8LIER+VERGMWGS5IEOW
Flagstaff’s largest volcano, which lends its name
to the entire field, is a horseshoe-shaped massif
crowned with five summits collectively known
as the San Francisco Peaks. The tallest of the five
summits, 3,850-meter-high Humphrey’s Peak, is the
highest point in Arizona and is a popular hiking and
skiing destination.
To the Navajo, whose reservation is less than an
hour’s drive north of Flagstaff, the massif is one of
four sacred mountains that mark the boundaries of
their traditional lands as well as the cardinal direc-
tions. The San Francisco Peaks represent the Sacred
Mountain of the West — the direction of the sunset
and hence the accomplishment of each day’s plans,
which cumulatively correspond to a person’s life.This symbolism is fitting because the peaks are
built from layer upon layer of alternating andesitic
volcanic ash and lava flows that erupted between
about 1.8 million and 400,000 years ago. This layer-
ing — and its andesite composition — identifies the
peaks as remnants of a stratovolcano that, according
to geological reconstructions, once soared to heights
of 4,500 to 4,800 meters prior to the major landslide
that formed today’s horseshoe. The horseshoe encir-
cles a central valley called the Inner Basin, where
snowmelt percolates through the porous volcanic
rocks to form one of Flagstaff’s most important
sources of water.
Today, Sugarloaf, a 300-meter-high, loaf-shaped
dome of rhyolite, blocks the Inner Basin’s mouth.
Because rhyolite is highly viscous and resists flow,
such lava domes typically do not spread out and
flatten, but instead grow by inflation: As more lava
rises toward the surface, the dome expands like an
inflating balloon. But because the rocky margins
around the dome are rigid — and not made of
stretchy rubber — it shatters rather than stretches as
it cools, gradually wrapping the dome in a distinctive
cloak of angular, broken rock called breccia. Given
its position, Sugarloaf, which erupted 200,000 years
ago, must have formed after the landslide occurred,
meaning the landslide happened between 400,000
and 200,000 years ago.
Through the cleft in the landscape left by this
landslide, visitors can access the Inner Basin during
the summer and fall via a scenic dirt road. Within
the basin are beautiful, alpine-like terrain and hiking
trails through stands of aspen trees that make for
colorful, leaf-peeping experiences in the fall and defy
stereotypes of Arizona scenery.3SVXLIVR &VM^SRE LSWXW ER
YRYWYEPHMZIVWMX]SJQEKQEW
[LMGLZEV]JVSQHEVOFEWEPX
EWWIIRMRSRISJXLIZSPGE
RMG JMIPHƶW QER] PEZE JPS[W
PIJX XS XLI PMKLXGSPSVIH
WMPMGEVMGLVL]SPMXISFWIVZIH
MRXLIZSPGERMGHSQIORS[R
EWYKEVPSEJ
XST
(VIHMX FSXL 8IVVM(SSOERH1SR
&FFSXX']JEVXLIPEVKIWXZSPGERSMRRSVXLIVR&VM^SREMWXLIQEWWMJGEPPIHXLIER+VERGMWGS5IEOWERI\XMRGXWXVEXSZSPGERSXLEXGSPPETWIH
MREQEWWMZIPERHWPMHIFIX[IIRȏȉȉȉȉȉERHȶȉȉȉȉȉ]IEVWEKS
(VIHMX lMXSGOTLSXSGSQ.VSRQER+MVIQER
TEKIȏȉ•4GXSFIVȶȉȦȮ• EARTH • [[[IEVXLQEKE^MRISVKFeature