroclastic surge. Katia and Maurice Krafft were at Mount Unzen in Japan videotaping the eruption from what they thought was a safe place. They were well aware of the dangers and had been filming volcanic eruptions for 20 years. As a matter of fact, they were at Unzen to get new footage of pyroclastic flows and surges for a United Nations video designed to warn people of volcanic hazards. A pyroclastic flow came down a valley and followed a turn in the valley, but a pyroclastic surge above the flow did not turn, and overwhelmed the Kraffts and 39 other people on level ground above the valley. Ironically, an earlier video that they had made for the United Nations is credited with saving thousands of lives at Mount Pinatubo in the Philippines only 2 weeks after the Kraffts died; this video convinced people living on the flanks of Pinatubo to leave for safer places.

So what triggers this destruction? Two processes can generate pyroclastic flows and surges. First, during an explosive eruption a heavy, hot cloud composed of hot particles and gas forms above the volcanic vent. Air mixes with the other two constituents, becomes heated, and expands rapidly. If the cloud is hot enough and contains a small proportion of particles, it is less dense than the surrounding air and rises like a hot-air balloon. In some cases the volcanic cloud can climb into the stratosphere (more than 6 miles!). But if the cloud is heavily loaded with particles, it may rise only a short distance, driven only by the force with which it was expelled from the volcano. Gravity then forces the cloud to fall back onto the ground surface, where it rapidly spreads as a dense pyroclastic flow or surge. A second process that generates flows and surges is the collapse of the steep front of a lava flow; the crumbling hot lava can rapidly release its dissolved gas, which mixes with air to form a violently expanding, dense cloud loaded with lava particles. These two processes produce different characteristics in pyroclastic deposits, especially the sizes of particles and the proportions of particles and gas. The rather small size and extremely frothy nature of the particles in Tuff Canyon indicate that these pyroclastic flows and surges formed by the first process, collapse of an eruption cloud.

It's hard to believe that this quiet spot was once the scene of such violence. You may want to take a few minutes to absorb what you have seen at this platform, but don't take too long. It's time to move on. You're headed west again. From the central observation platform take the trail to the west for a short distance (about a 3-minute walk). You have now reached the third observation platform (#3 on the map).