Geologic Studies in the Pinacate Volcanic Field

Geologic Studies in the Pinacate Volcanic Field JAMES T. GUTMANN The Pinacate is a special place thanks largely to its geology. At its core lies the Sierra Pinacate, a central massif of old lava flows that is mantled with and surrounded by younger flows and cinder cones. This rugged volcanic terrain made the Pinacate a bastion of relative safety for many creatures, including the bighorn sheep that figured so prominently in Hornaday’s 1907 expedition. Cinder cones, the most common type of volcanic edifice in the world, are remarkably abundant here; more than four hundred are concentrated in the 1,500-square-kilometer area of lavas. More unusual geologically are the great Pinacate craters. These are maars, craters formed by steam explosions when rising molten lava encountered water in the ground. There are eight maars, plus a tuff cone (Cerro Colorado, which has its own large crater) and a tuff ring (Diaz Crater). The maars are unusual for their size and number, as well as for the record of volcanic history exposed in their steep walls. Looking closely at the lava flows and cinder cones reveals other unusual features, including large crystals of feldspar so clear and glassy that few would recognize them as such. Indeed, one prominent mineralogist, when shown a large Pinacate feldspar crystal, thought it was a fragment of a totally different, common mineral (fluorite); red-faced, he was convinced only after physical testing that the specimen actually was plagioclase feldspar, the most common mineral in the crust of the Earth. The Pinacate has drawn geologists and groups of university geology students from as far away as northern New England. But the rough terrain makes access difficult, and field work in the Pinacate is time-consuming. JIM GUTMANN grew up in Maine and became interested in pegmatite minerals at a young age when exploring cellar excavations near his home. He majored in geology at Amherst College. Graduate school at Stanford was followed by thirtysix years (and counting) in the Department of Earth & Environmental Sciences at Wesleyan University, Middletown, Conn., where he taught physical geology, mineralogy, petrology, and volcanology. Now nominally retired, he continues research in the Pinacate, hiking, and traveling with his wife, Peggy. Journal of the Southwest 49, 2 (Summer 2007) : 189-243 190 ✜ JOURNAL OF THE SOUTHWEST Vehicular access is limited. More than one geologist has declined to work in the region out of concern for personal safety along the border. The logistics of geologic field work in the Pinacate can be challenging and much remains to be done. But the area is changing fast, and the Pinacate may receive much more attention in the near future. This report outlines most of the geologic studies concerning the Pinacate volcanic field more or less in chronological order. The coverage is not encyclopedic. Journal articles primarily compiling data from other volcanic regions are not included, nor are news reports, published abstracts that summarize material presented in detail elsewhere, or articles that use images of Pinacate features as examples to help understand landforms elsewhere. Otherwise, the vast majority of reports published in the geologic literature are noted here, together with some remarks on field aspects of the underlying work and personal observations of geologists. GEOLOGIC ADVANCES PRIOR TO THE OPENING OF MEXICO ROUTE 2 IN 1956 Native inhabitants of the region may have seen eruptions in the Pinacate, for these have occurred at least as recently as 12,000 years ago (Gutmann, Turrin, and Dohrenwend 2000), after the arrival of humans in North America (see Waters and Stafford 2007). But the first written record of geologic observations there may have been made by a Spanish military man in 1701. Captain Juan Mateo Manje accompanied Jesuit Padre Eusebio Francisco Kino on some 7,500 miles of his travels in Pimería Alta, and Manje was “a most excellent diarist” (Bolton 1960:272). According to Karns (in Manje 1954:xii), chapter 8 of Manje’s 1716 manuscript is believed to represent the complete notes Manje recorded during his travels. These notes include description of a journey made by Manje, Kino, and Padre Juan María Salvatierra on March 20, 1701, around the southwestern base of the Sierra Pinacate from near...