Geology 101 Pima Community College Name______

Geology 101 Pima Community College Name______

Dr. Jan Rasmussen ate______

Geology of “A”Mountain (SentinelPeak)

“A” Mountain Field Trip Outline

This lab exercise involves taking a geologic field trip to “A”Mountain, or SentinelPeak, on Tucson's west side. The objective of the geologic field trip is to discover if “A” Mountain (Sentinel Peak) is a volcano or and erosional remnant of volcanic rocks. For additional information, go to the Arizona Geological Survey website and download DGM-29 (Bedrock geologic map of Sentinel Peak (A-Mountain) and Tumamoc Hill, Pima County, Arizona) and Open File Report 01-07, Field guide to “A” Mountain and description of surrounding region, Pima County, Arizona.

“A” Mountain Field Trip Stops – Describe these in your report.

Stop 1–If there is too much traffic, this stop will be omitted

Location: Mission road at base of “A”Mountain on southwest. Parking area on east side of road.

Notes: Safety meeting: Potential traffic dangers. You should take care to watch for traffic. Skip this stop when doing the trip on your own. Road cut geology.Describe nature of rock, color, rock type, composition, texture, environment of deposition, origin.

Rock type: Porphyritic andesite, rock name Turkey Track andesite, age 28.6 Ma (million years).

Stop 2– Location: Parking area just after entrance to “A” MountainPark.

Notes: Walk down to west to stream channel to look at channel, caliche in eroded stream bank. Describe origin of caliche.Describe nature of rock, color, rock type, composition, texture, environment of deposition, origin.What is rock type of boulders in parking area? Describe nature of rock, color, rock type, composition, texture, environment of deposition, origin. Was the rock deposited from area or from outside area?

Stop 3 – Location: Along side road just after “one-way” traffic starts.

Notes: On west/south side of road, note the black rock and describe it. This is Tb1 - A Mountain basaltic andesite (27.6 Ma).

Then walk across road to outcrop of conglomerate – small pebbles of rounded cinders (vesicular basalt) in matrix of silt, sand, ash. This Tc conglomerate is a mudflow/debris flow, matrix supported, volcanic clasts (particles). Describe nature of rock, color, rock type, composition, texture, environment of deposition, origin.

Stop 4 –Location: Further up road at contact of tan tuff with agglomerate.

Notes: Notice nature of contact – eroded, baked, type of material at contact, etc. Describe tan to white tuff (Tumamoc tuff (27.4 Ma)) – note pumice fragments in tuff, black mica (biotite) flakes, crystals – feldspar cleavage faces. Describe nature of rock, color, rock type, composition, texture, environment of deposition, origin.

Stop 5 – Location:Slightly further up road at contact of pink to gray tuff over tan tuff.

Notes: This is also called Tumamoc tuff (Tt), with(26.4 Ma). Describe nature of contact. Describe nature of rock, color, rock type, composition, texture, environment of deposition, origin.

Stop 6 –Location:At parking area at east side of “A” Mountainunder the “A.”

Notes: Look at the monument. What is the white stuff? Caliche or whitewash?Find a rock that is in place and not disturbed by man. Describe nature of rock, color, rock type, composition, texture, environment of deposition, origin.This is Tumamoc basaltic andesite (23.7 Ma).

Discuss surrounding mountains and rivers. Label a map (use matching to assign names to mountains and rivers on a map. Rincon Mountains, Agua Caliente Hills, Redington Pass, Santa Catalina Mountains to East and North; Santa Rita Mountains and Sierrita Mountains to the South; Tortilla and Tortolita Mountains to North; Picacho Peak and Picacho Mountains to the Northeast; Tucson Mountains to Northwest and West.Discuss basin formation, erosion of mountains, differences between Tucson Mountains and Santa Catalina Mountains and effect on water supply, farming.

Stop 7 – Location: At parking area around top.

Notes: Walk up to outcrop and discuss flattened vesicles, rock composition, etc. Describe nature of rock, color, rock type, composition, texture, environment of deposition, origin.

From parking area near cars, discuss geology of Tucson Mountains – Black Mountain and Martinez Hill near San Xavier Mission to the South, Cat Mountain Rhyolite (74 Ma) to the west and at Gates Pass, Safford Peak (Sombrero Peak informally) as possible source of ash for tan tuff and pink tuff.Discuss crater – quarry for foundations and rock walls in homes built around 1920 around the University of Arizona.Discuss Basin and Range basin forming event, complexity of rock types and structures in mountains surrounding Tucson.

Color and label the geologic map

Mountain Ranges surrounding Tucson-

North:

East:

South:

West:

Rivers in the Tucson valley-

Name of river at the base of A Mountain:

Location:

Direction of flow:

Inlet:

Outlet:

Name or river at the base of the Rincon Mountains:

Location:

Direction of flow:

Inlet:

Outlet:

Name of river at the base of the Santa Catalina Mountains:

Location:

Direction of flow:

Inlet:

Outlet:

TucsonBasin

Composition:

Depth:

Caliche

Composition:

Source

Method of transportation

Method of deposition

Extent:

“A”MountainRock Sequence

Summarize the Geology of “A”Mountain

Conclusions concerning the origin of “A” Mountain as a volcano and evidences for this.

Is “A” Mountain a volcano? Or is it the erosional remnant of lava flows and ash falls?

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