Los Volcanes de Pinacate

Pinacate has two kinds of volcano, a composite volcanic mountain that began erupting before two (maybe three) million years ago and ceased at one million years, and a distributed monogenetic volcanism that scattered nearly 500 cone volcanoes across the mountain and surrounding desert starting before the mountain was extinct. The monogenetic volcanoes, also called “cinder cones,” were each built in a single, continuous, short eruption - weeks to months, years at most - from its own unique magma, through its own conduit, with thousands of years between eruptions. A few of the cone eruptions ended with magma mixing into ground water to create steam explosions that blasted large craters into the landscape, maar-calderas far larger than any of the monogenetic cones.

Volcan Santa Clara (Sierra del Pinacate on maps) is the volcanic mountain built by numerous, sequential eruptions through a central conduit system from the same, evolving magma, the opposite of monogenetic. Its geographic name, “Mountains (Sierra) of the Pinacate,” originated in 1854, more than a century before geologists recognized that it was a unique geologic feature, different from the Pinacate cones. Santa Clara is the name Father Kino gave it in 1701 and resurrecting his lost name was a good way to make a geological distinction.

The Pinacate volcanism comprises hundreds of monogenetic basalt volcanoes on a distinctive gray landscape of their own creation that almost completely hides the Santa Clara series rocks.beneath. Quaternary basalt fields like Pinacate, fields with recognizable volcanic landforms, are scattered across North America’s western cordillera; fifty are within 1000 km. Few of those other fields are as well defined as Pinacate, with sharp field boundaries that contain all but a half-dozen of the cones out of the picture to the northwest.

Low albedo basalt flows and cinder fields on the eastern side are younger than 50 thousand years (Ka); the northern- and southern-most of them have dates of 12 ± 6 and 13 ± 4 Ka respectively.

Pinacate’s volcanoes are all on continental crust of the Southern Basin & Range geologic province; a colorful background of preCambrian gneisses, Permian and Laramide granites, and mid-Tertiary andesites. The field covers 1800 km2 of desert. A major plate boundary with seafloor spreading and transform faulting is hidden beneath sand and water on the western (far) side of the Gulf (toward the lower left) but Pinacate’s volcanoes are alkali basalts and completely unrelated.

Reddish-tan sand and dust, blown in from the Colorado River delta, contrasts with gray detritus from crystalline rock mountains; the red color comes from red-bed mud-stones eroded from the Colorado Plateau.

Cone is the natural shape for a pile of loose fragments that are added to it from a fixed source - the pile of gravel under a conveyor belt, sawdust under a termite tube, volcano cone. Foreground is the classic crater-in-cone shape of a young basalt volcano whose fragments were projected upward from a central conduit under the cone, blowing-out the crater-bowl in the center. That bowl, as a remnant, is a common feature of eroding Pinacate volcanoes like Quiroz (next).

La Laja, the mined cone in the background, is an actual cinder cone composed of hyper-expanded basalt froth broken into cm-sized clasts that retained their integrity and did not weld. Its loose and naturally sized cinder was exploited to build Highway 2 across northern Sonora. After the highway was finished, cinder was shipped to Phoenix for landscaping. Architectural grade cinder is the only economic product of Quaternary basalt volcanic fields, few of them have escaped being scarred by prospecting or mining.

Quiroz, most distant and most voluminous of the separated northwestern volcanoes, is typical of the older, deeply eroded and castellated Pinacate cones. That tilted, shallow bowl at its top is all that remains of the original cone after half a million years (K-Ar age). The conduit appears to have come out near the summit of this crystalline rock mountain and is less than 50 m inside the castle. The rock that became the castle is composed of pyroclasts that traveled very short distances and were more liquid and ready to weld than those flying farther out on the cone, the result is the vertical walls. 

Pinacate cones on the top of Volcan Santa Clara are about the same size as in other places and some have the intact crater-bowls of relatively young volcanoes. Pinacate Peak is highest (1192 m) and Carnegie Peak to its right is second (1105 m). All were covered by a cinder blanket from the Carnegie eruption that is seen partially stripped away on upper cone slopes.

Carnegie Peak is one of Pinacate’s interesting volcanoes (not to denigrate those whose complexity has yet to be appreciated). Its cone collapsed down the flank of Santa Clara in a three quarter km long debris flow. The eruption produced gray basalt before cone building, a cinder blanket during, and black-brown basalt after.

An argon-isotope radiometric age of 38 ± 6 Ka (some time between 32 and 44 thousand years ago) was determined on the black basalt. Erosion of the cinder blanket is highly variable from none on the cone itself to substantial on cones to its south. There’s a story here.

Young cones, like black Julian Hayden in this group of three, have sharp cone rims and no slope erosion. Hayden erupted onto the north side of a larger, older cone that has a rounded crater rim, an intact crater that is not a bowl, and prominent rilles eroded into the slope on the left. This volcano, in turn, had erupted next to, or onto the side of, an even older cone to its right whose crater has been eroded into an open “U.”

Volcancito Coyote is the smallest Pinacate volcano yet identified, a N60W, 280-meter-long fissure, decorated by a dozen little conelets that are barely visible along the left side. A small sheet of basalt extends downslope 60-90 m: giving an estimated volume of less than 50 000 m³. Based on weathering and erosion, this is probably one to two hundred thousand years old. Magma volumes have their own Gaussian distribution - some large, some small, and surely that tried were too small to reach the surface.

Cerro Colorado is a tuff-cone, a hydro-volcano without a maar-caldera. It sits on the edge of the field across the east side drainage, a wanna-be wash with a 2 m/km gradient, where it has impounded Diaz Playa upstream. This cone is tuff-breccia, a mixture of quenched basalt glass, sand, and whatever gravel is the basin-fill beneath it. When water and magma mix, the water is converted to steam and the magma quenches to become a yellow-tan glass called (variously) hyaloclastite or palagonite.

The Ives flow on the south is the largest (75 km²), nearly contemporaneous with Laja, and its surface is almost entirely pahoehoe. It lacks a major pyroclastic cone, having poured out of fissures, three of which are marked with spatter ramparts in the center.

Tecolote volcano is more complex than Carnegie, and has its own story. Lines along the highest (right) ridge-crest (Graben ridge) are faults along which the crest sank into the ridge owing to internal compaction of cone scoria at the end of its eruption, however that occurred. The valleys cutting the slope in the foreground appear to have resulted from subsidence; chemical analyses of the lava flows fit the group of all analyzed rocks. The cone top and slopes are covered with fields of meter-sized bombs.

This is the world’s goddamndest volcano LINK.