SF peaks activity sheet
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Dec 6, 2023
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Activity: A Tour of the San Francisco Peaks Volcanic Field of Northern Arizona
Not all active volcanoes are found in close proximity to plate margins – some occur at oceanic Hot Spots, like Hawaii, and others can occur in the centers of continents. The San Francisco Peaks area of northern Arizona is considered to be an active volcanic province, with its most recent eruption, around 1064 AD, producing Sunset Crater, a large Cinder Cone
, and the Bonito lava flow. However these are only the most recent volcanic features in the field – so we’re going to explore it in Google Earth to learn more about its extent and the variety of eruptive features found there. Download the San Francisco Peaks kmz file, as well as the individual files for all the photos.
The photo positions are identified in the kmz file, but to “see”
the photos in Google Earth you may have to load them in yourself (not essential!).
Go to the photo position for the San Francisco Peaks Volcano – it’s at a pullout on the way to the Sunset Crater National Monument. You can look at the photo attached, as well as the Google Street View version (that was taken in wintertime). This volcano was once >16000 feet (>5000 meters), but has since lost a good bit of its height.
--
Go to Mount St. Helens in Google Earth, and compare what you see there to what the San Francisco volcano looks like. Mount St Helens lost its top during its 1980 eruption, which created the devastation all around it. What do you think might be a reason that the San Francisco Volcano lost its height?
The San Francisco Volcano has lost height over time primarily due to erosion. Erosional processes, including wind, water, and ice, wear away at the mountain's surface and gradually reduce its elevation. Additionally, volcanic activity, tectonic movements, and gravitational collapses can also contribute to the reduction in a volcano's height.
Click on the O’Leary Peak trail path link. This trail runs along the margin of the Bonito lava flow, which erupted from Sunset Crater in 1064 AD. Four photographs of the flow are included from four different locations. The Bonito flow is a classic A’a lava flow, in that it the lava is very rough and spiny, and the flow is thick.
Sunset Crater is marked with a placemark. It is a young Cinder Cone
, which means that it formed by a moderately explosive (called Strombolean
) kind of eruption. Sunset Crater lies at one end of a linear feature called the Cinder Hills Rift, which has been the locus of the most recent volcanism in the San Francisco field.
--Look along and around the Cinder Hills Rift. What kinds of volcanic edifices and structures do you see?
The Cinder Hills Rift in the San Francisco volcanic field is characterized by a concentration of recent volcanic features. Around the Cinder Hills Rift, one can observe cinder cones like Sunset Crater, which are formed from explosive Strombolian eruptions. Additionally, there are lava flows and spatter cones, indicative of both explosive and effusive volcanic activity.
--The Bonito lava flow, and the Kana’a lava flow (about 50,000 years old) are
basaltic lavas both associated with activity at Sunset Crater. Also marked is SP Crater to the north, which is associated with a prominent ~50,000 year
old lava flow. Looking at these craters and their associated lavas, how do you think these lavas erupted from these craters (or did they?). Provide a short explanation of your thinking.
The basaltic lavas likely erupted in an effusive manner from the base or flanks of these cinder cones, forming the extensive lava flows observed today.
Given the nature of basaltic lavas, it's likely that they erupted effusively rather than explosively. In such eruptions, magma rises through the conduit and overflows the crater, pouring out onto the surrounding terrain and forming lava flows. The presence of these extensive lava flows suggests that the magma found a pathway to the surface and flowed out, rather than being trapped and causing a more explosive eruption. The cinder cones, like Sunset Crater and SP Crater, would have formed from the explosive ejection of fragmented lava (cinders) during Strombolian eruptions, but the subsequent or concurrent effusive lava flows would have emerged from the base or flanks of these cones rather than from their summits.
Return to the O’Leary Peak trail, and follow it up that mountain. O’Leary Peak is a different kind of volcanic feature, called a Lava Dome
, which develop when a very viscous, silica-rich lava, called Rhyolite
erupts – basically this lava, unlike basalt, doesn’t flow, but instead piles up where it erupts. Several other Lava domes are noted, including Elden Peak and Sugarloaf Mountain.
--How are Lava Domes different from Cinder Cones? Look at both of them from above and in profile view, and compare their elevations to answer!
Lava domes are typically round to oval in shape and have a "spherical" or "rounded" appearance. Its surface is rugged because the lava moves slowly. It may not be as tall as a cinder cone, but it has a larger area. Cinder cones are also typically rounded, with typically steep sides, giving them a more "symmetrical" appearance compared to lava domes.
Climbing up O’Leary Peak, there are a series of photographs showing both aerial
views of Sunset Crater and the Bonito Flow, and views of the different kinds of volcanic rocks in the dome: Rhyolites
, and Ash Tuffs
. In terms of their chemistries, these rocks are similar, but they erupt differently: Ash Tuffs erupt explosively, while Rhyolites extrude very slowly.
--Looking at the photos provided of O’Leary rhyolites and ash tuffs, how do
these rocks compare? Which is more coherent and “strong”? The O’Leary Rhyolites appear to be more coherent and "strong" compared to the
ash tuffs. Rhyolites, being solidified lava, tend to have a more consolidated structure, making them harder and more resistant to erosion and mechanical breakdown. In contrast, ash tuffs are formed from volcanic ash and other fragments that are compacted over time, and they can be less solid and more prone to disintegration.
{Just for fun – check out the photos of the view from the top of O’Leary Peak, at 8912 feet. What volcanic features do you see?}
These are small, steep-sided volcanoes that erupt gas-rich magma. They can be seen as round or conical hills in the landscape. The dark color and distinctive cone shape indicate a cinder cone volcano. The vast dark areas may be ancient basaltic lava flows. Lava flows are molten rocks that flow from volcanic vents. These flows can cover large areas and create unique patterns on the landscape. Go to Lenox Crater (placemarked and shaded) – this is an older, smaller cinder cone near Sunset Crater. Zoom in on it, and look at it in some detail, as it is typical of older cinder cones.
--Now, I want you to try and estimate the total geographic extent of the San Francisco volcanic field. We’re going to do this by looking for the cinder cones! Zoom out from Sunset crater, and zoom around, both north and west of San Francisco Peaks, and south and east. Put your own placemarks on all the cinder cones you can identify, and then use the Polygon tool to shade in the area that these features cover. Save your placemarks and polygon as your own .kmz file, and send that along with your written answers!
--Do you see any other features in this area that you think might be lava domes? If so, highlight these as polygons!
--Cinder cones and lava flows are predominantly Basaltic eruption types, while stratovolcanoes and lava domes are higher silica/rhyolitic eruption types. Judging from your look around, what kind of lava erupts most frequently in the San Francisco Peaks volcanic field?
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The most common type of lava erupted in this volcanic field is typically rhyolitic or
andesitic in composition, which contains higher levels of silica compared to basaltic lava.