Instead, minerals with higher silica contents, which have lower melting points, melt first, resulting in a “partial melt” that contains a higher proportion of silica than the parent material. As rock heats up, it doesn’t usually melt completely, or all at once. The reason why so many different types of magma erupted in the San Francisco Volcanic Field lies in a process called partial melting. Stickier, higher-silica magmas - like those in stratovolcanoes such as Washington’s Mount Rainier - trap gases more tightly, leading to buildups of pressure and more explosive eruptions. Because of its low viscosity, basalt eruptions tend to be relatively tame compared to other types. Basalt is relatively runny, so basalt lava flows can travel tens of kilometers across the landscape, slowly releasing pent-up gases, a process that leaves behind empty, spherical holes called vesicles in the lava. The silica content of magma controls its viscosity, which is the most important factor in determining a volcano’s style of eruption, and hence its size and shape. Sugarloaf, a small dome that sits at the feet of the San Francisco Peaks, consists of rhyolite. Finally, rhyolite, the most silica-rich lava, is light gray. Andesite, which forms the region’s largest volcano, is dark gray in color thanks to its intermediate silica content. Most of the San Francisco Volcanic Field’s volcanoes consist of basalt. Basalt, the lava type richest in iron and magnesium and most deficient in silica, is black. Volcanic rocks are classified by their silica composition, for which color serves as a loose guide. The diverse volcanic features on display here stem from the variety of magmas that have erupted. This process brought warm mantle rock toward the surface, and the consequent heating and depressurization of this rock led to volcanism around what’s now Flagstaff, starting about 8 million years ago. After a long-lived subduction zone off the West Coast shut down about 30 million years ago, the stress field in the American Southwest became extensional, triggering thinning of the crust from Southern California to Arizona to form the Basin and Range. It is one of several volcanic fields that perch on the very edge of the Colorado Plateau, next to the actively stretching Basin and Range Province. Unlike most volcanic fields, the San Francisco field lies far from any tectonic plate boundary. Although none of the volcanoes that currently exist are expected to erupt again, the field is still considered active.
Approaching the town of 70,000 from any direction entails a gradual climb of at least 1,000 meters, from parched plains and pinyon-juniper forest to Flagstaff’s tranquil, ponderosa pine-studded volcanoes. Located on the southwestern edge of the Colorado Plateau, Flagstaff sits at nearly 2,100 meters elevation. Many neat geologic sites are located close to Flagstaff. But Flagstaff’s cones and craters, and the region’s spectacular scenery, also offer non-astronauts an out-of-this-world experience. This great diversity led NASA to select the area as a training ground for the Apollo astronauts in the 1960s and ‘70s. Few places in the country exhibit so many types of volcanic features, including jagged lava flows, crumbly cinder cones and the remnants of a towering stratovolcano, in such a compact region. Near the outdoorsy college town of Flagstaff in northern Arizona, more than 600 mounds and mountains - every one of them a volcano in the San Francisco Volcanic Field - are sprinkled across the high plateau on which the town is perched. But Arizona also hosts an impressive volcanic field. Ask them about Arizona, and images of the Grand Canyon, red rock deserts and saguaros baking in the sun probably come to mind. Helens and Mount Rainier in the Pacific Northwest. volcanoes, most people think of Hawaii, Alaska, or Mount St. This waterfall, which only flows at certain times of the year when snows melt or monsoon rains fall, was created when a lava flow from Merriam Crater blocked the channel of the Little Colorado River. Although better known for its parched deserts and graceful saguaros, Arizona also hosts a wide variety of spectacular volcanic features, including Grand Falls.
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