| San Francisco volcano field | |
|---|---|
 Satellite image of Black Point lava flow | |
| Highest point | |
| Peak | Humphreys Peak |
| Coordinates | 35°20′49″N111°40′41″W / 35.347°N 111.678°W /35.347; -111.678 |
| Dimensions | |
| Length | 50 miles (80 km)[1] |
| Area | 5,000 square kilometres (1,900 mi2) |
| Geography | |
 | |
| Location | just north ofFlagstaff, Arizona |
| Country | United States |
| Region | Arizona |
| Geology | |
| Rock age | ~6 Ma |
| Mountain type | Volcano field |
| Rock types | |
TheSan Francisco volcanic field is an area ofvolcanoes in northernArizona, north ofFlagstaff, US. The field covers 1,800 square miles (4,700 km2) of the southern boundary of theColorado Plateau. The field contains over 600 volcanoes ranging in age from nearly 6 million years old to less than 1,000 years (Miocene toHolocene), of whichSunset Crater is the youngest.[2] The highest peak in the field isHumphreys Peak, at Flagstaff's northern perimeter: the peak is Arizona's highest at 12,633 feet (3,851.5 m) and is a part of theSan Francisco Peaks, an activestratovolcano complex.[3]
The first volcanoes in the San Francisco Volcanic Field began to erupt about 6 million years ago, in an area where the town ofWilliams, Arizona is now. Subsequently, a several-mile-wide belt of successively younger eruptions migrated eastward, to the area of modernFlagstaff, Arizona, and beyond toward the valley of theLittle Colorado River. Today, this belt of volcanoes extends about 50 miles (80 km) from west to east.[1]
Most volcanoes are located near boundaries of the Earth's tectonic plates, but Arizona is well within the interior of the North American Plate. Some geologists suggest that there is a site oflocalized melting fixed deep within theEarth’s mantle beneath northern Arizona. As the North American Plate moves slowly westward over a stationary source ofmagma, eruptions produce volcanoes that are strung out progressively eastward.[3][4] By looking atseismic waves and studying elements andisotopes within rocks from the region, scientists now believe that the layers of molten material in the mantle circulate beneath the Colorado Plateau. Warm material from theasthenosphere is able to invade thelithosphere. The influx of hotter asthenospheric material below the crust provides the heat necessary forvolcanism.[1]
Most of the more than 600 volcanoes in the San Francisco Volcanic Field aremonogeneticbasaltcinder cones. The cinder cones are relatively small, usually less than 1,000 ft (300 m) tall, and formed within months to years.[1] They are built when gas-charged frothy blobs of basalt magma are erupted as alava fountain. During flight, these lava blobs cool and fall back to the ground as dark volcanic rock containing cavities created by trapped gas bubbles. If small, these fragments of rock are called “cinders” and, if larger, “bombs.” As the fragments accumulate, they build a cone-shaped hill. Once sufficient gas pressure has been released from the supply of magma, lava oozes quietly out to form alava flow. This lava typically squeezes out from the base of the cone and tends to flow away for a substantial distance because of its low viscosity.[3]SP Crater, 25 mi (40 km) north of Flagstaff, is an excellent example of a cinder cone, and its associated lava flow extends for 4.3 mi (6.9 km) to the north of the cone. Several zones of concentrated eruptive activity have been identified – the moresilicic volcanic centers appear to have begun with basaltic activity and then evolved to more silicic compositions.[1]
San Francisco Mountain is the onlystratovolcano in the San Francisco Volcanic Field. Stratovolcanoes have moderately steep slopes and form by the accumulation of layer upon layer ofandesite lava flows, cinders, and ash, interspersed with deposits fromvolcanic mudflows at lower elevations. San Francisco Mountain was built by eruptions between about 1 and 0.4 million years ago. Since then, much of the mountain has been removed to create the “Inner Basin.” The missing material may have been removed quickly and explosively by an eruption similar to the1980 eruption of Mount St. Helens, Washington, or it may have been removed slowly and incrementally by a combination of largelandslides, watererosion, andglacial scouring.[3]
The San Francisco Volcanic Field also includes severallava domes, formed by highly-viscousdacite andrhyolite magmas. These magmas are so viscous that they tend to pile up and form very steep-sided bulbous domes at the site of eruption. Domes can be active for decades or sometimes centuries.Mount Elden, at the eastern outskirts of Flagstaff, is an excellent example of an exogenous dacite dome and consists of several overlapping lobes of lava.Sugarloaf Peak, at the entrance to San Francisco Mountain’s Inner Basin, is arhyolite lava dome.[3]
Although there has been no eruption for nearly 1,000 years, it is likely that eruptions will occur again in the San Francisco Volcanic Field. With an average interval of several thousand years between past periods of volcanic activity, it is impossible to forecast when the next eruption will occur. USGS scientists believe that the most probable sites of future eruptions are in the eastern part of the field and that the eruptions are likely to be small. These future eruptions may provide spectacular volcanic displays but should pose little hazard because of their small size and the relative remoteness of the area.[3]
Popular tourist and hiking destinations in the volcanic field include theKendrick Mountain Wilderness, 20 miles northwest of Flagstaff; andSunset Crater. Sunset Crater has a hiking trail along anʻaʻā lava flow to its base.[5]
Areas of the volcanic field have been used byNASA for testing techniques for exploration in a simulated extraterrestrial terrain environment.[6]NASA has also conducted theDesert Research and Technology Studies (DRATS) tests here.
This article incorporatespublic domain material from"Geology and History of San Francisco Volcanic Field". USGS. October 6, 2023. This article incorporatespublic domain material fromPriest, SS; Duffield, WA; Malis-Clark, K; Hendley, JW, II; Stauffer, PH (April 16, 2001).The San Francisco Volcanic Field, Arizona (Report). U.S. Geological Survey. Fact Sheet 017-01.{{cite report}}: CS1 maint: multiple names: authors list (link)| International | |
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