ENCHANTED ROCK

FIELD TRIP STOP -- ENCHANTED ROCK STATE NATURAL AREA, TEXAS

(PreCambrian Granite Dome)

 

LOCATION:  In central Texas, off of Route 965 about 15 miles north of Fredericksbury, TX.

GEOLOGIC FEATURES: Exfoliation Dome; Sheeting; Batholith; Jointing

DESCRIPTION:  Enchanted Rock is the largest granite dome in Texas.  It reaches an elevation of 1825 ft., rising 445 ft. above Sandy Creek found at road level.  The dome is just a small part of a huge batholith composed of the 1.1-billion-year-old pink Town Mountain Granite. This batholith represents an intrusive igneous rock that rose through the previously emplaced Packsaddle Schist and Gneiss. The granite is thought to have been created as a result of melting at a once-present subduction zone.

 

After cooling slowly deep underground for over 1000s of years, this granite experienced repeated episodes of uplift and erosion.  Eventually rapid erosion of fractured granite divided the granite bedrock into separate exfoliation domes, exhibiting sheeting (slabs) due to the release in pressure accompanying uplift and erosion.  In addition to sheeting, the dome incorporates xenoliths, pegmatite dikes, and diamond-shaped jointing patterns.

 

STUDENT QUESTIONS:

(1) Part of Enchanted rock exhibits a nonconformity, that is a contact between an igneous or metamorphic rock and sedimentary rock. Explain how this might have occurred.

(2) Describe the role of uplift and erosion on the presence of sheeting.

(3) What is a batholith?

(4) How is it known that the Enchanted Rock granite is 1.1 billion years old?

(5) CHALLENGE: How might xenoliths be incorporated within the Town Mountain Granite?

(6) CHALLENGE: What might be a cause of jointing at Enchanted Rock?

SELETED REFERENCES:

-Peterson, James F. 1984. Enchanted Rock State Natural Area – A Guide to the Landforms. Terra Cognita Press, San Marcos, TX 78666. 56 pp.

 

-Spearing, Darwin. 1991. Roadside Geology of Texas. Mountain Press Publishing Company, Missoula, Montana. 418 pp.

PHOTOS:

Figure 1 - Smooth surfaced Enchanted Rock (left) and angular Turkey Peak (a "Castle Tor") (right). These two features are separated by a fracture zone.

Figure 2 -  The large, smooth exfoliation dome of Enchanted Rock looking toward the northwest.

Figure 3 - Pink granite of Enchanted Rock showing slabs resulting from Sheeting of the Dome.

Figure 4 - Enchanted Rock showing some exfoliation slabs. The base of the dome trending from left to right is part of a Fracture Zone.

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