San Francisco Volcanic Field
Arizona's Hotspot
• Continental Hotspot Volcanism: Hotspots are
thought to be caused by a narrow stream of hot
mantle convecting up from the Earth's core-mantle
boundary called a mantle plume
• The San Francisco Volcanic Field is comprised of
more than 600 volcanoes-most of them relatively
small cinder cones-scattered over an area of about
1,800 square miles.
Arizona's Hotspot
• Most hotspot volcanoes are basaltic because they
erupt through oceanic lithosphere (e.g., Hawaii,
Tahiti). As a result, they are less explosive than
subduction zone volcanoes, in which water is
trapped under the overriding plate.
• Where hotspots occur under continental crust,
basaltic magma is trapped in the less dense
continental crust, which is heated and melts to
form rhyolites. These rhyolites can be quite hot
and form violent eruptions, despite their low water
content.
Geologic Background:
• The San
Francisco
volcanic field
ranges in
composition
from basalt to
rhyolite and
began activity
about
6,000,000.
•Sunset Crater, northeast of
San Francisco Mountain, is
the youngest feature in the
volcanic field.
Volcanoes and Types of
Magma
• 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 are built when gascharged frothy blobs of basalt magma are
erupted as an upward spray, or lava
fountain.
Volcanoes and Types of
Magma
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.
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.
Stratovolcanoes
• 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.
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.
Lava Domes
• 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.
Dacite lava
consists of about
63 to 68 percent
silica (SiO2). It is
one of the most
common rock
types associated
with enormous
Plinian-style
eruptions. When
relatively gas-poor
dacite erupts onto
a volcano's
surface, it
typically forms
thick rounded lava
flow in the shape
of a dome.
• Humphreys Peak is the highest mountain
in Arizona (12,633 feet): Humphreys Peak
was named after General A. A. Humphreys
who was a US chief of Engineers.
• Many volcanologists believe that the scoopedout shape of the San Francisco Peaks may be the
result of a catastrophic sideways blast like that of
Mount St. Helens.
•Sugarloaf
Mountain, the
small domeshaped hill in the
foreground, is a
rhyolite dome.
Quarries at the
base of the dome
(bottom-right)
mine pumice.
The quarries are
in an older dome
that predates
Sugarloaf
Mountain by
about 500,000
years.
Major Volcanic Events in Northern
Arizona
• 6 million yrs ago: the first volcanoes begin to
erupt in the San Francisco Volcanic Field.
• 3 million yrs ago: Kendrick Peak forms north of
Flagstaff.
• 430,000 yrs ago: the Peaks reach their highest
elevation.
• 400,000-200,000 yrs ago: San Francisco Mountain
erupts- decapitating the peak and forming the
Basin.
• 200,000 yrs ago: O’Leary Peak north of Sunset
Crater erupts.
Major Volcanic Events in Northern
Arizona
• 70,000 yrs ago: SP Crater north of Flagstaff
erupts.
• 933 yrs ago: Sunset Crater erupts (last
active eruption in the volcanic field).
Aa flow near to
Sunset Crater
Red Cinder Cone
near Sunset Crater
Collapsed Lava
Tube
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.
Volcanoes get younger to the east. This is consistent
with the westward motion of the North American
Plate over a fixed hotspot.
Map of Volcanic Rock Distribution near the Colorado Plateau
The geochronology of the San Francisco
volcanic field has been studied fairly extensively
beginning with Robinson ( 1913) who divided
the rocks into three periods of eruption:
•basaltic volcanics, probably
of the late Pliocene,
•rhyolitic to andesitic
volcanics, probably of the
early Pleistocene, and
•basaltic volcanism,
probably of the latter part of
the Quaternary.
The Hopi Buttes volcanic field is in the southern part of Black
Mesa Basin. Igneous rocks in the Hopi Buttes were intruded as
dikes and sills, and erupted pyroclastics, lava flows and lava
domes. About 200 volcanoes erupted during the Pliocene
(Sutton, 1974).
Depending on the
materials that filled
volcano vents after
eruption, three topographic
forms evolved that
characterize the Hopi
Buttes field:
The Hopi Buttes volcanic field is in the southern part of
Black Mesa Basin.
The Hopi Buttes volcanic field is in the southern part of
Black Mesa Basin.
•prominent necks, or plugs, that
rise above the landscape as narrow,
nearly circular and steep-sided
buttes surrounded by talus slopes,
•lava-capped mesas that resulted
from the erosion of lava domes on
flows that overlie maar craters, and
•maar craters that have no
protective covering of lava,
massive breccia or agglomerate.