The Hopi Buttes volcanic field consist of about 300 discrete vents; mostly maars and diatremes. Volcanic features and lava flows are heavily eroded. The field was largely active during the Miocene from 8.5 to 6 Ma, but the most recent activity occurred in Pliocene about 4 Ma. The field covers about 2,500 sq kilometers, about 1/2 that of the nearby San Francisco volcanic field to the west. Interestingly, volcanic alignments so common in the San Francisco field are largely absent in the Hopi Butte field. The vent distribution looks more like a shotgun blast. (Image from Google Earth)
Lavas of the Hopi Buttes - basanites, nephelinites, and monchiquites (all three are silica undersaturated with high alkali values basalts) - are markedly different in chemical composition than other volcanic field of Arizona.
See Mallory Zelawski's Arizona Geology article, 'The Hopi Buttes Volcanic Field', for details and some field photos: http://www.azgs.az.gov/arizona_geology/spring10/article_earthscience%20.... .
