SHIPROCK NATIONAL LANDMARK
(and other Volcanics of the Navajo Volcanic Field)
FIELD TRIP STOP – VOLCANIC NECK WITH RADIATING DIKES OF MAGMA (OLIGOCENE)
LOCATION: Shiprock is located in far northwest New Mexico, just west of Highway 491 and about 5 miles south of the town of Shiprock.
GEOLOGIC FEATURES: Erosional remnant of a Volcanic Neck and Dikes radiating from the neck (throat) and composed of Lamprophyre (var. Minette)
DESCRIPTION: Shiprock is a 27 myo volcanic remnant (Tertiary: Oligocene) whose neck is composed of a fractured volcanic breccia with several volcanic dikes radiating from the neck. It rises over 1500 ft above ground level. The rock composing the volcano has been identified as Lamprophyre (var. Minette). This is a porphyritic alkaline igneous rock dominated by biotite and/or amphibole with potassium feldspar as a groundmass with lesser amounts of pyroxene. Small phenocrysts are composed of mica and amphibole whose individual crystals readily reflect light (glitter). The eruption is thought to have been Phreatomagmatic (due to steam pressure created by ground water heated by the magma).
Several similar erosional remnants dot the land (e.g. Bennett Peak) and make-up the northeast part of the Navajo Volcanic Field. Agathia Peak (El Capitan) is another prominent Volcanic Plug within the Navajo Volcanic Field found west of Shiprock with a similar composition. Surrounding the Shiprock Volcano, the exposed host rock is the Cretaceous Mancos Shale. (See Photos below of Bennett and Agathia Peaks.)
STUDENT QUESTIONS:
(1) What is a Dike?
(2) The Mancos Shale is referred to as a Host Rock. What is meant by that?
(3) What chemical elements are considered to make a magma Alkaline?
(4) Define a Volcanic Neck. How does it differ from a Vent and Pipe.
(5) Why might the flanks of the volcano have disappeared while the Neck and Dikes still stand tall?
(6) CHALLENGE: What is a Porphyritic Texture? A Phenocryst?
(7) CHALLENGE: If crystal size in an igneous rock is largely due to the rate of cooling, what geologic events can account for a Porphyritic Texture?
SELECTED REFERENCES:
-St. John, J. Ship Rock. Accessed on Feb. 25, 2020 from: jsjgeology.net/Navajo-Volcanic-Field.htm
-New Mexico Bureau of Geology and Mineral Resources. The Ship Rock Landform. Accessed on Feb. 25, 2020: geoinfo.nmt.edu/tour/landmarks/shiprock/home.html
PHOTOS:
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Figure 1 - Upon turning west off of Hwy 491, we see the Shiprock volcanic neck to the right and the southern trending dike to the left.
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Figure 2 - The vertical dike composed of lamprophyre igneous rock stands tall as it radiates away from the Shiprock Volcanic Neck
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Figure 3 - Access to the Volcanic Neck is on private property making this the closest approach.
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Figure 4 - The Dike and Volcanic Neck protrude through the Cretaceous Mancos Shale.
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Figure 5 - A close-up view of the Volcanic Neck composed of a volcanic breccia.
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Figure 6 - Looking on edge at part of the vertical Volcanic Dike as it merges into the Neck to the north.
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Figure 7 - The western facing side of the Dike.
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Figure 8 - A view perpendicular to the southern dike wall.
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Figure 9 - A close-up view of the dike wall. The distance from left to right is 5 inches.
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Figure 10 - South of Shiprock are a number of Volcanic Necks including Bennett Peak.
Figure 11 - South of Monument Valley (along Route 163) and west of Shiprock is the eroded volcanic plug Agathia Peak (El Capitan), the second most prominent diatreme (volcanic pipe formed by gaseous explosion) next to Shiprock. It is also a part of the Navajo Volcanic Field.