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Article Excerpt
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. 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 Pimeria 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 Maria Salvatierra on March 20, 1701, around the southwestern base of the Sierra Pinacate from near Tinaja del Cuervo west to the general vicinity ofTinaja Chivos. In the words of Manje, via Karns' 1954 translation, the group passed "mounds and ravines of burnt, molten rock" (Manje 1954:160); and "We judge (although this is not credible) that this, pcrhaps, was a volcano which was active for several years" (Manje 1954:161). He then wrote that "Other volcanos of Europe and those of the two Americas have lasted four or five centuries from such underground conflagrations" and that he and his companions saw slag-like material "which runs from the top of the Santa Clara Mountain" (Manje 1954:161). Ives (1942) provided a different translation of Manje's words and noted that Manje had even attempted to explain the mechanics of volcanism.
Kino had first climbed to the summit of Santa Clara Mountain, as he called the Sierra Pinacate, on October 9, 1698 (Bolton 1919); and he was the first European to do so (Ives 1966). There seems to be no indication that he recognized Santa Clara as an ancient volcano at that time. Kino's first written reference to the volcanic origin of Santa Clara may have been in a part of his memoir penned six years after that ascent (see Bolton 1919:1:229; see also p. 124). We may never know who first realized the volcanic origin of the Pinacate but clearly the team of Kino, Manje, and Salvatierra made a significant discovery as they traveled west that day.
Ives wrote that "This identification of an extinct volcano in 1701 appears to be one of the first, if not the first, of such reports from North America, so far as can be determined from available literature, and antedates the publication of the works of Guettard and Desmarest, frequently called the pioneers ofvolcanology, by about half a century" (Ives 1942:237). It is true that this identification did precede the works of Guettard and Desmarest by about fifty years. But Manje and the padres had an advantage over those pioneering French field volcanologists. The latter recognized the igneous origin of some older rocks that were much less obviously volcanic. In contrast, the route followed by Kino and his companions took them right beside, if not actually over, part of the Ives flow. This enormous basalt flow is among the youngest in the Pinacate and recently has been dated by Turrin, Gutmann, and Swisher (in review) at 13,000 [+ or -] 3,000 years of age. It is perhaps the most spectacular ropey ("pahoehoe") lava in the volcanic field, and is ornamented with an array of flow features, spatter cones, and other structures that bespeak its origin from molten volcanic material. The travelers could hardly have missed it. Nevertheless, the men who journeyed from Cuervo to Chivos that day included very well informed and thoughtful observers of nature.
In November 1706, Kino and others rode to the summit platform atop the Sierra Pinacate and camped on top of one of the three cinder cones there. These cones are located on the northwest, northeast, and south edges of the platform, as at the corners of a triangle. They camped on the cone that was south sloping and the easiest to climb of the three (Bolton 1919:2:211). This description indicates it was the one now called Bonillas Cone (Lynch and Gutmann 1982). The next significant advance in geologic understanding of the Pinacate was made by Ygnacio S. Bonillas some 204 years later.
Ygnacio Safford Bonillas was the son oflgnacio Bonillas, a Mexican mining engineer who went on to serve as mayor of Nogales, Mexican ambassador to the United States, and briefly, candidate for the presidency of Mexico. Ignacio, the father, kalew of the Pinacate and had visited Sykes Crater in 1882 (Lumholtz 1912). His son clearly had been well trained in mineralogy and geochemistry and was an engineer at the Instituto Geologic de Mexico in Mexico City when he published a brief paper on the large feldspar crystals of the Pinacate.
In that paper, Bonillas (1910) recognized that Pinacate lavas are mostly basalts (lavas relatively rich in iron and magnesium) and that they contain coarse crystals of olivine and labradorite (one of the plagioclase feldspars). He went on to describe the morphology of the labradorites and to give details of the optical crystallography and chemical composition of one crystal, data that in many respects could not be improved on today. He recognized their high iron content and correctly deduced that this iron was dissolved in the crystal structure rather than being present as minute bits of iron oxide; and he suggested that iron caused the citrine color these extraordinary crystals sometimes display. He also noted that the orientation of the optic vibration directions in the crystals seemed to indicate a chemical composition richer in sodium than shown by his chemical analysis, and he suggested that this anomaly might possibly have been caused by impurities in the crystals. This paper is a superb piece of science and remarkably sophisticated for its day.
The next steps forward in geologic knowledge of the Pinacate began in 1931, when a young man from New Jersey made his first visit to the Pinacate (Ives 1956). Ronald L. Ives read Camp-Fires on Desert and Lava as a youth, became enthralled with the area, and explored it extensively over several decades. The tracks that were to become Mexico Routes 2 and 8 were rough at best until Route 8 was opened during World War II, so access even to the periphery of the lavas was no easy matter. And Ives traveled through the Pinacate without benefit of the mules that carried Kino to the summit platform, or the horses and wagons that carried the members and supplies of the Hornaday and Lumholtz expeditions about the lava field. He covered vast distances on foot, alone, traveling very light. He was a prolific writer, and by 1981 this intrepid desert explorer had published 130 articles on some aspect or another of the Pinacate region (Dahood 1989).
Ives received an MS in geology from the University of Colorado and a PhD in geography from Indiana University. Although he evidently worked in many places during the course of his long and varied career, he visited the Pinacate frequently and hiked over a great deal of the area, all in the days before topographic maps of the region existed. A professor of geography at Northern Arizona University beginning in 1968, Ives published widely oil the Pinacate, chiefly on its geography, history, and climate; but he also addressed geology.
Ives (1935) used Kino's name, "Santa Clara," for the volcanic massif in the southern part of the Pinacate field; and he recognized that one could see only the "erosional remnants" of this "former great volcano." (The name Santa Clara predates Kino, however, for he evidently adopted it because the Sierra Pinacate overlooks Adair Bay on the Gulf of California, a bay that "the ancient geographers called the Puerto de Santa Clara" (Bolton 1919:1:187). Ives also pointed out the presence of "a number of great calderas," although the term "caldera" was then used without genetic connotation. And he noted the numerous cinder cones and ash beds. Volcanologic field terms thus began to make their appearance with Ives' 1935 paper.
In his publication on desert floods in the Sonoyta Valley, Ives (1936) presented the first map showing the approximate extent of Pinacate lavas; and in the same paper, he suggested that the Sonoyta River had once flowed westward to the vicinity of Adair Bay but had been diverted to its present course by eruption of the Pinacate lavas.
Ives (1956) discussed the age of Cerro Colorado, a question that remains unsettled. He believed that Cerro Colorado was no older than the most recent ash falls in the Pinacate, and he wrote that potsherds probably not older than 1,000 years were contained in ash at two of the tinajas (natural plunge-pool tanks cut in bedrock); thus, he concluded that Cerro Colorado was not more than 1,000 years old (Ives 1956). However, Shakel and Harris (1972) reported potsherds resting on eroded tuff (volcanic ash) at Cerro Colorado that indicate an age of at least 1,000 years and probably much more. And Gutmann, Turrin, and Dohrenwend (2000) reported that cinders have been deposited on Cerro Colorado tuff. These must have come from a cinder cone or cones nearby, and the youngest reasonable candidates are La Laja and Tecolote cones. The younger of these, La Laja, evidently is some 12,000 years old (Gutmann, Turrin, and Dohrenwend 2000). Thus, Cerro Colorado must be at least that old and may be a good deal older. In any case, it is certainly not the youngest volcano in the Pinacate. Its sharp-edged crater and the crumbly slopes of its smooth northern rim can leave a misleading impression that Cerro Colorado is easily eroded and young; but they contrast sharply with the terrain high on the southern slopes of the tuff cone, where deep gullies have been cut over millennia in tuff that seems almost as well indurated in places as concrete.
In a summary paper, Ives (1964) assembled a great deal of information about the Pinacate, including the observation that lava flowing from the bases of its cinder cones commonly led to collapse of the cone rim, leaving breaches in the walls of many Pinacate cones. The complete version of this paper also includes geologic observations about the craters (see Jahns 1959) and numerous historical and geographic observations.
Now let us go back to October 1950. Early that month, Randall Henderson and three others left El Centro, California, to visit Crater Elegante. They drove east to Sonoyta in two Jeeps, largely via the Camino del Diablo (Henderson 1951). From Sonoyta they proceeded down Route 8, which reportedly was paved in about 1946, and followed directions to Crater Elegante via woodcutters' roads. Elegante's rim is low and its presence is not obvious from the desert floor to the east, but the group found the crater on their second day in the area.
Crater Elegante, named by Lumholtz in 1910, is the biggest of the Pinacate craters, measuring eight hundred feet deep and nearly one mile across from rim crest to rim crest (figure 6). Henderson's party was determined to make it to the floor of the "stupendous" crater, did so, and came back to report that Elegante "shows no evidence of volcanic action" (Henderson 1951 : 13). Avowedly not a geologist, Henderson hazarded a guess that Elegante had been caused by a great subterranean gas explosion. As it turns out, this was not such a bad guess.
At about this same time, word reached Richard H. Jahns, a professor of geology at California Institute of Technology, that "a crater rather like Meteor Crater" lay in the desert of northern Sonora (R. H. Jahns, personal communication 1969). In January 1951, Jahns drove down to investigate. Like Ronald Ires before him, he sought out Alberto Celaya in Sonoyta for directions and information about the Pinacate. He drove to Elegante via Route 8, taking the turnoff at Batemote. Using a plane table and alidade to determine precise topographic locations of key points, he drew a geologic map of the crater and worked out the major elements of Elegante's eruptive history. Most important, he recognized that the tan, bedded deposits around its rim resulted from repeated volcanic explosions involving basaltic magma; and he judged that the crater formed by collapse following removal of underlying material.
The term "caldera" has been used by geologists for volcanic depressions formed by collapse, by erosion, or by explosion. But when Jahns published a report the following year on the origin of the Pinacate craters and called them calderas (Jahns 1952), he clearly meant they were calderas formed by collapse.
In the spring of 1951, Jahns returned to the Pinacate. Together with some Cal Tech students, he made a detailed geologic map of Cerro Colorado, again controlled by surveying with a plane table. He also mapped Sykes and MacDougal craters that spring, using his plane table and alidade, at least at MacDougal. And he worked out the essential geologic history at all three of these, finally finishing the map of MacDougal in February 1952.
Jahns, who later became president of the Geological Society of America, was a remarkable field geologist. He evidently had been told by Alberto Celaya that in order to reach Sykes Crater, he could simply hike northwest from the end of the road that led in that direction from the north side of Elegante. Jahns' field notes from April 1951 include his odometer readings on an exploratory side trip west along that road, and they include another trip shortly thereafter to the end of that road a few miles west of his camp at Elegante. His field notes further indicate that after a stint of field work at Cerro Colorado, he went on a "traverse" (a geologist's term for a hike with geological objectives) that he referred to as the "Sykes Traverse." This is the only traverse recorded in any of his Pinacate field notes, which demonstrate great economy of verbiage. The record of this traverse begins with a set of compass bearings taken from the top of a cinder cone northwest of Elegante. When plotted on a map, these bearings indicate that he...
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