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14.2: Assignment: A Virtual Field Trip Guide to Exploring the Volcanic History and Evidence of Mass Wasting at Sentinel Peak, Tucson, Arizona

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    Figure 1. View from the top of Sentinel Peak


    This field activity is significantly modified from work by J. Rasmussen, Ph.D., Pima Community College, with permission


    Sentinel Peak, also known as “A Mountain”, is one of a number of peaks in the Tucson Mountains, located on the west side of the City of Tucson. There is a rich geologic history surrounding Sentinel Peak and the Tucson Mountains. Your mission will be to follow this guide, collect “virtual” evidence, and ultimately decide if Sentinel Peak (A Mountain) is a volcano! In addition, there is an interesting conglomerate layer and you will have an opportunity to decide what sort of mass wasting event might have caused its deposition. I have outlined a series of short stops (see below) that will guide you in this quest.

    Figure 1. Summit of Sentinel Peak (A-Mountain). There are beautiful views of the city from the top!

    Maps and Publications

    Before you begin your virtual field trip, it will be a good idea to do a little background research on Sentinel Peak, and on the Tucson Mountains, in general. You will include this information in your final write-up, so spend some time reading about its geologic history. It is recommended to review the following documents to start your research:

    Map of bedrock geology of Sentinel Peak and Tumamoc Hill (courtesy of AZGS)

    Field Guide to “A” Mountain and Surrounding Region (courtesy of AZGS)

    There is also a ArcGIS Story Map that you can review. This application will help you see where all the stops are on map imagery. Included is some of the text from this document so you will have descriptions of each rock formation to accompany your virtual field trip.

    What you need

    You will need the following items when you virtually visit Sentinel Peak:

    1. AZGS bedrock geology map
    2. Table 1
    3. Field Guide to “A” Mountain and Surrounding Region


    StoryMap Stop # 1

    Consult the geologic map on StoryMap.

    StoryMap Stop # 2

    Location: Stream channel (a dry wash) to the west of the parking lot. Note the caliche in the eroded stream bank. Caliche, also known as hardpan, is a calcium carbonate cement that forms within a few feet of the surface.

    Figure 2. Caliche in stream channel at StoryMap #2. Look for white, highly weathered, crumbly material on the bank.

    StoryMap Stop # 3

    Location: Approximately 0.4 miles from Stop 1. Observe the dark, almost black, rocks scattered along the ground. This is an igneous rock called the basaltic andesite (Tb1 on the geologic map). If you crack open these rocks with a hammer, you would find a much lighter greyish color inside of them. The black on the outside is more of a varnish due to exposure in the desert environment.

    • Describe this rock according to the characteristics listed in Table 1 for igneous rocks.
    Figure 3. Basaltic andesite along trailhead at StoryMap Stop #3. Note the vesicular nature of the rock.

    StoryMap Stops # 4 and 5

    Location: Approximately 70 paces up the one-way road (to the southeast). On the left side of the road is an outcrop of conglomerate (Tc on the geologic map), which contains small pebbles of vesicular basalt in a matrix of silt, sand, and ash. This conglomerate may have been deposited by a mudflow or debris flow during an intense rainfall that resulted in a wet slurry of sand, mud, and volcanic clasts that it picked up along its path.

    Figure 4. Volcanic conglomerate on the left side of the road, just after the fork.
    • Describe this rock according to the characteristics listed in Table 1 for sedimentary rocks.
    • In the questions at the end of this field activity, you are asked to take a closer look at the clast size and composition of the conglomerate.
    • Please refer to Question 3 at end of this activity to guide you in your assessment of this formation while you are visiting this stop.
    Figure 5. Volcanic clasts of vesicular basalt populate the conglomerate. The tip of the pen points to a basalt clast.

    StoryMap Stops # 6 and 7

    Location: Approximately 0.2 miles further up the road. On the left (north/northwest) side of the road you will find a tan- to white-colored igneous formation called tuff. This is the Tumamoc Tuff (Tt on the geologic map). After some observation, you may find fragments of pumice, and possibly some biotite and feldspar in the tuff. Can you spot these?

    Figure 6. Tan material is Tumamoc tuff.
    • Describe this rock according to the characteristics listed in Table 1 for igneous rocks.

    Location: Approximately 10 paces up the road from where you observed the Tumamoc Tuff at Stop 4. Look for the contact between a pink- to grey-colored tuff and the tan tuff underneath. The pink/grey tuff is also part of the Tumamoc Tuff formation, but it is called a different member of the Tumamoc Tuff.

    Figure 7. Pumice fragment embedded in the Tumamoc tuff.
    • Describe this rock according to the characteristics listed in Table 1 for igneous rocks.

    StoryMap Stops # 8 and 9

    Location: Approximately 0.3 miles up the road to the parking area on the east side of the mountain, overlooking the city, and just under the “A” near the top.

    Figure 8. Tumamoc basaltic andesite under the “A” near the top of the mountain.

    Observe the rock formation at road level, under the A. This is basaltic andesite again, but part of a different formation called Tumamoc Basaltic Andesite (Tb2 on the geologic map). How is it different from the Tb1 formation you observed at Stop 2?

    • Describe this rock according to the characteristics listed in Table 1 for igneous rocks.
    Figure 9. Note the vasicular nature of the rock, similar to the basaltic andesite at StoryMap Stop #3.

    StoryMap Stop # 10

    (Optional, for the purpose of observation only)

    Location: Approximately 0.2 miles to the parking area. Here you will find another outcrop of Tumamoc Basaltic Andesite that is visible from the parking area, near the top of the peak. Look for the dark red-brown color rock near the top.

    Now you are finished with your field work!

    Figure 10: Though it is difficult to see in this photo, near the top is another outcrop of Tumamoc basaltic andesite, with the trailhead for the Sentinel Summit in the foreground.


    You will need to submit the following to the Assignments tool:

    1. Table 1 completed (this can be submitted as a separate document, and can be scanned in if you have hand-written your observations);
    2. A short paragraph (3 – 5 sentences) addressing the origin of clasts in the conglomerate layer at Stop 3. Where did these materials come from if they were deposited by a mudflow or debris flow? How might you determine the original direction of flow in the conglomerate? What visual cues can you find that might help you determine this?
    3. A short paragraph (3 – 4 sentences) on whether Sentinel Peak complies with the Law of Superposition, and why or why not;
    4. A short paragraph (3 – 4 sentences) on whether Sentinel Peak is considered a volcano. What is your final assessment?

    Please submit your observations to the Assignment 14 folder. You will need to upload two separate files— one with Table 1, and one with your written assessment addressing items 1 – 5 above.

    Grading Rubric

    20 points: Table was complete, photos were included, and paragraphs addressed the questions thoroughly.

    17 points: Table was mostly complete, photos were included, and paragraphs addressed questions thoroughly but may have been a bit too brief.

    15 points: Table was incomplete and a couple photos were missing. Paragraphs addressed questions but were too brief.

    10 points: Only very partial information was given. Photos were not included, and paragraphs missing or too brief.

    0 points: Assignment was not turned in.