Big Bend National Park

Pyroclastic Igneous Processes

Pyroclastics rocks are formed from fragments of rock and dust that are ejected from active volcanoes into the air. Given the volcanic history of Big Bend National park, it is highly likely that the park contains some pyroclastic rocks.

In fact, there are many different layers of pyroclastic rocks that stretch extensively through the park. Mule Ear Spring Tuff, Wasp Spring Flow Breccia, the Chisos formation, and the South Rim formation clearly contain thick tuff deposits as well as thin interbedded layers of tuff. Tuffaceous clays and sandstone are also evidence of the volcanic history of the park.

Two different ash flows can be found near the pouroff of Burro Mesa. The bottom layer of light gray rock on the side of the mesa is a member of the Chisos tuff formation. The next layer above gray tuff is a dark layer of trachyandesite that resulted from a massive lava flow. Above the trachyandesite, the yellowish orange layer of rock is a the Wasp Spring Flow Breccia member of the South Rim formation. That layer is composed mainly of tuff with interbedded breccia.

	Tuff Canyon lies along Ross Maxwell Drive on the midwest side of the park. A small stream cut this 100+ foot deep canyon through a thick layer of tuff. This photograph was taken from the overlook. There is a hike that one can take through the canyon which gives an excellent close view of tuff and its properties.

This photograph shows a portion of one of the canyon walls. Notice the texture of the wall, a result of the way tuff erodes. Since tuff can contain both fine powder, large granules of ash, and small bits of rocks all cemented together, the way tuff erodes is quite unique. Portions of the rock that are more loosely cemented together wash away leaving rounded cavities and crevices in the rock face. This form of erosion is called differential erosion.

	The photograph to the left is an up close shot of tuff, using a dime for scale. As mentioned above, tuff is a combination of fine powder, larger granules of ash, and small chunks of rock. Once this mixture has settled down on the surface of the earth, over the period of several millions years the ash and debris is covered up. Given pressure and time, the ash and debris lithifies to become tuff.

An ash flow is advancing, threatening to engulf a 1984 Nissan pickup. Ok, maybe that is a bit dramatic considering that this ash flow has been here many millions of years before man ever set foot in the park. This tuff deposit can be found on Ross Maxwell drive directly across from Cerro Castolon. Some vesicular basalt can be found scattered atop this deposit, indicating an ash flow followed by a lava flow at some later date.