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Arizona Volcanos

Volcanoes and Types of Magma

A volcano is an opening where magma erupts onto the surface as lava after rising from deep within the Earth. Not all magma is the same. Some magma contains as much as 75% silica (SiO2), whereas other magma contains as little as about 50%. The more silica in a magma, the higher its viscosity, or resistance to flow. Viscosity controls the type of volcano that forms. Eruptions of high-viscosity magma build very steep-sided lava domes. Low-viscosity magma produces cinder cones and thin sheet-like lava flows, and intermediate-viscosity magma creates moderately steep mountains called stratovolcanoes.

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.

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.

Stratovolcanoes

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.

Lava Domes

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

Elden Mountain is a steep-sided lava dome in the San Francisco Volcanic Field. 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

 

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Map of Arizona

 

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Map of the reservations and Four Corners 

 

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