America's Volcanic Past
|"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|
Volcanic Highlights and Features:
|[NOTE: This list is just a sample of various Arizona features or events and is by no means inclusive. All information presented here was gathered from other online websites and each excerpt is attributed back to the original source. Please use those sources in referencing any information on this webpage, and please visit those websites for more information on the Geology of Arizona.]|
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.
Arizona Geological Survey Website, 2001
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.
|Arizona's Volcanic Gemstone|
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.
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.
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]
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, 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.
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, 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, 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]
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, 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.
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.
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.
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.
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.
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]
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]
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
America's Volcanic Past - States and Regions]
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