History of Field Observations on Volcanic Rocks of Western Mexico, Pre-Columbian to Recent

Abstract

By the time the first detailed reports on western Mexico's volcanic rocks had begun to appear in the 1970s, most of the earlier observations on these rocks and most knowledge of those who made these observations were all but forgotten. A review of previous field observations in this region shows, however, a long history of geologic discovery reflecting or even preceding developments elsewhere.Ethnological studies suggest that the Pre-Columbian inhabitants of the Sierra Madre Occidental (SMO) observed the characteristics of rock formations in their sierra homeland and understood something of the regional stratigraphic relationships. Late sixteenth and early seventeenth century explorers of the Spanish Colonial Period singled out volcanic rock known to them as piedra de malpaís for special recognition, and Padre Kino and his fellow explorers clearly recognized the volcanic origin of piedra de malpaís decades prior to similar observations in Europe. As the Spanish Colonial Period came to a close, Andrés Manuel del Río help organize a state-of-theart mining college in Mexico City where students were instructed in Werner's geognosy prior to their taking positions in Mexico's mining industry, most of it located in western Mexico's volcanic rocks.Although the first part of the tumultuous period between Mexico's revolutions of 1810 and 1910 saw few advances in geological knowledge, the reign of President Porfirio Díaz produced a geologic map of Mexico, the founding of the Instituto de Geología, and an ‘American invasion’ of geologists and mining engineers who locally gathered information on the nature of volcanic rocks of western Mexico. During the same period, Instituto geologist Ezequiel Ordóñez established the general stratigraphic sequence in the SMO, recognized the widespread occurrence of rhyolite there, and applied the petrographic microscope to the study of SMO volcanic rocks. The first identification of ignimbrites in the SMO came as a result of the World War II-era search for strategic minerals, and NASA's push to put a man on the Moon supported a series of student mapping projects producing the SMO's first geologic maps showing individual ignimbrite units and calderas.