A HISTORY OF FIELD STUDIES AND OBSERVATIONS ON VOLCANIC ROCKS OF THE SIERRA MADRE OCCIDENTAL, MEXICO

Although the Sierra Madre Occidental (SMO) remains mostly unmapped, the history of SMO field observations is surprisingly lengthy. Missionary zeal and a quest for precious metal rapidly propelled Europeans to the far reaches of northwestern Mexico where extinct volcanoes were recognized as early as 1701. Malpaís, the name still commonly used for areas of young lava, was applied as early as 1606 to volcanic rocks in the central SMO.

The Royal Mining College, a state-of-the art facility, was established toward the end of the Spanish Colonial Era, and its staff included Andrés Manuel del Río, who had studied under Werner, Lavoisier and Haüy. In 1795, Del Río began training skilled mining geologists and published the America's first mineralogy text. For perspective, Benjamin Silliman, with few scientific credentials, was still years away from accepting the first professorship of chemistry and natural science at Yale. Del Río introduced Neptunist ideas to Mexico, ideas reinforced by Baron von Humboldt, who spent a month mapping SMO volcanic rocks.

The Revolution of 1810 liberated Mexico from Spain, but it also produced a period of economic chaos that persisted until the reign of Porfirio Díaz (1876-1911). Díaz supported the Instituto de Geología and encouraged foreign capitol. Instituto geologist, Ezequiel Ordoñez, produced a map showing the distribution of rhyolite of the SMO and he noted the special characteristics of rocks now known as ignimbrites. Economic stability also encouraged the influx of American mining engineers and geologists. Numerous mining reports published between 1888 and the Revolution of 1910, reveal considerable bewilderment over ignimbrites. SMO rocks were occasionally called tuffs; more frequently porphyry; porphyritic breccia; or even porphyritic, metamorphic sandstone. The origin of ignimbrites remained an enigma to such eminent geologists as R. T. Hill and E. O. Hovey, although both had witnessed results of the 1902 eruption of Mt. Pelée. In 1946, Robert Smith, later the author of seminal papers on ignimbrites and calderas, became the first to describe SMO welded tuff. The first identification of ignimbrite cooling units and SMO calderas took place beginning in the late 1960's and early 1970's as a result of mapping projects by students of the University of Texas at Austin.