DESCRIPTION:
Arizona Volcanoes and Volcanics

Twenty seven million years ago a volcanic eruption of immense proportions shook the land around Chiricahua National Monument. One thousand times greater than the 1980 eruption of Mount St. Helens, the (nearby) Turkey Creek Caldera eruption eventually laid down two thousand feet of highly silicious ash and pumice. This mixture fused into a rock called rhyolitic tuff and eventually eroded into the spires and unusual rock formations of today. ... Erosion carved along weak vertical and horizontal cracks forming the fascinating rock forms preserved today in Chiricahua National Monument.

Late Cenozoic volcanism extends across a broad region from southwestern Utah to the north rim of the Grand Canyon in western Arizona. ... The relative ages of lavas can be readily established from morphological relations, with the oldest capping mesas or buttes, and the most recent occupying present drainage valleys. Many of these young flows have no soils developed on them and have well preserved flow features and associated cinder cones. The youngest measured age is 12,500 years ago for a young flow at the Grand Canyon.

The Uinkaret volcanic field (1.2 million years ago to 12,500 years ago) at the north rim of the Grand Canyon in the Grand Canyon National Park is especially noteworthy. The lavas are alkalic and commonly contain periodotite inclusions.

Vulcan's Throne, a Quaternary cinder cone on the rim of the Canyon is cut by recent fault movement on the Toroweap fault. Late Cenozoic lava flows have repeatedly flowed down Toroweap Valley and several adjacent valleys into the Grand Canyon, at times forming large lava dams. Flows are exposed on the walls of the Grand Canyon, often interbedded with fluvial and lacustrine sediments, up to 600 meters above present river level. Several of the dams are estimated to have been at least 200 meters high. The most recent flows in the Grand Canyon have cascaded over the rim of the Esplanade to the river 1,000 meters below.

Within the river itself is a volcanic neck, Vulcan's Forge, 25 meters in diameter and rising 15 meters above the river.

Northern Arizona's San Francisco Volcanic Field, much of which lies within Coconino and Kaibab National Forests, is an area of young volcanoes along the southern margin of the Colorado Plateau. During its 6-million-year history, this field has produced more than 600 volcanoes. Their activity has created a topographically varied landscape with forests that extend from the Pinon-Juniper up to the Bristlecone Pine life zones. The most prominent landmark is San Francisco Mountain, a stratovolcano that rises to 12,633 feet and serves as a scenic backdrop to the city of Flagstaff.

The San Francisco Volcanic Field, which covers about 1,800 square miles, is part of northern Arizona's spectacular landscape. Much of the field lies within Coconino and Kaibab National Forests. Forest life zones in the region range from Pinon-Juniper at lower elevations through Ponderosa Pine to Fir and Bristlecone Pine at the highest elevations. The varied forests and geologic features of the San Francisco Volcanic Field offer diverse recreational opportunities, including camping, hiking, mountain biking, wildlife viewing, and winter sports.

Almost all hills and mountains between Flagstaff and the Grand Canyon are geologically young but extinct volcanoes of the San Francisco Volcanic Field. Without the volcanoes, this region would be a flat, arid plateau. Instead, the area includes both Arizona.s highest mountain.San Francisco Mountain, with peaks rising to 12,633 feet.and the State's youngest volcano -- Sunset Crater, which erupted less than 1,000 years ago and has been a National Monument since 1930.

Why does Northern Arizona have so many geologically young volcanoes? 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 of localized melting, or "hot spot," fixed deep within the Earth's mantle beneath northern Arizona. As the North American Plate moves slowly westward over this stationary source of molten rock (magma), eruptions produce volcanoes that are strung out progressively eastward.

The first volcanoes in the San Francisco Volcanic Field began to erupt about 6 million years ago, in an area where the town of Williams is now. Subsequently, a several-mile-wide belt of successively younger eruptions migrated eastward, to the area of modern Flagstaff, and even a bit beyond, toward the valley of the Little Colorado River. Today, this belt of volcanoes extends about 50 miles from west to east.

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. U.S. Geological Survey (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.

Humphreys Peak

The peaks of San Francisco Mountain, an eroded stratovolcano.which includes Arizona's highest point, Humphreys Peak at 12,633 feet -- tower over the ruins of an ancient Native American pueblo in Wupatki National Monument. The ancient inhabitants of this area must have witnessed the eruption of nearby Sunset Crater, the State.s youngest volcano, which erupted in about A.D. 1064. San Francisco Mountain and Sunset Crater are only two of the hundreds of volcanoes in the San Francisco Volcanic Field, which covers about 1,800 square miles of northern Arizona.

San Francisco Mountain

Stratovolcanoes have moderately steep slopes and form by the accumulation of layer upon layer of intermediate-viscosity (andesite) lava flows, cinders, and ash, interspersed with deposits from volcanic mudflows (lahars) at lower elevations. These tall, cone-shaped volcanoes, such as Mount Rainier, Washington, and Mount Fuji, Japan, normally rise to a central peak and are built up by countless eruptions over hundreds of thousands of years. San Francisco Mountain is the only stratovolcano in the San Francisco Volcanic Field and 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 the 1980 eruption of Mount St. Helens, Washington, or it may have been removed slowly and incrementally by a combination of large landslides, water erosion, and glacial scouring.

Eldon Mountain and Sugarloaf Mountain

The San Francisco Volcanic Field also includes several lava domes. Lava domes are formed by dacite and rhyolite magmas, which have high silica contents. Dacite and rhyolite are so viscous that they tend to pile up and form very steep-sided bulbous masses (domes) at the site of eruption. Domes can be active for decades or sometimes centuries. If a lava dome grows entirely by internal inflation, similar to a balloon, it is called an endogenous dome. If, however, magma breaks out through a dome.s flank during inflation and adds new lava layers to the outer surface, the final dome is called exogenous. Elden Mountain, at the eastern outskirts of Flagstaff, is an excellent example of an exogenous dacite dome and consists of several overlapping lobes of lava. Sugarloaf Mountain, at the entrance to San Francisco Mountain.s Inner Basin, is a rhyolite lava dome. This dome is thought to be endogenous, but its forest cover hides direct evidence of its internal structure.

SP Crater

Most of the more than 600 volcanoes in the San Francisco Volcanic Field are basalt cinder cones. Basalt has the lowest viscosity of all common magmas. Cinder cones are relatively small, usually less than 1,000 feet tall, and form within months to years. They are built when gas-charged frothy blobs of basalt magma are erupted as an upward spray, or lava 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 a lava 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. SP Crater, 25 miles north of Flagstaff, is an excellent example of a cinder cone and its associated lava flow.

San Francisco Peaks are the remnants of the only stratovolcano in the San Francisco volcanic field. For decades, volcanologists suggested that the mountain now called San Francisco Peaks had simply worn away over time, eroded bit by bit to form its current bowl-shaped top. Then, in 1980, the catastrophic explosion of Mount St. Helens forced us to rethink our ideas about volcanoes. Many volcanologists now feel that the scooped-out shape of the San Francisco Peaks may be the result of a catastrophic sideways blast like that of Mount St. Helens.

The cones and lava flows of the San Francisco volcanic field, which covers about 2,000 square miles of the southwestern Colorado Plateau, result from several million years of volcanic activity. These powerful underground forces changed the landscape dramatically beginning in the winter of AD 1064-65. Sunset Crater appeared when molten rock sprayed out of a crack in the ground high into the air, solidified, then fell to earth as large bombs or smaller cinders. ...

The San Francisco volcanic field is one of several dominantly basaltic volcanic fields of late Cenozoic age near the south margin of the Colorado Plateau. The volcanic field, which is predominantly of Pliocene and Pleistocene age, is just north of a broad transition zone between the Colorado Plateau and Basin and Range provinces. K-Ar ages indicate a general north-eastward progression of volcanism during the past 15 million years across the transition zone from central Arizona onto the plateau.

The San Francisco field lavas, ranging in composition from basalt to rhyolite, erupted through Precambrian basement and the overlying kilometer-thick cover of nearly horizontal sedimentary rocks, mostly of Paleozoic age. The volume of volcanic rocks is approximately 500 cubic kilometers; they cover an area of about 5,000 square kilometers. ...

Most of the San Francisco field is covered by basalt flows ranging in age from approximately 6 million years to less than 1,000 years. Flows erupted from approximately 600 individual vents, most of which are marked by cinder cones. The basalt is predominantly alkali-olivine basalt and associated plagioclase-rich basalt of hawaiitic composition. ...

The western centers consist of closely spaced silicic domes and some viscous flows. Bill Williams Mountain (4.2-2.8 million years ago), the oldest of the western centers, is composed primarily of andesite and dacite. Sitgreaves Mountain (2.8-1.9 million years ago), consists almost entirely of rhyolite domes. Kendrick Peak (2.7-1.4 million years ago), consists dominantly of dacite and rhyolite domes and flows that were intruded and partly buried by andesite. ...

Two additional intermediate to silicic centers, San Francisco Mountain and O'Leary Peak in the eastern part of the volcanic field ... and are mostly younger than the western centers.

The upper part of the San Francisco Mountain is a truncated stratovolcano built primarily of porphyritic andesitic and dacitic flows and pyroclastic deposits erupted approximately 1 million to 400,000 thousand years ago. These lavas overlie still older dacite and rhyolite, however, that are as old as 2.8 million years. Except for a small volume of plagioclase-rich basalt, basaltic lavas are conspicuously absent within San Francisco Mountain, even though basalt of the same general age was erupted abundantly to the west, north, and east. This virtual absence of basalt may reflect the presence of an evolved magma chamber beneath the stratovolcano that blocked ascent of basaltic magma to the surface. ...

The Sugarloaf rhyolite dome erupted on the northeast flank of San Francisco Mountain approximately 220,000 years ago. About the same time, the O'Leary Peak volcanic center, which includes a pair of dacite-porphyry domes, several dacite flows, an andesite flow, and small rhyolite domes, erupted approximately 9 kilometers to the northeast. One of the dacite-porphyry domes has been dated at 240,000 years, and a dacite flow at 170,000 years. The O'Leary Peak volcanic center and Sugarloaf are the youngest major silicic eruptives in the San Francisco volcanic field.

A deep linear valley transecting the northeast quadrant of San Francisco Mountain was formed between 400,000 years ago, when the youngest unit of the composite cone was erupted, and 220,000 years ago, when Sugarloaf dome was emplaced at the valley's northeast end. After the Sugarloaf eruption, glaciation further sculptured the valley. ...

The youngest (less than 200,000 years) basaltic lavas occur north and east of San Francisco Mountain. The most recent eruption, which began about 1065, A.D., produced the Sunset Crater Cinder Cone, three lava flows, and an extensive air-fall tephra sheet.

Sunset Crater
Location: Arizona
Latitude: 35.37N
Longitude: 111.50W
Height: 2,447 meters (300 meters above ground surface)
Type: Scoria Cone
Remarks: Formed 1064(?)-1068 A.D.

The cones and lava flows of the San Francisco volcanic field, which covers about 2,000 square miles of the southwestern Colorado Plateau, result from several million years of volcanic activity. These powerful underground forces changed the landscape dramatically beginning in the winter of AD 1064-65. Sunset Crater appeared when molten rock sprayed out of a crack in the ground high into the air, solidified, then fell to earth as large bombs or smaller cinders. As periodic eruptions continued over the next 200 years, the heavier debris accumulated around the vent creating a 1,000-foot cone. The lightest, smallest particles blew the farthest, dusting 800 square miles of northern Arizona with ash. Perhaps as spectacular as the original pyrotechnics were two subsequent lava flows: the Kana-A flow in 1064 and the Bonito flow in 1180. They destroyed all living things in their paths. ... In a final burst of activity, around 1250, lava containing iron and sulfur shot out of the vent. The red and yellow oxidized particles fell back onto the rim as a permanent "sunset" so bright that the cone appears still to glow from intense volcanic heat.

Sunset Crater is one of the youngest scoria cones in the contiguous United States. The cone is named for the topmost cap of oxidized, red spatter which makes it appear bathed in the light of the sunset. In the 1920's H.S. Colton saved the cone from severe damage by averting the attempt of a Hollywood movie company to blow it up in order to simulate an eruption. This led to the establishment of the National Monument at Sunset Crater.

The Sunset eruption products are a classic example of a monogenetic strombolian volcanism. The eruption began with the opening of a 15-kilometer-long fissure, accompanied by curtain of fire activity and the growth of a small lava flow at the southeast end. Strombolian fountaining then localized near the northwest end and Sunset scoria cone grew, with the simultaneous deposition of a widespread scoria fall layer. At the same time the 11-kilometer-long Kana-a lava flow issured from the cone. This was followed by further cone building and the production of the Bonito lava flow. This flow may have come from the base of the cone, as it has rafted portions of the cone on its surface. The last phase of the eruption featured low-level fountaining that repaired the cone and deposited the cap of scoria and spatter, which oxidized bright red due to retained heat. ...

The age of the eruption is known from an archaeological site in the vicinity of the crater.

How to get there: From U.S. Route 66 in Flagstaff, take U. S. Route 89 north toward Cameron. After approximately 15 kilometers look for the right turn for Sunset Crater National Monument.

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