Earth_Magazine_October_2017

Flagstaff’s tranquil, ponderosa pine-stud-

ded volcanoes. Although none of the

volcanoes that currently exist are expected

to erupt again, the field is still consid-

ered active.

Unlike most volcanic fields, the San

Francisco field lies far from any tectonic

plate boundary. It is one of several volca-

nic fields that perch on the very edge of

the Colorado Plateau, next to the actively

stretching Basin and Range Province.

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, trigger-

ing thinning of the crust from Southern

California to Arizona to form the Basin

and Range. 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.

The diverse volcanic features on display here stem

from the variety of magmas that have erupted. Vol-

canic rocks are classified by their silica composition,

for which color serves as a loose guide. Basalt, the

lava type richest in iron and magnesium and most

deficient in silica, is black. Most of the San Fran-

cisco Volcanic Field’s volcanoes consist of basalt.

Andesite, which forms the region’s largest volcano,

is dark gray in color thanks to its intermediate silica

content. Finally, rhyolite, the most silica-rich lava,

is light gray. Sugarloaf, a small dome that sits at the

feet of the San Francisco Peaks, consists of rhyolite.

The silica content of magma controls its vis-

cosity, which is the most important factor in

determining a volcano’s style of eruption, and

hence its size and shape. 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. Because of its low

viscosity, basalt eruptions tend to be relatively tame

compared to other types. 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.

The reason why so many different types of magma

erupted in the San Francisco Volcanic Field lies in

a process called partial melting. As rock heats up,

it doesn’t usually melt completely, or all at once.

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. Because this melt is

less dense than the surrounding solid rock, it grad-

ually rises toward Earth’s surface.

Partial melting of the upper mantle, which con-

tains a very low proportion of silica, results in

basalt, the material that makes up most of Flagstaff’s

volcanoes. As the basalt magma rises, its heat can

partially melt the continental crust through which it

passes. And because continental crust contains more

silica than upper mantle rock or basalt, the resulting

partial melts form subsidiary magma chambers that

are even richer in silica, producing lighter volcanic

rocks like andesite and rhyolite.

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Grand Falls

Navajo Nation

Reservation

Merriam Crater

Wupatki National

Monument

Sugarloaf

Meteor Crater

Sunset Crater

National Monument

Cinder Lake

Humphrey’s

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