Geology of the Pinacate Volcanic Field

Pinacate is a distributed vent monogenetic volcanic field, one of dozens in western North America. Monogenetic volcanoes are usually small cinder cones, the most common volcanic landform on earth. Cones are built in single, continuous eruptions of weeks to, at most, years duration. Cinder cones in various stages of erosion like those seen in this and accompanying photographs are randomly dispersed across 2000 sq km of Sonoran Desert. These eruptions occurred sporadically over more than a million years. Cinder cone erosion is a process as complex as the eruption.







This field is most famous for its huge maar craters, collapse depressions created in the aftermath of steam explosions generated when normal basalt magma traded heat with ground water in the sands beneath the field. Only a dozen of more than 400 volcanoes here have hyaloclastic tuff, the product of interaction between liquid rock and liquid water. Three of these are spectacular calderas; Sykes Crater (to the left) and MacDougal Crater in the middle distance. Those who are pure of heart can find three-lobed Molina Maar in front of MacDougal on its left.

Volcan Santa Clara is the shield volcano on the horizon. Its lava flows are related to each other by successive eruption from a long-lived magma body as it changed its composition through fractional crystallization.

The young and atypical Ives lava flow covers 75 sq km of desert extending from the sand in the foreground to the base of Santa Clara. One need not be pure of heart to see its boundaries, to pick it out from the rocks around it. One reason it stands out is the distribution of sand on its surface. Ives is in intimate contact with the dune sands of the Gran Desierto on both sides. Sand is an overlooked element of Pinacate geology.

The long Julian Hayden flow can be seen on the flank of Santa Clara.

Ives is not only the most voluminous and widespread lava field in Pinacate, it is one of the youngest at 13 ± 3 thousand years. Unique among the other flows, it is entirely a tube-fed pahoehoe flow that erupted from three or four N-S fissures with very little pyroclastic material. Ives's composition also differs from the other Pinacate basalts; it is a tholeiite, richer in silica and poorer in sodium and potassium.

The Ives surface here shows classic pahoehoe spatter tubes in a sag-flowout structure.

Carnegie volcano on the summit platform is somewhat unusual. Its lava flows (the eastern is longest in the field) issued from three places on a 1.5 km fissure. The north section of cinder cone wall collapsed during the last part of the eruption creating a beautiful debris flow down the flank of Santa Clara.

Tecolote is unique! It's the anarchist volcano because the eruption threw so many bombs. It has seven lava flows and their surface features show a range from very low to very high viscosity. Large parts of the cone are altered and welded with fumarolic sulfur. If all of that is not enough, the cone is sliced by numerous faults.

Weird as it is, there is nothing to suggest that either the basalt rock or the nature of eruptive processes at Tecolote are any different from those at far less complex volcanoes elsewhere. This story will be a BIG chunk of the website.