San Francisco Volcanic Field

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					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   •Sunset Crater, northeast of
  about            San Francisco Mountain, is
  6,000,000.       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 gas-
  charged 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 scooped-
  out 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 dome-
                                                         shaped 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.

				
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posted:2/11/2012
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