Sierra Blanca Photo L. Crumpler
Sierra is an older volcano consisting of intrusive stocks and dikes and contemporaneous ash, breccia, and flows. It is not a modern volcano like Mount Taylor or the Valles Caldera, but is an old one that has been greatly eroded. Based on the attitude of existing volcanic units the bulk volcano may have been originally on the order of 3000 ft above the surrrounding terrain, 20 miles in diameter and contained about 185 cubic miles of volcanic material. Estimates of the age of the initial volcano is 38 to 26 million years. This means that it erupted at about the same time that the volcanoes of the Gila and Mogollon Ranges in western New Mexico were erupting.There was considerable volcanism in the Southwest during this interval of time, most of which just preceded the formation of the Basin and Range Province. Volcanism of this age is often referred to as "mid-Tertiary" volcanism, which in the lingo of southwestern volcanology means "old". It is a testament to the original great size of the volcano, together with later block-faulting that it remains as a significant mountain even today.
Photo L. Crumpler
Chronology of Construction
Volcanism began in Eocene time with intrusion of viscous alkalic magmas that domed pre-existing strata to form the northeast-trending, doubly plunging Black Mountain anticine. Emplacement of alkali gabbro and monzogabbto dikes in late Eocene time resulted in numerous shallow intrusions that fan out from the complex over much of the surrrounding terrain and account for numerous dikes in road cuts along highway 380 near Lincoln. Nearly 200 dikes may be counted along the road in a distance of 10 miles. The intensity of the intrusions is such that dilation of the region may have been as much as 0.5 miles. Folding of rocks observed near Lincoln may be related to emplacement of these intrusive masses.
Following a half-million year interval of erosion that left a lowland along the axial trough, eruptions began to form the Sierra Blanca volcanic pile. Erupted rocks consist mostly of intermediate trachytic (syenitic), basaltic, alkali-calcic andesite, monzonite, and locally rhyoltic compositions. The volcano itself developed between 38 Ma and 26 Ma with the eruption of a series of volcanoclastic materials, trachybasalt flows, trachyte and phonotephrite flows, and trachyphonolite flow breccias. The whole mass was intruded much later (26 million years ago) by numerous dikes and sills, and finally by 16 large igneous (dominantly syenite and monzonite) stocks. Allen and Kottlowski (1981) show that the the age progression is from north to south at about 2.5 inches per year. Sierra Blanca Peak is itself one of these stocks, as is the Capitan Mountains to the north.
The estimated thickness of the volcanic pile following main cone construction is 2500 m. Most of the volcanic materials are concentric to a core of intrusive rocks, possibly filling an arcuate depression interpreted as a central sag.
Subsequent erosion has removed hundreds of cubic kilometers of rock from the volcano, precursor stocks, and Paleozoic and Mesozoic strata of the surrounding areas.The current elevation and shape is largely due to fault block uplift and erosion, including glaciation in the Pleistocene (past two million years).The summit of Sierra Blanca is an intrusive stock.
Sierra Blanca is said to be the eastern-most mountain associated with the Basin and Range Province, and the southern-most mountain in the U.S. with relief that extends upward into the Arctic-Alpine life zone.
View Sierra Blanca Volcano in a larger map
Petrology/ General Geology:
Allen, J. E., and Kottlowski, F.E., Scenic Trips to the Geologic Past No. 3: Roswell-Ruidoso-Valley of Fires, 3rd Edition. New Mexico Bureau of Mines and Mineral Resources, Socorro, 96p, 1981.
Moore, S. L., Thompson, T.B. and Foord E.E., Structure and Igneous Rocks of the Ruidoso region, New Mexico. New Mexico Geological Society Guidebook, 42nd Field Conference, 137-145, 1991.
Thompson, T. B., Sierra Blanca Igenous Complex, New Mexico. Geological Society America Bulletin, 83, 2341-2356, 1972.
See the New Mexico Geological Society Guidebook for the 15th Field Conference, 1964, and 42nd Field Conference, 1991 for general information about the surrounding region.