Earth_Magazine_October_2017

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Most of the San Francisco Volcanic Field’s vol-
canoes are basaltic cinder cones. Just 17 kilometers
east of the San Francisco Peaks stands Sunset Cra-
ter, a cinder cone that erupted just over 950 years
ago, making it the field’s youngest volcano and the
featured attraction in its namesake national monu-
ment. Although visitors aren’t allowed to hike up
it, there are great views of its perfect cone shape
and oxidized, orange-red rim from the monument’s
Lava Flow Trail, which crosses the fresh-looking
Bonito lava flow.
Members of the Sinaguan culture likely witnessed
Sunset Crater’s eruption; the visitor center displays
molds of Sinaguan corncobs enveloped by lava. From
trees living at that time, as well as the beams from
Sinaguan pit houses buried by lava, scientists have
determined that Sunset Crater erupted in either late
A.D. 1064 or early 1065. If it was similar to modern
volcanic cone-forming eruptions, the Sunset erup-
tion would have begun with glowing lava fountains
spewing bits of liquefied rock that cooled in mid-air.
These bits then fell back to the ground as cinders,
which piled up around the central volcanic vent to
build the 300-meter-high cone.
About a century later, another, less-gaseous erup-
tion seeped from the cone’s northwest side to form
the Bonito lava flow, which crumpled and cracked
as it cooled. While walking across the flow on the
Lava Flow interpretive trail, you can see many ves-
icles as well as volcanic “bombs” — blobs of ejected
molten rock that stretched into elongated shapes
while falling back to Earth. The sixth stop on the
1.5 kilometer-long trail marks a lava tube, a conduit
that forms as a crust builds up around still-flowing

magma. Once an eruption ceases and

the magma inside the conduit drains

away, all that remains is a hollow tube.

Such tubes can stretch for kilometers.

Visitors used to be able to tour the lava

tube, but it was closed after a roof col-

lapse. It is home to an ice cave in which

the temperature remains below freezing

year-round; at the lava tube trail stop,

visitors can feel the cold air rushing out

of the cave.

Several more volcanoes can be seen

along the 55-kilometer scenic drive that loops

through both Sunset Crater National Monument

and Wupatki National Monument, which protects

settlements built by the Sinagua and other Ancestral

Puebloan cultures. The most photogenic of these is

Wupatki Pueblo, a multistory Sinaguan dwelling

with more than 100 rooms built from blocks of

red Moenkopi Sandstone and the occasional black

basalt. This area didn’t become densely populated

until about a century after Sunset Crater erupted, by

which time the Ancestral Puebloans had learned that

the ash and cinders from the eruption had increased

the soils’ ability to hold water. This population surge,

however, was brief. By 1225, the pueblo, and much

of the surrounding region, was deserted, most likely

due to drought.

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