Geology of Chiricahua National Monument: A Review for the Non-Specialist

Pallister, John; Du Bray, Edward; Hall, Douglas

We gather clues about the geologic history of Chiricahua National Monument by examining rocks in the field and in the laboratory and by plotting the distribution of the various rock types to produce geologic maps that show where ancient landforms (rivers, lakes, and volcanoes) used to be. Just as a detective reconstructs the scene of the crime and a sequence of events, we have reconstructed how and when the rocks of the Monument area formed. We are fortunate because these rocks record the natural environment of Southeast Arizona throughout much of Earth history. We see evidence that the region was alternately flooded by ancient seas, uplifted along great fault zones, and covered by lavas and ash from volcanoes. The most dramatic geologic event took place 27 million years ago when a giant volcano, called the Turkey Creek caldera, erupted just south of the National Monument (Figure 1). This eruption was 1000 times larger than the 1980 eruption of Mount St. Helens and five to ten times the size of the great explosion of Krakatoa in 1883. Similar and still potentially hazardous giant volcanoes exist in the United States (e.g., within Long Valley, California and in Yellowstone National Park) and in a number of other places around the Pacific rim; luckily for us, they erupt infrequently. Yet, the very fact that eruptions are infrequent at these calderas makes predicting how and when they will next erupt difficult. This is one of the reasons we study extinct volcanoes like Turkey Creek. We have determined the size and frequency of ancient eruptions that took place here and we can estimate the size of the area that was devastated by the eruptions. Because erosion has cut deep valleys into the volcano, we can also directly examine parts of the subterranean magma chamber that fed the eruptions, something that is impossible at active, uneroded, volcanoes. In this way, we develop and refine a general model for caldera-forming eruptions and we compile a series of "case examples" that allow us to better understand the hazards posed by still-active calderas. Shortly after eruption, the Turkey Creek volcano would have been similar in appearance to Crater Lake caldera, in Crater Lake National Park, Oregon... but twice as large. The Turkey Creek eruption buried the region with a thick layer of hot ash and pumice, producing a deposit known as tuff. The ash and pumice layer was hot enough to fuse together, or weld. As it cooled, the welded taj[f contracted, forming cracks or joints. Water then seeped into the cracks and began eroding out the columns for which the Monument is best known. Assisted by the wedging action of freezing water, stream flow, and erosion by the wind, the cliffs and columns were slowly carved into the thick layer of welded tuff. We will discuss the volcanic rocks of the National Monument in more detail later, but first we will review the more ancient part of the geologic story.

Geology of Chiricahua National Monument: A Review for the Non-Specialist

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