# The San Francisco Volcanic Field-Arizona's Hotspot

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A "hotspot" is not just a place where you can surf the Web. A hotspot is also a long-lasting place on the earth where an unusual amount of heat, rising from the mantle, melts the overlying crust to form volcanoes.

SP Crater, in the San Francisco Volcanic Field, is an excellent example of a cinder cone and associated lava flow. This flow extends 4 miles from the cone and is only about 100 feet thick.

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As many of you know, the Hawaiian Islands are the result of this type of volcanism, called hotspot volcanism. The Big Island has active volcanoes because it is currently situated on top of the Hawaiian hotspot. The older Hawaiian islands formerly held this distinction, but, captive to the oceanic crust on which they sit, they drifted off toward the northwest along with the rest of the Pacific Plate because of plate motion.

The world's ocean basins are dotted with islands that, like Hawaii, exist because they sit on top of a hotspot. Examples include places like the Galapagos, Samoa, and Iceland. The continents, too, host hotspots, but these are far less common, and far less commonly known. The most widely recognized of the continental hotspots is the one beneath the Yellowstone caldera. The Yellowstone hotspot is responsible for Old Faithful, the Norris Geyser Basin, and all the rest of the hot springs, geysers, and mud pots for which Yellowstone is famous.

The paucity of hotspot volcanism within continental crust is not fully understood. The thickness of the continental crust, which is, on average, about four times as thick as the oceanic crust, could be a contributing factor. Another could be that, since about two-thirds of the earth's crust is oceanic crust, there simply is less continental crust available to override a hotspot.

Other examples of continental hotspot volcanism do, however, exist. One of these is the San Francisco Volcanic Field in northern Arizona. There is some debate about whether or not this Volcanic Field truly resides over a hotspot, but it does get younger to the east. This is consistent with the westward motion of the North American Plate over a fixed hotspot.

The San Francisco Volcanic Field is comprised of more than 600 volcanoes-most of them relatively small cinder cones-scattered over an area of about 1,800 square miles. The oldest volcanoes formed about six million years ago, which, geologically speaking, is fairly young. At the other end of the spectrum, the most recent eruption, of which Sunset Crater near Flagstaff, Arizona, is the most prominent vent, occurred less than a thousand years ago.

With an onset dated between A.D. 1064 and 1065, this most recent eruption falls well within the period when this area was inhabited by the indigenous Sinagua people. The eruption forced them to abandon their homes at Wupatki Pueblo for several decades. This archeological site is now part of the Wupatki National Monument.

Most of the volcanoes in the San Francisco Volcanic Field are basalt cinder cones. Located on the arid Colorado Plateau, many of these cones have experienced little erosion and remain largely vegetation-free. Some of the finest examples of cinder cones anywhere, such as the picturesque SP Crater, can be found here.

In addition to the basalt cinder cones, the San Francisco Volcanic Field is home to a stratovolcano, composed of andesite, and several lava domes, composed of rhyolite and dacite. These types of volcanic rocks have a higher silica content and viscosity than the basalt we are used to here in Hawaii.

The erosional remnants of the stratovolcano, known as the San Francisco Peaks, tower above the surrounding landscape. At 12,633 feet, the San Francisco Peaks are among the most prominent landmarks in northern Arizona and are sacred to the Native American people who occupy the region.

Unlike Hawaii's volcanoes, which are among the most active volcanoes on the planet, the average interval between periods of volcanic activity in the San Francisco Volcanic Field is on the time-scale of several thousand years. If another eruption does occur, which is likely, it will probably happen in the remote eastern part of the field, away from populated areas.

Like the eruption that formed Sunset Crater, the next eruption will probably put on a spectacular display of lava fountains and lava flows similar to Puu Oo in its early years. But don't buy your plane ticket to Flagstaff yet-the next eruption is probably centuries, or even millennia, away.

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### Volcano Activity Update

This past week, activity levels at the summit of Kīlauea Volcano have remained at background levels. The number of earthquakes located in the summit area is low (usually less than 10 per day are large enough to locate).

Eruptive activity at Puu Oo continues. On clear nights, glow is visible from several vents within the crater. Lava is fed through the PKK lava tube from its source on the southwest flank of Puu Oo to the ocean. About 1 kilometer south of Puu Oo, the Campout flow branches off from the PKK tube. The PKK and Campout tubes feed two widely separated ocean entries, at East Laeapuki and East Kailiili, respectively. Both entries are located inside Hawaii Volcanoes National Park.

A third entry, fed by an offshoot of the Campout flow, has been active since December 26. It is located at Kamokuna, about midway between the two older entries. In the last week, intermittent breakouts from the Campout tube have continued on the slope of Pulama pali and on the coastal plain near Kamokuna. A new breakout from the main PKK tube began to advance down the pali in the past week, more than a kilometer west of the Campout tube. The terminus was at the 1100-ft elevation on January 25.

Access to the sea cliff near the ocean entries is closed, due to significant hazards. The surrounding area, however, is open. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

Two earthquakes beneath Hawaii Island were reported felt within the past week. A magnitude-2.2 earthquake at 3:18 p.m. H.s.t. on Sunday, January 21 was located 8 km (5 miles) southwest of Honokaa at a depth of 13 km (8 miles). A magnitude-2.9 earthquake at 4:43 p.m. on Tuesday, January 23 occurred 13 km (8 miles) north-northwest of the summit of Kīlauea Volcano at a depth of 12 km (7 miles).

Mauna Loa is not erupting. During the past week, earthquake activity remained low beneath the volcano's summit (one earthquake was located). Extension of distances between locations spanning the summit, indicating inflation, continues at slow rates.