Hey!—you could have skied “Arizona Snowbowl East” year-round 20,000 years ago, but watch out for those deep crevasses! Geology. But we geologists don’t have an accepted theory to explain why the Colorado Plateau has risen the way it has over the last 30 million years, and until we understand that, we probably won’t be able to explain the origin of the several volcanic fields in our region that are one piece of that larger puzzle. The Inner Basin of the San Francisco Peaks in Arizona was created. However, the most recent and scientifically rigorous study of the pre-collapse SFM, based on measurements of slope angles of uneroded portions of the mountain’s flanks (see figure below) suggests that there were two distinct vents up on top of the volcano back when SFM was at its tallest. San Francisco Peaks: Beautiful volcano! Competing Theories: In the years that shortly followed the notorious “sideways eruption” of Mount St. Helens in Washington state in 1980 (see computer model graphic of this event below), the collapse of the top and northeast flank of SFM was interpreted by a number of local geologists as having occurred during a similar “sideways eruption”. Buy Now. The multiplicity of volcanic fields cannot be credited to a multiplicity of hot spots, or to the effects of a single large hot spot. Stratovolcanoes are large conical volcanoes composed of many alternating layers of hardened lava, and looser tephra (cinders, and larger volcanic “bombs”), plus pumice and ash, deposited during multiple eruptions, typically over millions of years. The gentle slopes at far right and far left project upward toward the center, giving an idea of how high this mountain used to be, … The Southern Cone was older, the Northern Cone a bit younger, and topped out higher, at 14,633 ft. This horseshoe-shaped recess into the core of the mountainisattheapexofalarge,mostlyburied fan of bouldery debris derived from the composite volcano. And finally, not only can you see what remains of SFM today, you can also still see portions of the huge avalanche deposit that were created when the top and northeast flank of the mountain came tumbling down. And finally, San Francisco Mountain is by no means alone as a volcano in our vicinity. Over the last 2.6 million years, there have been 13 major continental glaciations (“Ice Ages”), each accompanied by growth and expansion of mountain glaciers, including on SFM. Northern Arizona is best known for the Grand Canyon. Flagstaff, AZ 86001 In the 17th century, members of the Spanish mission at Oraibi named the summits after the founder of their order, St. Francis of Assisi. This is a deeply eroded valley, where the topmost portion of SFM once stood (see figure below). In which country are the Southern Alps located? In round numbers, it’s accurate enough to say that SFM dates back as far as 1 million years, and became dormant 400k years ago. Northern Arizona is best known for the Grand Canyon. You are riding down the sloping top surface of the giant avalanche deposit, which was formerly the top and northeast flank of SFM. San Francisco Peaks: Beautiful volcano! Mt St Helens is this type of volcano: Composite cone. Areas between lava-flow ridges were surfaced predominantly by looser cinder and ash deposits, and consequently, these have eroded fairly deeply between the ridges. Introduction: The San Francisco Peaks are the eroded remains of a single, once-much-higher, stratovolcano called San Francisco Mountain (‘SFM’). Kachina Peaks Wilderness is part of a large composite volcano that last erupted roughly two million years ago. Sunset Crater, the State's youngest volcano, erupted in about A.D. 1085 and must have been witnessed by native inhabitants who lived very nearby. Humphreys. You are looking at glacial moraines that once flowed down the Inner Basin. San Francisco Mountain is a composite volcano comprising layers of weak, unconsolidated of volcanic tephra (ash, lapilli and bombs) alternating with competent, dense lava flows. In the years that shortly followed the notorious “sideways eruption” of Mount St. Helens in Washington state in 1980 (see computer model graphic of this event below), the collapse of the top and northeast flank of SFM was interpreted by a number of local geologists as having occurred during a similar “sideways eruption”. The geologic map below shows—in shades of purples and greens—the major lava flows that radiated outward and downward on all the flanks of SFM, with the different hues distinguishing the different ages and mineral compositions of the flows. 9300 North Snowbowl Road They rise to Arizona's highest point of 12,633 feet. The Schultz Fire burned several peaks on the east side of the San Francisco Peaks composite volcano. Stay tuned! The tallest of these are Humphreys at 12,637 feet and Agassiz at 12,356 feet. So rather than being related to movement over a hot spot, the origin of these several volcanic fields must somehow be related to the uplift of this, the southwestern edge of the Colorado Plateau. Lava Plateau. Start studying Geoscience Homework 3. Although this idea was attractive, to date no hard geologic evidence has been found that directly supports the occurrence of the final eruptions of SFM being simultaneous with, and triggering, the northeastward collapse of the mountain. You can see that the light tan of the Inner Basin extends toward the NE, interrupted only by Sugarloaf lava dome, depicted by an orange circular area in the middle of the northeastward light tan, which represents the upper portion of the avalanche deposit. The San Francisco volcanic field is an area of volcanoes in northern Arizona, north of Flagstaff, US.The field covers 1,800 square miles (4,700 km²) of the southern boundary of the Colorado Plateau.The field contains 600 volcanoes ranging in age from nearly 6 million years old to less than 1,000 years (Miocene to Holocene), of which Sunset Crater is the youngest. Less widely known are the hundreds of geologically young volcanoes scattered across the southern portion of the Colorado Plateau at the eastern foothills of the San Francisco Peaks. Less widely known are the hundreds of geologically young volcanoes scattered across the southern portion of the Colorado Plateau at the eastern foothills of the San Francisco Peaks. The San Francisco Volcanic Field, which covers about 1,800 square miles, is part of northern Arizona’s spectacular landscape. The large hole at the top of a volcano formed when the roof of a volcano's magma chamber collapses. The San Francisco Peaks are actually the remains of an extinct volcano, which mllions of years ago shattered and reshaped these great peaks, forming four of the highest individual peaks in Arizona and is home to the only region of tundra in Arizona. (2010). The Lenox Crater Trail hike is a strenuous walk up the side of the Lenox Crater Volcano. Enjoy amazing views of the lava field and the San Francisco Peaks … San Francisco Peaks: Beautiful volcano! In round numbers, it’s accurate enough to say that SFM dates back as far as 1 million years, and became dormant 400k years ago. The broad snow-capped mountain in the background to the west, behind and to the right of Sunset Crater, is San Francisco Mountain. Computer graphic depictions of the May 18, 1980 eruption of Mount St. Helens. Courtesy US Geologic Survey. Mt. The most prominent landmark is San Francisco Mountain, a stratovolcano that rises to 3,850 m (12,633 ft) and serves as a scenic backdrop to the city of Flagstaff. 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