Diablo Canyon Recreational Area is located to the northwest of the city of Santa Fe, NM. Diablo Canyon Recreational Area is famous for its incredible basalt columns and is a popular hiking and climbing location. The area appears in numerous film sets. This post is a section of text from my upcoming book, the Roadside Geology of New Mexico.
Diablo Canyon Geologic Setting
Diablo Canyon is located at the northern end of the Cerros del Rio volcanic field. The Cerros del Rio volcanic field is located near the meeting of the Rio Grande rift and Jemez Lineament. Volcanic activity occurred here between 3-1 Ma, with the majority of the basaltic to andesitic lavas erupting about 2.5 Ma.
The source of the Diablo Canyon Recreational Area basalt that make the vertical walls of the canyon area is a maar volcano. The lavas that erupted occurred in three main phases. First, basaltic magma began rising through the sediments of the Tesuque Formation. This unit is porous and is historically (and to modern day) and aquifer; within the Tesuque, the basalt intercepted the water table. This interaction of molten lava and cold water resulted in an explosive blast, forming a tuff ring a mile in diameter. The second phase of volcanism produced cinder, scoria, and bombs as well as small lava flows, filling in much of the cone produced in phase one.
Lastly, large volumes of basalt erupted both onto the surface (extrusive volcanism), as well as intrusively, with basaltic magmas squeezing between existing rocks and cooling in large sills, dikes, and plugs. At this time, a lava lake formed, pooling around central scoria cone and held in place by a moat of earlier erupted lavas.
Formation of Columnar Jointed Basalt
The columnar basalts for which the area is known are a cross section through the solidified lava lake, exposed in Cañada Ancha. Exposed in a near-vertical wall, over 300 feet of spectacularly jointed, vertical to slightly curved basalt columns are visible. Columns form in basalt, lava flows, and ash flow tuffs as the magma cools slowly over tens- to hundreds of years (or more). The hexagonal cracks form as magma cools and contracts.
The hexagonal shape forms because contractional stress is best relieved at 120 degree angles. Cracks form while the magma is still hot: not at it’s original temperatures of 1100 to 1250 ° C, but not fully cooled.
Fractures form at the top and bottom surfaces of a flow or intrusion, where the hot magma interacts with cooler host rock or the atmosphere. Cracks then propogate into the magma bod, and exist perpendicular to these cooling surfaces. As the cracks form and propogate through the magma body, cooling becomes more rapid, as the cracks allow for increased heat transfer.
Erosion & Modern Exposure
The incredible cross sections of magmatic/volcanic activity can be seen in Cañada Ancha, a canyon carved by an ephemeral stream/tributary to the Rio Grande. The Rio Grande carved through the plateau just to the north of the Diablo Canyon Recreational Area, and the deep erosion driven by the Rio Grande has in turn resulted in the rapid carving of many tributary canyons, giving spectacular insight into the complex geology of the Espanola Basin.
While Diablo Canyon Recreational Area is known for its incredible basalt columns, many other US locations also display incredible for their basalt formations. These include the Devil’s Tower National Monument (WY), Devils Postpile (CA), John Day Fossil Bed (OR), and even Shenendoh National Park (VA). International examples include the Giants Causeway and Fingal’s Cave in Northern Ireland, the Basaltic Prisms of Santa Maria Regla in Mexico, and Svartifoss in Iceland and several others.
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To learn about the amazing geology of Colorado, please check out my book Colorado Rocks:A Guide to Geologic Sites in the Centennial State, available signed here, or at booksellers nationwide (like Amazon).