America's Volcanic Past -
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| "Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994 |
Learn More About
Arizona Volcanoes
Volcanic Highlights and Features:
[This list is just a sample of
various Arizona volcanic features or events and is by no means inclusive.]
Volcanic eruptions took place sporadically throughout Arizona, but especially during the last 40 million years. During the first 15 million years of this time period volcanism generally progressed across the State from southeast to northwest. Most of the volcanic rocks younger than 15 million years are located near the southwestern boundary of the Colorado Plateau physiographic province. The youngest volcanic rocks in Arizona, those that are four million years old or less, are clustered in six areas throughout the State. Those that crop out along the International Border southeast of Yuma are on the northernmost edge of the Pinacate volcanic field, which is a large, spectacular, young field that is almost entirely in northern Sonora, Mexico.
Most eruptions during the past 15 million years
had comparatively minor explosive activity.
The lava flowed like hot tar and cooled to form basalt.
During the past two million years lava with that
consistency flowed into the western part of the
Grand Canyon near Vulcan's Throne at least a dozen times,
temporarily damming the Colorado River. Lava also
flowed into and along the Little Colorado River and
dammed it in two places. Grand Falls, on the
Navajo Reservation about 40 miles northeast of Flagstaff,
is the most easily accessible place to view the results.
Several times from about 8 to 2 million years ago lava
spilled into what was then a large lake but is now the
Verde River Valley between Clarkdale and Camp Verde.
Basalt there is interbedded with lake-bed sediment.
Between Camp Verde and Strawberry lava cascaded southward
over the Mogollon Rim and completely obscured it.
Many eruptions were explosive. The lavas that accompanied them were
richer in silica and, therefore, more viscous (less fluid)
than the basaltic flows. Viscosity of the lava was determined
by its composition, which, in turn, determined the ease with which
trapped gases could escape. Much of this lava solidified as
rhyolite or associated rock types. Ash, pumice, and rock fragments were
ejected into the air. San Francisco Mountain, 10 miles north of Flagstaff,
is the remnant of one of Arizona's best-known explosive volcanoes.
Humphreys Peak, which has an elevation of 12,633 feet,
is the highest point on
San Francisco Mountain and the highest
point in Arizona. Sometime between
200,000 and 400,000 years ago, however,
San Francisco Mountain was
15,000-16,000 feet high before the top and
northeast side blew off
during a violent eruption. About 2,500-3,000 feet
of the summit was destroyed.
(That's essentially what happened to Mount St. Helens
in Washington in May 1980,
except that elevation of St. Helens prior to
the eruption was 9,677 feet and
1,300 feet of its summit was blown off.)
Pumice and volcanic ash from
San Francisco Mountain were scattered toward
the northeast and accumulated in low places.
Excerpts from: Arizona Geological Survey Website, 2001 |
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Colorado Plateau:9
Basin and Range:9 The Basin and Range province has a characteristic topography that is familiar to anyone who is lucky enough to venture across it. Steep climbs up elongate mountain ranges alternate with long treks across flat, dry deserts, over and over and over again! This basic topographic pattern extends from eastern California to central Utah, and from southern Idaho into the state of Sonora in Mexico. Within the Basin and Range Province, the Earth's crust (and upper mantle) has been stretched up to 100% of its original width. The entire region has been subjected to extension that thinned and cracked the crust as it was pulled apart, creating large faults. Along these roughly north-south-trending faults mountains were uplifted and valleys down-dropped, producing the distinctive alternating pattern of linear mountain ranges and valleys of the Basin and Range province. |
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| Arizona's Volcanic Gemstone |
Peridot:11
Currently, the United States is the basic
suppler of peridot to the world gemstone industry.
Deposits in Arizona are the major source of
U.S. peridot. Faceted Arizona peridot is
highly prized locally, but also enjoys
widespread popularity for the
manufacture of birthstone and other
jewelry. The bright green, yellow-green
to olive-green peridot is used both as a
faceted and tumbled gem.
The faceted stones are used in rings,
earrings, pendants, bracelets, and
as accent stones. The tumbled gems
can be set in baroque jewelry, drilled
and strung as beads, used in mosaics,
and in the manufacture of gem trees.
| Chiricahua National Monument |
Chiricahua National Monument:1,3
Twenty seven million years ago a volcanic eruption of immense proportions shook
the land around Chiricahua National Monument. One thousand times greater than the
1980 eruption of Mount St. Helens, the (nearby) Turkey Creek Caldera eruption
eventually laid down two thousand feet of highly siliceous ash and pumice.
This mixture fused into a rock called rhyolitic tuff (Rhyolite Canyon Tuff).
These units were subsequently eroded to form spectacular pinnacles
and columns.
| Elden Mountain |
Elden Mountain:10
The San Francisco Volcanic Field
includes several lava domes, 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. Domes can be active for decades or sometimes centuries. If a lava dome
grows entirely by internal inflation, similar to a balloon, it is called an endogenous dome. If,
however, magma breaks out through a dome's flank during inflation and adds new lava
layers to the outer surface, the final dome is called exogenous.
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. Sugarloaf
Mountain, at the entrance to San Francisco Mountain's Inner Basin, is a rhyolite lava dome.
This dome is thought to be endogenous, but its forest cover hides direct evidence of its
internal structure.
[See San Francisco Volcanic Field below]
|
Grand Canyon -
Grand Canyon National Park |
Grand Canyon Geologic Background:5
The rocks at Grand Canyon are not inherently unique;
similar rocks are found throughout the world. What is unique
about the geologic record at Grand Canyon is the great
variety of rocks present, the clarity with which they're
exposed, and the complex geologic story they tell.
These rocks provide a remarkable (but incomplete) record of
the Paleozoic Era (550 to 250 million years ago),
as well as scattered remnants of Precambrian rocks
as old as 2,000 million years (2 billion). The story these rocks tell
is far older than the canyon itself. Mesozoic and
Cenozoic rocks (250 million years old to the present)
are largely missing at
Grand Canyon (they've either
been worn away or were never deposited).
Although the rocks exposed in the walls of the canyon are
geologically old, the Canyon itself is a fairly young
feature. The oldest rocks at the canyon bottom are close to 2 billion years old.
The Canyon itself -- an
erosional feature that owes its existence to the Colorado River
(which is largely responsible for the depth of
the canyon) -- has formed only in the past 5 or 6 million years.
Geologically speaking, Grand Canyon is very young.
Toroweap Valley:8
Late Cenozoic lava flows have repeatedly flowed down
Toroweap Valley and several adjacent valleys into the Grand
Canyon, at times forming large lava dams. Flows are
exposed on the walls of the Grand Canyon, often interbedded
with fluvial and lacustrine sediments, up to 600 meters
above present river level. Several of the dams are
estimated to have been at least 200 meters high.
The most recent flows in the Grand Canyon have cascaded over the
rim of the Esplanade to the river 1,000 meters below.
Uinkaret Volcanic Field:8
The Uinkaret volcanic field (1.2 million years ago to 12,500 years ago)
at the north rim of the Grand Canyon in the Grand Canyon National Park is
especially noteworthy. The lavas are alkalic and
commonly contain peridotite inclusions.
[see Uinkaret Volcanic Field below]
Vulcan's Forge:8
Within the river itself is a volcanic neck,
Vulcan's Forge, 25 meters in diameter and rising 15 meters above
the river.
Vulcan's Throne:8
Vulcan's Throne, a
Quaternary cinder cone on the rim of the Canyon is cut
by recent fault movement on the Toroweap fault.
Young Lava Flows:8
Late Cenozoic volcanism extends across a broad
region from southwestern Utah
to the north rim of the Grand Canyon
in western Arizona.
The relative ages of lavas can be readily
established from morphological relations, with
the oldest capping mesas or buttes, and the most recent occupying
present drainage valleys. Many of these young
flows have no soils developed on them and have well
preserved flow features and associated cinder cones. The
youngest measured age is 12,500 years ago for a young flow at the Grand Canyon.
Zoroaster Granite:6
The oldest rocks exposed in the Grand Canyon are the Proterozoic Vishnu schist.
Approximately 2 billion years ago, 25,000 feet of sediments and
volcanic material were deposited on the sea floor. These sediments were
metamorphosed
during a mountain building episode 1.7 billion years ago.
The rocks were folded, uplifted, and
intruded by granites
(Zoroaster Granite), resulting in the formation
of a mountain range that is believed to have
been 5 to 6 miles high. Between 1.7 and 1.2 billion years ago
these mountains were eroded till only the
roots remained.
| Humphreys Peak |
Humphreys Peak:1
Humphreys Peak, which has an elevation of 12,633 feet,
is the highest point on
San Francisco Mountain and the highest
point in Arizona. Sometime between
200,000 and 400,000 years ago, however,
San Francisco Mountain was
15,000-16,000 feet high before the top and
northeast side blew off
during a violent eruption. About 2,500-3,000 feet
of the summit was destroyed.
Pumice and volcanic ash from
San Francisco Mountain were scattered toward
the northeast and accumulated in low places.
[See San Francisco Mountains below]
| Organ Pipe Cactus National Monument |
Organ Pipe Cactus National Monument:1
The Organ Pipe Cactus National Monument
is an excellent laboratory in which to observe rhyolite,
volcanic ash, and other rocks associated with explosive volcanism.
| Peridot Mesa |
Peridot Mesa:11
Peridot Mesa, located on the
San Carlos Apache Indian Reservation east
of Globe in Gila County, is the most
productive locality for peridot in the
world. A second Arizona location from
which peridot is recovered is
Buell Park Apache County,
Arizona, about 16 kilometers north of Fort
Defiance. However, it is estimated that 80%
to 95% of the world's production of peridot
comes from the San Carlos Reservation.
The peridot occurs as individual grains
and aggregates of grains in a basalt
which is about 3 to 35 meters thick that
forms the top and sides of Peridot
Mesa. The material is recovered after
it has weathered out of the basalt, in
the gullies and canyons which lead down
the sides of the Mesa or it is
recovered in place in the basalt.
The latter method of recovery requires
drilling and blasting. The peridot grains
that are commercially recovered are
typically 6 to 13 millimeters in size.
Therefore, the faceted stones produced
from this material are generally about
1 carat in size; 2 to 3 carat stones
are not uncommon, but stones, particularly
flawless ones, 5 carats and over are unusual.
Stones as large as 15 and 22 carats have been cut from San
Carlos peridot.
| San Francisco Peaks |
San Francisco Peaks:9
San Francisco Peaks are the remnants of the only
stratovolcano
in the San Francisco volcanic field.
For decades, volcanologists suggested
that the mountain now called San Francisco Peaks had simply worn
away over time, eroded bit by bit to form its current bowl-shaped top. Then, in 1980, the catastrophic explosion of Mount St. Helens forced us to rethink our ideas about volcanoes.
Many volcanologists now feel 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.
[See San Francisco Volcanic Field below]
| San Francisco Volcanic Field |
San Francisco Volcanic Field:10
Northern Arizona's San Francisco Volcanic Field, much of
which lies within Coconino and Kaibab National Forests, is
an area of young volcanoes along the southern margin of
the Colorado Plateau. During its 6-million-year history,
this field has produced more than 600 volcanoes. Their
activity has created a topographically varied landscape
with forests that extend from the Pinon-Juniper up to the
Bristlecone Pine life zones. The most prominent landmark
is San Francisco Mountain (San Francisco Peaks), a stratovolcano
that rises to 12,633 feet and serves as a scenic backdrop to the city of Flagstaff.
[see Elden Mountain above]
[see San Francisco Peaks above]
[see Sugarloaf Mountain below]
[see Sunset Crater Volcano National Monument below]
[See Williams, Arizona, below]
| Sugarloaf Mountain |
Sugarloaf Mountain:10
The San Francisco Volcanic Field includes several lava domes, 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. Domes can be active for decades or sometimes centuries. If a lava dome
grows entirely by internal inflation, similar to a balloon, it is called an endogenous dome. If,
however, magma breaks out through a dome's flank during inflation and adds new lava
layers to the outer surface, the final dome is called exogenous.
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. Sugarloaf
Mountain, at the entrance to San Francisco Mountain's Inner Basin, is a rhyolite lava dome.
This dome is thought to be endogenous, but its forest cover hides direct evidence of its
internal structure.
[See San Francisco Volcanic Field above]
| Sunset Crater Volcano National Monument |
Sunset Crater Volcano National Monument:7
Sunset Crater is one of the youngest scoria cones
in the contiguous United States.
The cone is named for the topmost cap of
oxidized, red spatter which makes it appear bathed in the light of the sunset.
In the 1920's H. S. Colton saved the cone from severe damage by averting the
attempt of a Hollywood movie company to blow it up in order to simulate an
eruption. This led to the establishment of the
National Monument at Sunset Crater.
Sunset Crater Eruptive History:4
The cones and lava flows of the San Francisco volcanic field,
which covers about 2,000 square miles of the southwestern Colorado Plateau,
result from several
million years of volcanic activity. These powerful underground
forces changed the landscape dramatically
beginning in the winter of AD 1064-65. Sunset Crater appeared
when molten rock sprayed out of a crack in the
ground high into the air, solidified, then fell to earth as
large bombs or smaller cinders.
As periodic eruptions continued over the next 200 years, the
heavier debris accumulated around the vent
creating a 1,000-foot cone. The lightest, smallest particles
blew the farthest, dusting 800 square miles of
northern Arizona with ash. Perhaps as spectacular as the original
pyrotechnics were two subsequent lava flows:
the Kana-A flow in 1064 and the Bonito flow in 1180. They
destroyed all living things in their paths.
In a final burst of activity, around 1250, lava containing
iron and sulfur shot out of the vent. The red and yellow
oxidized particles fell back onto the rim as a permanent
"sunset" so bright that the cone appears still to
glow from intense volcanic heat.
Bonito Lava Flow:9
The Bonito Lava Flow is one of several flows that streamed
out from the base of Sunset Crater Volcano about 930 years ago.
The 1200 degrees C (2200 degrees Fahrenheit) liquid formed a river of
black lava that inundated over four and a half square kilometers
(almost two square miles) of the landscape before it cooled and solidified.
Cinder Cones:4
Cinder cones, such as Sunset Crater, are formed by explosive eruptions.
Magma, a mixture of molten rock and highly compressed gases,
rises upward from its underground source. As the magma ascends,
the extreme pressure drops and gases are released. The relatively
thick magma and the high gas percentage causes an explosion out
of the central vent. Solidified rock pieces - of various sizes -
fall back down around the vent, creating a mound. Another
kind of eruption, involving thinner magma with a lower gas content,
produces lava flows that may issue from the side or base of the cone.
Colors9
Sunset Crater Volcano was originally named Sunset Mountain by that
intrepid explorer of the Colorado River, John Wesley Powell,
for the bright sunset reds and yellows of its summit.
Of all the cinder cones of the San Francisco volcanic field,
Sunset Crater Volcano is one of the most colorful and pristine.
Most of the rock you see at Sunset Crater Volcano National Monument
is dark gray to black basalt. Basalt is rich in iron and magnesium-bearing minerals,
which give it its dark color. While the base of Sunset Crater Volcano is
mantled with dark gray cinders, the summit of the cinder cone is a striking rusty red.
Although you might speculate that the rocks at the top are of a different type,
they are not! What makes these basalt cinders red?
Searing basalt lava was not the only material to erupt from Sunset Crater Volcano!
The cinder cone also belched forth hot gasses. The cinders on the rim of the
cinder cone were bathed in these vapors and chemically reacted with
them to form iron oxide (rust), sulfur compounds, and gypsum.
The resulting red, yellow, purple, and green-colored basalt cinders
decorate Sunset Crater Volcano's summit.
Lava Tubes:9
Hidden beneath Sunset Crater's thick volcanic crust are
elongate caves called lava tubes. Lava tubes are remnants
of the Bonito Lava Flow's plumbing system. These pipes first
form while the lava is actively streaming downhill.
The scorching, runny basalt lava cools and hardens
quickest on the surface of the flow where it contacts air.
The first solid rock forms plate-like sheets that are swept
along like rafts on this swiftly-flowing stream of lava.
As the surface continues to cool the plates will pile up
and fuse together to form a kind of crusty roof over the
gushing lava river below. The solid roof insulates the
still-molten lava below from the cooling effects of the air.
The long, straight tubes may extend miles from the vent
where the lava emerges, emptying their molten contents far downstream.
Eventually the vent exhausts its lava supply or simply
becomes plugged up. Lava already in the tube drains out
at the down stream end, leaving an empty lava tube behind.
Sometimes part of the thin, crusty roof collapses, and an
entrance to the tube opens up as it did here at Sunset Crater.
Spatter Cones:9
One to fifteen meter (3-50 feet) high spatter cones
(or hornitos -- 'little ovens' in Spanish) form a string
of lumpy beads along a once-active vent system near the base of
Sunset Crater Volcano. Like bubbling spaghetti sauce, they form
when gasses escape from molten lava beneath the crusty, solid surface of a flow.
If you look closely at this spatter cone you can easily see
where individual 'spatters' fell with a SPLAT! onto the ring of
solidified lava. Their heat welded them to the older spatters,
gradually building the miniature volcanoes.
| Superstition Mountains |
Superstition Mountains:12
The consolidated deposits
of pyroclastic flows, lava flows and domes,
and lahars in the Superstition
Mountains and adjacent areas testify to a
period of intense volcanism about
17 to 25 million years ago in central Arizona.
Apache Trail:1
In the Superstition Mountains,
about 23 miles east of downtown Phoenix,
a volcano exploded about 18 million years ago.
Volcanic ash beds associated with these eruptions
are exposed spectacularly along the Apache Trail,
which follows the Salt River northeast of Apache Junction.
Geologists named the thickest and
most widely distributed ash deposit the Apache Leap Tuff.
| Tonto Natural Bridge State Park |
Tonto Natural Bridge:2
Tucked away in a tiny valley surrounded by a
forest of pine trees, Tonto Natural Bridge has
been in the making for thousands of years. It
is believed to be the largest natural travertine
bridge in the world. The bridge stands 183 feet
high over a 400-foot long tunnel that
measures 150 feet at its widest point.
According to state geologists, the formation of
Tonto Natural Bridge went through several
stages of development.
The west side of Pine Creek was formed by a
flow of lava in the form of rhyolite. The rock
eroded, leaving behind purple quartz sandstone.
The rock layers were then lithified, tilted
and faulted.
The area was then covered by sea water, leaving
behind a sediment of sand and mud.
Volcanic eruptions covered the rock layers with
lava, forming a basalt cap. Through erosion,
the basalt cap broke down and was shifted by faults,
creating Pine Creek Canyon.
Precipitation began seeping underground through
fractures and weak points in the rock,
resulting in limestone aquifers. Springs emerged
as a result of these aquifers, carrying the
dissolved limestone and depositing calcium
carbonate to form a travertine dam. The waters
of Pine Creek then eroded through the travertine
and formed the Natural Bridge.
| Uinkaret Volcanic Field |
Uinkaret Volcanic Field:8
The Uinkaret volcanic field at the north rim of the Grand Canyon in the
Grand Canyon National Park is especially noteworthy. The lavas are
alkalic and commonly contain peridotite inclusions.
[See Grand Canyon above]
| Williams, Arizona |
Williams, Arizona:10
The first volcanoes in the San Francisco Volcanic Field began to erupt about 6 million years
ago, in an area where the town of Williams is now. Subsequently, a several-mile-wide belt
of successively younger eruptions migrated eastward, to the area of modern Flagstaff, and
even a bit beyond, toward the valley of the Little Colorado River. Today, this belt of
volcanoes extends about 50 miles from west to east.
[See San Francisco Volcanic Field above]
Excerpts from:
1) Arizona Geological Survey Website, 2002
2) Arizona State Parks Website, 2002
3) U.S. National Park Service Website,
Chiricahua National Monument, 2000
4) U.S. National Park Service Website,
Sunset Crater Volcano National Monument, 2000
5) U.S. National Park Service Website,
Grand Canyon National Park, 2000
6) U.S. National Park Service Website -
Geology of Grand Canyon National Park, North Rim,
Geology Field Trip guides by Anabelle Foos,
University of Akron, April 2000;
7) Self, 1990, IN: Wood and Kienle, 1990,
Volcanoes of North America: Cambridge University Press
8) Nash, 1990, IN: Wood and Kienle, 1990,
Volcanoes of North America: Cambridge University Press
9) USGS/NPS Geology of the National Parks Website, 2001
10) Priest, Duffield, Malis-Clark, Hendley II, and Stauffer, 2001,
The San Francisco Volcanic Field, Arizona:
USGS Fact Sheet 017-01
11) Gemstones, An Overview of Production of Specific U.S. Gemstones:
U.S. Bureau of Mines Special Publication 14-95
12) Brantley, 1994,
Volcanoes of the United States:
USGS General Interest Publication
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