The Pinacate Volcanic Field
Quaternary basalt volcanism in Northwestern Sonora, Mexico
Pinacate is a superb example of the monogenetic basalt volcanic fields erupted in tectonically active regions of the world during the Quaternary (the last 2.8 Ma). It contains hundreds of "cinder cone" volcanoes, a composite volcanic mountain, and the most extraordinary maar-calderas (steam explosion craters) outside of Africa. Although Father Francisco Eusebio Kino, who explored Pimaria Alta saving souls in 1698, gave the name "Santa Clara" to the volcanic mountain that towers over the scattered mountains of the Southern Basin and Range, people eventually referred to the volcanoes using the name "Pinacat'l," the black eleodes armata beetle common in the Sonoran Desert.
Pinacate and Santa Clara, differing volcanismsThe hundreds of Pinacate "cinder cones" (few have actual cinder) and maar-calderas are monogenetic volcanoes, each was built independently in a single, short duration eruption from a unique magma body and is related to its neighbors only by its location and by the uniform processes of mantle partial melting. They cover older Santa Clara, a composite volcanic mountain built by numerous, sequential eruptions over a long time period from the same magma body as its composition evolved from basalt to trachyte. Lavas flowed easily away from a central conduit system to build a shield volcano. Monogenetic basalt volcanism began before the last Santa Clara trachyte erupted and is dormant, not extinct.
The dramatic maar-calderasMost visitors come to Pinacate to see the deep craters that were excavated into the land surface by powerful steam explosions generated when hot magma mixed with ground water in the valley-fill sediments beneath the lava flows. Maar craters are found in a quarter of the nearby monogenetic volcanic fields but Pinacate's are more spectacular than any of these and are exceeded only by some in Africa. The maar-calderas are surrounded by rings of tuff-breccia; a mixture of gravel, old basalt, and palagonite (quenched hydrated glass) expelled from the crater by the explosions.
Why is Pinacate here?Monogenetic volcanic fields, some with volcanic mountains, are found in all geologically active regions of the Earth. Each marks a heat source that has partially melted upper mantle peridotite to form magma (ion soup) that collected into bodies of sufficient size to make volcanoes. When the pressure in the magma chamber exceeds the minimum horizontal compressive stress in the rock above it, magma intrudes as a dike through the rock to find its way to the surface. That eruption ends when the weight of the magma in the conduit equals magma chamber pressure. Nobody knows why some magmas collect into bodies large enough to erupt sequentially and build composite volcanic mountains.
This link will take you to Google Earth where you can zoom in on any of the surrounding fields.
Plate tectonics of the Pinacate regionPinacate is on the edge of the continental crust next to an active transform plate boundary with seafloor spreading. The juxtaposition of Pinacate volcanism with the magma-genesis associated with seafloor spreading is too convenient to ignore but the two are not related. Pinacate, with the exception of Ives, has the same alkali-olivine basalt found in the other monogenetic fields whereas ocean ridge basalt is tholeiite.
Pinacate and the sandOpening of the Gulf of California was the most far reaching geologic event in this part of North America in the last 7 million years. Prior to this event, Baja California acted as a dam that held back a vast sea of detritus that had accumulated in the Southern Basin and Range province. Whatever rivers ran across this land left no trace but their base level was the Pacific Ocean far to the west. The new Gulf changed the dynamics of regional drainage bringing base level inland, causing drainage on the Colorado Plateau to organize into the Colorado River system, and creating a sink for local sediment. Sand and silt eroded from the Grand Canyon and other parts of the Plateau was carried to the river's end, picked up by the wind, and blown eastward onto the Gran Desierto de Altar. There, the tan sand makes such a contrast with Pinacate's black basalt that astronaut Harrison Schmidt claims to have been able to recognize it from the Moon.
The Great DenudationThe pre-Gulf landscape in the Pinacate region probably had few mountains but it did have some kind of drainage. When lowered base level rejuvenated the desert washes, they excavated the detritus that had accumulated around the mountain blocks and carried it into the opening Gulf in what I call "the Great Denudation." We find many places where these washes cut down through mountain ranges, washes superimposed on the landscape, their sizes far to small to account for the major down cutting they have inherited.
Biosphere Reserve:Pinacate and the surrounding Gran Desierto de Altar dune fields were given Biosphere Reserve status under the United Nations in 1993 and are now administered by the Comision de Areas Naturales Protegedas. Their headquarters is at Ejido Nayarit near the Highway 8 bridge over the Rio de Sonoyta and they have a spiffy new visitor's center, Schuk T'oak, on the Ives flow south of Sierra Blanca.