Organ Mountains–Desert Peaks National Monument

Organ Mountains–Desert Peaks National Monument is a large, diverse and recently established park. It has four separate geographic areas: the Organ Mountains, the Doña Ana Mountains, the Potrillo Mountains, and the “Desert Peaks." This tour deals only with the Organ Mountains. The geology of the Doña Mountains area is briefly discussed in the tour of Fort Selden–Leasburg Dam. The Potrillo Mountains include the remote Potrillo volcanic field and are covered in the Kilbourne Hole tour. The "Desert Peaks" includes the Robledo Mountains and Sierra de las Uvas. The Robledo region is described in the tour of the "Prehistoric Trackways National Monument" (a separate, but contiguous national monument).

Location

Overview

Rising nearly a mile above Las Cruces is the jagged crest of the Organ Needles, the backbone of the Organ Mountains. Few skylines in southern New Mexico are as inspirational to artists, climbers, and outdoor enthusiasts. Although views of the Organ Mountains reflecting the glow of the setting sun can induce a sense of tranquility, the scene here 36 million years ago was far from peaceful. Instead, three caldera-forming super-eruptions rocked the region, marking the beginning of a volcanic episode that would forever change the face of southwestern New Mexico. Following these explosive eruptions, the Organ caldera was ripped apart by large-scale faulting, providing geologists with a unique opportunity to study both the erupted volcanic rocks and the crystallized magma chambers beneath them.

An early name for the Organ Mountains was La Sierra de Soledad or “the mountain of solitude.” Their dramatic peaks and spires, reminiscent of a pipe organ, were an essential landmark along El Camino Real between the 15th and 18th centuries. Sporadic mining of precious metals and minerals occurred in the Organ Mountains during the 18th and 19th centuries. In the late 19th century, Dripping Springs Resort was established as a reclusive mountain getaway. It was later converted to a sanatorium. The Organ Mountains were designated as part of the Organ Mountains–Desert Peaks National Monument in 2014.

Regional Setting

The Organ Mountains are located in a complex zone of overlapping geologic regions. The Organ caldera marks the southeastern margin of the Mogollon–Datil volcanic field. Superimposed on the eastern margin of this field is the Rio Grande rift, the north-south trending series of down-dropped basins related to east-west stretching of the crust. The Organ Mountains are part of a 150-mile-long block of the Earth’s crust that was uplifted during rifting. The San Andres Mountains to the north and Franklin Mountains to the south are parts of this same west-tilted fault block. The Organ Mountains uplift is sandwiched by two basins, the Tularosa Basin to the east and Jornada del Muerto to the west.

The Rock Record

The majority of the rocks exposed in the Organ Mountains are intrusive or volcanic in origin, the latter including both ash-flow tuffs and lavas. The Organ caldera erupted three major ash-flow tuffs about 36 million years ago. Tuffs are composed mostly of volcanic ash (tiny particles of volcanic glass) deposited from an incandescent cloud of gas and ash as it flowed rapidly across the land surface. Lens-shaped features in the tuffs are flattened pumice, indicating that the ash was hot enough to fuse together after deposition and form a welded tuff. The three tuffs are exposed along the western flank of the mountain and dramatically thicken westward from less than one thousand feet in the east to nearly 10,000 feet in the west. The wedge-shape geometry of the tuffs resulted from deposition within a trapdoor-style caldera, formed when the western part of the circular caldera subsided more deeply along faults than did the eastern side. Similar to most calderas, lavas erupted prior to and after the caldera formed. The best lavaexposures are near the head of Fillmore Canyon, close to the crest of the mountain.

World-class examples of granitic intrusions are found in the Organ Mountains. Granite forms when silica-rich magma cools and crystallizes completely within the crust. Crystals in granite range in size from a fraction of an inch to several inches. Typically, smaller crystals form when magma cools quickly. Granite contains mostly quartz and feldspar, with lesser amounts of mica and hornblende. Some granites of the Organ Mountains were emplaced as magma chambers that were themselves the sources of the ash-flow tuffs. Other granites are younger, recording the slow cessation of magmatism. Together, this group of intrusions is called the Organ batholith. The Organ batholith is similar to the tip of an iceberg in that most of it is hidden in the subsurface. Distinguishing the intrusive rocks from volcanic rocks here is easy; the intrusions typically weather into serrated spires that dominate the northern part of the range, whereas the volcanic rocks usually weather into rounded massifs that dominate the southern range.

Rocks not related to the Organ caldera are also exposed in the Organ Mountains. The oldest rocks in this area are 1.4 billion-year-old granites. These are texturally similar to the younger intrusive rocks but are typically darker in color and more eroded. Exposed along the western flank of the Needles are variably altered Paleozoic sedimentary rocks, chiefly limestones, sandstones, and siltstones. The rift-related Pliocene–Pleistocene Camp Rice Formation is the youngest sedimentary unit in the area. Thin layers of Pleistocene volcanic ash in the upper part of the Camp Rice Formation include those from theYellowstone caldera in Wyoming and the Long Valley caldera in California. These far-travelled ashes are a testament to the explosive power of these modern calderas, which were much like the long-extinct one exposed in the Organ Mountains.

Geologic History

The earliest geologic history of the Organ Mountains region is characterized by multiple mountain-building events and deposition of enormous volumes of sediments during Precambrian time. Between 1.7 and 1.1 billion years ago, volcanic islands and continents collided with the fledgling North American continent to build what is now much of the Southwest. Silicic magmas, such as those preserved as Precambrian granites in the Organ Mountains and in the Sandia Mountains, were intruded into the crust and crystallized during this time, contributing to the regional metamorphism of the earlier volcanic and sedimentary deposits.

Following a long episode of regional uplift and erosion, vast Paleozoic seas and rivers deposited many thousands of feet of sediment between 500 and 250 million years ago. The Laramide Orogeny thrust both the Precambrian and Paleozoic rocks upward, deforming the once horizontal sedimentary rocks into a series of folds and fault-bounded blocks during the Late Cretaceous and Early Cenozoic (about 75 to 45 million years ago).

This was followed by three caldera-forming eruptions in the Organ Mountains about 36 million years ago. Geologic mapping suggests that each successive caldera was superimposed on the previous one, thereby obscuring the structure of all but the youngest caldera. Following the caldera formation, lava flows erupted for at least 1 million years. As thecalderas erupted, and for as much as 10 million years thereafter, magma was emplaced into the crust at a depth of about 3 miles where it would eventually crystallize to form the Organ batholith. Fluids associated with the magma caused alteration and mineralization of rocks adjacent to the magma chamber.

Rift-related faulting, basin subsidence, and sedimentation are the youngest events in the Organ Mountains area. Faulting and basin development likely began during the waning stages of magmatism, as some of the intrusions appear to have been emplaced into fault zones. The combination of faulting, westward tilting of the fault block, and subsequent erosion has exposed the intrusive rocks beneath the caldera.

Geologic Features

Dripping Springs and Soledad Canyon — Intra-caldera tuffs (tuff that accumulated within the caldera) are well exposed at both Dripping Springs and in Soledad Canyon. The towering 2,000-foot walls that surround the ruins of Dripping Springs resort are composed almost entirely of intra-caldera Squaw Mountain tuff, deposited during a single eruption about 36 million years ago. The terrain north of Soledad Canyon is also composed of Squaw Mountain tuff, but the hills to the south are mostly intra-caldera Achenback Park tuff. Thin sediments between the tuffs were deposited between eruptions.

La Cueva — The intracaldera Cueva Tuff is exposed at the La Cueva shelter. The lower part visible here, in contrast to the younger tuffs, contains an abundance of rock fragments that are mostly volcanic in origin. The rock fragments were likely incorporated into the tuff when an extremely violent explosion initiated the caldera-forming eruption. Amazing exposures of various parts of the Cueva Tuff are also found near Peña Blanca, south of La Cueva.

Aguirre Springs — Intrusive rocks related to the Organ caldera are exposed along the east slopes and crest of the Organ Mountains. The highest peaks are a part of the Organ batholith and are composed of a rock called the Organ Needle Quartz Monzonite, which is similar to granite. The quartz monzonite also forms the iconic Rabbit Ears pinnacles. The age and chemistry of the quartz monzonite are similar to the Squaw Mountain tuff, suggesting a genetic relationship.

The 4-mile-long Pine Creek Loop Trail provides easy access to the Sugarloaf Peak Quartz Monzonite, which was emplaced 1 to 2 million years after the caldera eruptions. The crystals in this intrusion are very similar in size to one another, although some zones have larger crystals set in matrix of smaller crystals, a texture called porphyritic. Also seen in this intrusion are baseball- to beach-ball-sized masses of darker rock. These features record the mixing of hot (2,100°F) basaltic magma into the quartz monzonite magma. This mixing added new heat to the quartz monzonite magma, slowing its cooling and possibly triggering smaller eruptions.

Several debris-flow deposits are exposed near Aguirre Springs. In August 1991, heavy monsoon rains triggered a debris flow that roared down Anvil Creek and crossed the Loop Road just east of the Pine Creek Trailhead. Most of the debris-flow material has been cleared away, but remnants can be seen from the road. Parallel levees com­posed of large boulders delineate the margins of the flow.

Exfoliation Domes — Sugarloaf Peak, south of Aguirre Springs, and San Agustin Peak, just north of San Agustin Pass on NM–70, display rounded outcrops that result from a type of physical weathering common in granitic rocks called exfoliation. Granites are emplaced deep within the crust and are under pressure from the surrounding rocks.

When eventually exposed at the surface, decreased confining pressure causes the granite to expand, creating a series of concentric fractures parallel to the surface termed exfoliation joints. Exfoliation joints are subsequently enlarged by a variety of processes, including freezing/ thawing of water and thermal stresses caused by daily or seasonal tem­perature cycles. These processes incrementally enlarge the joints and pry the intervening curved slabs of rock apart, eventually producing rounded knobs like those of Sugarloaf Peak and San Agustin Peak.

Viewscape from the Crest of the Needles — Superb views of south-central New Mexico, Texas, and northern Mexico are a worthwhile reward for those willing to make the rugged hike and climb to the Organ Needle summit. The endless array of peaks and valleys are characteristic of the southern Rio Grande rift. On a clear day, the vista stretches for at least 100 miles and includes many locations discussed in this book, such as Sierra Blanca and White Sands National Park to the northeast, Kilbourne Hole to the southwest, the Florida Mountains, Rockhound State Park, Cookes Peak to the west, and the Robledo Mountains and Mogollon–Datil volcanic field to the northwest.

For more information:

Organ Mountains–Desert Peaks National Monument (BLM)

Bureau of Land Management
Las Cruces District Office
1800 Marquess Street
Las Cruces, NM 88005-3370
(575) 525-4300

Dripping Springs Visitor Center
(575) 522-1219