Aerial view of Coliseum maar. The maar is about 2,600 feet (800 m) in diameter. The inward dip of the beds and circular shape of the maar resemble the Roman Coliseum. Photograph by George Ulrich, U.S. Geological Survey.
The maars are volcanic cones that form from phreatic explosions near the surface. The diatremes are subsurface pipes that fed the maars and were filled by volcanic material at the time of the eruption. They are now exposed because of lowering of the land surface by erosion. Most of the maars and diatremes at Hopi Buttes formed between 8.5 and 6 million years ago. The youngest volcanoes are 4.2 million years ago.
Two models have been proposed for the origin of the maars and diatremes. One model suggest that they formed from the rapid unmixing of gas from the magma as it ascended through the crust. The other model proposes that the maars and diatremes result from phreatic explosions caused by magma coming in contact with water saturated rock under a lake.
The tuff beds and lake deposits that make the maar dip about 35 degrees towards the center of the Coliseum. The dips may reflect the original deposits of the material or sagging of the center of the maar due to magma withdraw. Photo by Steve Mattox, 1989.
Gregory, H.E., 1917, Geology of the Navajo country; a reconnaissance of parts of Arizona, New Mexico, and Utah: U.S. Geological Survey, Professional Paper 93: 92-103.
Lorenz, V., 1986, On the growth of maars and diatremes and its relevance to the formation of tuff rings: Bulletin of Volcanology, v. 48, p. 265-274.
Shafiqullah, M., and Damon, P.E., 1985, Geochronology of the Hopi Buttes volcanic field, northeastern Arizona (abs); in Abstracts of the Symposium on Southwest Geology and Paleontology: Museum of Northern Arizona, Flagstaff, Arizona, p. 9.
Shoemaker, E.M., Roach, C.H., and Byers, F.M., 1962, Diatremes and uranium deposits in the Hopi Buttes, Arizona; in Petrologic studies - A volume in honor of A.F. Buddington: Geological Society of America, New York, p. 327-355.
Sutton, R.L., 1974, The geology of Hopi Buttes, Arizona; in Geology of northern Arizona with notes on archaeology and paleoclimate; Part II, Area studies and field guides; Hopi Buttes - Navajo Buttes area: Geological Society of America, Rocky Mountain Section, Guidebook 27: p. 647-671.
Ulrich, G.E., Condit, C.D., Wenrich, K.J., Wolfe, W.E., Holm, R.F., Nealey, L.D., Conway, F.M., Aubele, J.C., and Crumpler, L.S., 1989, Excursion 5A: Miocene to Holocene volcanism and tectonism of the southern Colorado Plateau, Arizona, in Chapin, C.E., and Zidek, J., eds., Field excursions to volcanic terranes in the western United States, Volume 1: Southern Rocky Mountain region: New Mexico Bureau of Mines & Mineral Resources, p. 1-41.
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