The purpose of this presentation is to examine the characteristics of volcanic "necks" in the Four Corners region of the Southwest, and to explore the tectonic forces that created the magnificent formations of the region, as well as to show off some of my photos.
Shiprock, located in northwestern New Mexico, is a most impressive example of a volcanic neck, or a central feeder pipe. The remnant of an eruption around 30 million years ago during the Oligocene, it is the basalt core of an extinct volcano. Near the main peak, one can see small pinnacles, the remains of smaller auxiliary volcanic vents. When the magma solidifies before ever reaching the surface, it is referred to as a "diatreme". The local Navajos consider it sacred, being a main character in their folklore. They call it Tse Bitai, meaning "the winged rock". The central part of Shiprock, visible from many kilometers away, is roughly 500 meters in diameter. Stretching 600 meters into the sky above the surrounding terrain, Shiprock is part of both the Navajo and Chuska volcanic fields in northeastern Arizona and northwestern New Mexico, extending north into Utah and Colorado. It is also within the boundaries of the vast Colorado Plateau. Known as the Four-Corners area, this region of extinct volcanic features covers approximately 20,000 square km. The following diagram illustrates the relationship between Shiprock and another well-known volcanic neck, Agathla, within the Navajo volcanic field in the Four Corners region.
In the above illustration, the red area is the approximate orientation of the Navajo volcanic field, and the letters are used to identify two of the more well-known diatremes, Shiprock(SR) and Agathla(AG). There are numerous other volcanic features throughout this region. Although Shiprock is believed to have been created as much as 1,000 meters below the Earth's surface, millions of years of differential erosion have uncovered the main volcanic neck, or plug, as well as associated smaller plugs and six dikes radiating out from the center. The following diagram shows the layout of these features from above.
Agathla, located on the first diagram as AG, was formed in a similar fashion as Shiprock. Both structures are believed to be the result of "maar" eruptions, where an explosive event produces a round crater at low relief, usually filling with water. Agathla can be seen near the entrance to Monument Valley in northern Arizona. The following figure illustrates how Agathla and other similar structures in the area were created, as well as how erosion plays a part in their appearances today.
The following photo demonstrates how Agathla and Shiprock have similar physical characteristics. The two monoliths are found approximately 120 km apart.
Shiprock is comprised of a form of magma known as volcanic breccia, pronounced "BRET-cha". This is a material composed of smaller rock fragments cemented together, similar to conglomerate but with sharper, more irregular pieces. The jagged fragments hint at the explosive nature of the eruption that created Shiprock. In fact there is an abundance of boulders consisting of tuff-breccia near the base of Shiprock, displaying components of varying sizes and colors. This is characteristic of a violent volcanic event, welding ash and rock together.Return to top
The magma of Shiprock is composed of an uncommon, highly potassic material referred to as "minette", or an orthoclase biotite lamprophyre, which is believed to result from extremely gradual melting of mantle rock. Minette consists of alkali feldspar, biotite or phlogopite, and diopside(pyroxene). This minette also contains crystals of augite and olivine as well as biotite mica flecks. Lacking plagioclase, it is dark brown to black with a density and texture similar to basalt. As the magma cooled, it formed vertical cracks producing an irregular columnar jointing. It is believed that the magma which fed Shiprock may have traveled as much as 35km from the Earth's mantle through igneous and metamorphic rock, including over 3000 meters of old sediments. Perhaps the release of pressurized gas and the interaction with sub-surface water produced a violent eruption called a "mantle blowout". A striking feature of Shiprock is the presence of dikes radiating out from the main formation. These formed as magma filled cracks in the ground during a period of eruption. The same process of erosion that uncovered the neck of Shiprock has also revealed these dikes as walls of lava. Scientists believe the tuff-breccia was first on the scene during the initial explosion, followed by the gas-poor minette lava as it slowly filled the dikes as well as the cracks in the neck. We can assume the minette lacked appreciable gas as the final product is void of vesicles, or air bubbles. Following are two photos of these remarkable examples of dikes.
What is the origin of the forces behind the formation of volcanic features as Shiprock, Agathla and so many others in the Four Corners region? It appears that around 30 million years ago, the approximate time of the Shiprock eruption, the land that is now New Mexico began to stretch apart, forming the Rio Grande Rift. Running roughly north to south from southern Colorado through New Mexico, this geological feature bisects the eastern edge of the Colorado Plateau. Previously, this region of the North American continent was actually believed to be a subduction zone. The transformation of a converging plate boundary to one of strike-slip, or transform faulting, appears to coincide with the beginnings of the rift valley.
There is some discrepancy between scientists as to whether magma rising up towards the Earth's surface leads to spreading apart of the crust, or if the thinning surface material "invites" the magma to enter through the path of least resistance. Either way, the result was large amounts of magma near the surface. As magma nears the crust, decreased pressure allows gas to escape the mixture, often explosively.
Throughout New Mexico are signs of stress within the Earth's crust. The Rio Grande Valley, to the southeast of Shiprock, is an example of a horst and graben formation. The Sandia Mountains on the east side of Albuquerque are the horst, lifted up along a fault line tens of thousands of kilometers above the city while the valley through which the Rio Grande River flows is the graben, having descended well below the valley floor. There is still some volcanic activity in the region, evidenced by geothermal hot springs and minor tremors.
Most people consider the formations of the Navajo Volcanic Field extinct, as they have not been active for millions of years (the formations, not the people). However, the existence of geothermal activity farther south as well as lava flows in the central part of New Mexico and Arizona that are only thousands of years old makes one wonder.
Ralph Lee Hopkins; Hiking the Southwest's Geology; 2002
Halka Chronic; Roadside Geology; 1987
Navajo Volcanic Field, Arizona; July 13, 2000; http://volcano.und.nodak.edu/vwdocs/volc_images/north_america/arizona/navajo.html
Dikes from Shiprock, New Mexico; http://www.geology.sdsu.edu/how_volcanoes_work/Thumblinks/dike_shprk_page.html
USGS Sites: Colorado Plateau; Chuska Volcanic Field; Shiprock; New Mexico; http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Places/volcanic_past_new_mexico.html
Shiprock and the Chuska Volcanic Field; http://www.nmmnh-abq.mus.nm.us/nmmnh/volcano/shiprock.html
NM Tech - NM Bureau of Geology and Mineral Resources; NM Geology Virtual Tour - Shiprock; http://geoinfo.nmt.edu/tour/shiprock.html
All Photos of Shiprock and Agathla by Larry Kuss.
All Diagrams by Larry Kuss.Return to top