DEVILS TOWER NATIONAL MONUMENT – An Igneous Intrusion
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FIELD TRIP STOP– DEVILS TOWER NATIONAL MONUMENT (An eroded remnant of a Laccolith, Stock, or Volcanic Neck)
LOCATION: From Route-24 in Northeast Wyoming, take Route-110 westbound into the Monument.
GEOLOGIC FEATURES: Butte; Laccolith; Stock; Volcanic Plug or Neck; Columnar Jointing; Phonolite; Porphyry.
DESCRIPTION: Devils Tower, rising 867 ft. from its base, is a 50 million-year-old igneous intrusion composed of a gray to greenish gray rock with large crystals of white feldspar called a Phonolite Porphyry.
The interpretations of its emplacement are varied. Some believe it is the result of an intrusion into sedimentary rocks (laccolith or stock) while others interpret it as a volcanic plug (neck). In either case the exposure of Devils Tower is the result of the differential erosion of surrounding softer sediments.
Particularly characteristic of Devils Tower are the tall vertical columns that are commonly 6-sided, but can be 4 to 7-sided. These columns are the result of cracking from cooling and shrinking of the magma producing Columnar Jointing.
Devils Tower is perhaps best known for its appearance in the movie Close Encounters of the Third Kind as the area where the alien “mothership” lands.
STUDENT QUESTIONS:
(1) What is the cause of Columnar Jointing?
(2) Name a place in California and a place in Ireland where columnar jointing is prominent.
(3) Compare a Laccolith with a Stock and a Volcanic Neck.
(4) How is Differential Erosion exhibited at Devils Tower?
(5) How does a Porphyritic texture in an Igneous rock differ from an Aphanitic (fine-grained) or Phaneritic (Coarse-grained) Texture in an Igneous rock?
(6) CHALLENGE: How might an igneous rock with a porphyritic texture be formed?
(7) CHALLENGE: What mountain building event occurring in the western U.S, might the formation of Devils Tower be associated?
SELECTED REFERENCES:
USGS. Geology and Ecology of National Parks, Devils Tower National Monument. Accessed Jan. 2, 2020: https://www.usgs.gov/science-support/osqi/yes/national-parks/devils-tower-national-monument
PHOTOS:
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Figure 1 - Devils Tower is an igneous intrusion, possible a stock, laccolith, or volcanic neck. Characteristic of Devils Tower is the Columnar Jointing producing polygonal vertical columns.
Figure 2 - A close-up view of the Columnar Jointing.
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Figure 3 - Devils Tower, which formed about 50 mya (Tertiary Period), rises 867 ft. from its base. Relatively recent erosion has wiped away the surrounding sediments exposing the more resistant igneous rock.
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Figure 4 - Prairie Dog colonies are found on the Monument grounds