Winter 2006 VOLCANOES OF NEW MEXICO Perhaps more than any other state in one of these enormous eruptions, as are the U.S., New Mexico offers an the striking landforms at City of Rocks enormously diverse record of volcanic State Park. activity. Many familiar and distinctive The shape of volcanoes, and the time New Mexico landscapes are composed that they take to form, vary widely with of volcanoes and volcanic rocks. The the style of eruption. Cinder cone vol- black, barren, lunar landscapes around canoes, such as Capulin Mountain and Grants and Carrizozo are recent lava Bandera Crater, are perfect cone-shaped flows, and the black flat-topped mesas hills. Cinder cones typically form around Albuquerque are remnants of quickly and are the result of a more-or- older lavas. Mount Taylor and Capulin less continuous single episode of Mountain are volcanoes, and Los eruption. An interesting example is the Alamos is built on the flank of a huge eruption that formed the famous volcano, one so large that it is best Mexican volcano, Paricutín. This vol- viewed from space. Ship Rock is the cano was born in a cornfield on eroded remnant of a volcano, as is February 20, 1943, following several Capulin Mountain in northeastern New Cabezon Peak. Although not as easy to Mexico is a classic cinder cone. The cutaway weeks of local earthquake activity. The recognize, many of the rocks in the view revealing the structure of a typical cinder volcano grew to a height of 150 feet on Gila Mountains and other southern cone (below) shows the layers of ash that its first day of eruption and was 500 and western New Mexico mountain accumulate around the central vent. The feet high 6 days later, reaching a final ranges are also volcanic. The youngest summit crater is often preserved in younger height of over 1,000 feet after 3 years features, such as Capulin Mountain, which volcanic rocks in New Mexico are a erupted about 55,000 years ago. Photo © of activity. scant 3,800 years old. At the other end Adriel Heisey. Most volcanoes, however, are larger of the age spectrum, volcanic rocks in and take longer to form. Stratovolcanoes, parts of northern New Mexico are more such as Mount Taylor and Sierra than a billion years old. Blanca, form from repeated eruptions Volcanoes are defined as vents over many thousands of years, and the through which magma (melted rock) resulting landforms are large, some- and gases erupt from below the earth’s times asymmetrical mountains. The surface. Volcanoes range widely in size, Jemez Mountains in north-central New from small cinder cones, such as duced several hundred cubic miles of Mexico are the geographic expression of Bandera Crater (near Grants), which volcanic rock in single events, hundreds a series of volcanic eruptions that can be climbed in a matter of minutes, of times larger than the May 18, 1980, occurred over the past 15 million years. to much larger landforms like the eruption of Mount St. Helens. The The two largest eruptions, which Jemez Mountains, and skyline-domi- Mogollon–Datil eruptions were larger occurred 1.2 and 1.6 million years ago, nating volcanoes like Mount Taylor and than any volcanic events that have produced the distinctive white-to- 12,000-foot-tall Sierra Blanca on the occurred in historic time, and the salmon-colored cliffs of Bandelier Tuff east edge of the Tularosa Basin. deposits from some of these events pro- that form a spectacular backdrop Eruptions from some of the largest duced sheets of rock several hundred around much of the area. These two New Mexico volcanoes, located in the miles across. The well-known very explosive eruptions poured hun- Mogollon–Datil volcanic field in the “Kneeling Nun” landform near Silver dreds of cubic miles of hot ash, pumice, southwestern part of the state, pro- City is formed from volcanic rock from and gas over the surrounding land- Published by the New Mexico Bureau of Geology and Mineral Resources • A Division of New Mexico Tech faults. The river flows are probably extinct, meaning that they through a series of broad are unlikely ever to erupt again. valleys created by this However, new volcanoes will certainly stretching process. The form within our state, though probably Jemez lineament is a not within our lifetimes. Dr. Allan more subtle crustal Sanford, a geophysicist at New Mexico feature, expressed as an Tech, has identified the Socorro Magma alignment of volcanic Body, a large body of molten rock vents, including (but not underlying the area from Bernardo to limited to) the south of Socorro. Many small earth- Zuni–Bandera, Mount quakes associated with the magma body Taylor, Jemez Mountain, have been detected using seismometers, and Raton–Clayton vol- and some have been felt by local resi- canic fields. This zone dents. These earthquakes are caused by may be the expression of swelling of the magma chamber, which a weakness formed fractures the overlying brittle rocks. where two very old The magma body is located 12 miles blocks of the earth’s beneath the earth’s surface but shows crust were pressed no signs of erupting soon. The swelling together. In addition to of the magma chamber is being studied various styles of crustal using high-precision global positioning weakness, volcanism in system (GPS) data and instruments New Mexico is also likely called tiltmeters. The combination of The major volcanic fields in New Mexico tend to follow two major zones of weakness in the crust and underlying mantle, related to upwelling these two instruments allows even tiny the Jemez lineament and the Rio Grande rift. of abnormally hot changes in the shape of the earth’s mantle material. surface over the magma body to be scape. The rocks were later eroded by detected. This research is being carried water, resulting in the beautiful land- Active, Dormant, or Extinct? out by Los Alamos National Laboratory scape we see today. It is difficult to Volcanoes may be classified as active, geologist Andy Newman, as well as recognize the Jemez Mountains as a dormant, or extinct. An active volcano bureau geologists David Love, Bruce volcanic landform because they are so is one that is emitting lava or volcanic Allen, and Richard Chamberlin. The large. However, on a digital elevation gas. A dormant volcano is not actively magma body’s activity is being closely map (like the one shown on page 3), erupting, but, based on patterns of past monitored by the New Mexico Tech the volcanic shape and large central activity, is likely to erupt again. With geophysics program (on the Web at crater (caldera) become evident. the possible exception of the Jemez www.ees.nmt.edu/Geop/NM_Seismolo Volcanism in New Mexico Mountains, all volcanoes in New Mexico gy.html), and any change in activity Volcanic activity occurs in New Mexico and elsewhere in the western United States because of the local nature of the earth’s crust and mantle. The crust in New Mexico is being stretched by regional plate tectonic forces. This stretching produces areas of weakness in the brittle crust, allowing magma to work its way upward. The very large volcanic field represented by the Jemez Mountains occurs at the intersection of two zones of crustal weakness, the Rio Grande rift and the Jemez lineament. The Rio Grande rift is a major zone of crustal weakness running north to south through New Mexico that formed in order to accommodate stretching of the crust in an east-to- west direction. The surface expression of the Rio Grande rift is a broad, shallow valley caused by down-dropped A complete section of Bandelier Tuff is exposed along NM–4 north of Bandelier National blocks of the crust along rift-bounding Monument. NEW MEXICO EARTH MATTERS 2 WINTER 2006 outward from the main Ship Rock landform are also part of the original volcano. These walls are actually volcanic dikes, formed when hot magma moved laterally through cracks in the earth’s surface at the time of the eruption, cooled, and solidifed while still in the cracks. Like the central part of the volcano, the dikes are harder than the sedimentary rocks surrounding them and remained standing while the sedimentary rocks weathered away. Similar dikes are found In this oblique digital in many places throughout New elevation model of the Jemez Mountains, the shape of the Valles Mexico, typically appearing as thick, caldera becomes very clear. The Jemez black walls running across the land- River occupies the steep canyon running scape. Some of these dikes can be south from the caldera rim in the fore- traced across the earth’s surface for as ground. Although there have been no major much as 45 miles. would be carefully analyzed to assess eruptions from the Jemez Mountains vol- the possibility of an eruption. canic field in the last million years, there Our Youngest Volcanoes have been a number of smaller eruptions, The large explosive events that In contrast to violent eruptions like formed the upper part of the Bandelier the most recent of which occurred about 60,000 years ago. Bandelier and Ship Rock, most of the Tuff in the Jemez Mountain volcanic recent volcanic activity in New Mexico field occurred in the relatively recent cold ground water. The resulting vio- has been of a more passive eruption geologic past. No eruptions this large lent steam explosions excavated a deep style. This includes the 3,800-year-old have occurred anywhere in the world in depression in the earth’s surface. McCartys and the 10,000-year-old historic time, and only six have Magma and a variety of rock fragments Bandera Crater lava flows in the occurred in the U.S.