The Hopi Buttes volcanic field comprises ~300 late Miocene volcanic centers distributed over ~ 1800 km2 in northeastern Arizona. Excellent cross-sectional exposures of well-preserved diatremes, vents, and related maar-crater deposits are exposed in the cliffs of the lava-capped mesas and buttes of the area. Vents in the eastern part of the field preserve surficial maar deposits while vents in the western part preserve the sub-volcanic “plumbing” system or diatreme.
The maars formed through explosive interaction of groundwater, liquefied lower Bidahochi sediments, and/or lake water with monchiquitic and nephelinitic magmas. The ratio of water to magma during the eruptions may have controlled the type of landform produced, which includes maars (including tuff rings and exposed diatremes), scoria cones, and lava flows.
The style of eruption ranged from phreatomagmatic to magmatic and the morphologic characteristics of some vents, particularly maars and tuff rings are typical of eruptions that occur in wet, low-lying areas. Clay-rich tephra of the maars indicate explosive interactions of magma with a clay-water slurry, inferring that molten fuel-coolant interactions involving magma and wet-sediment are important in maar and diatreme emplacement.
From Mallory Zelawski's Arizona Geology article, http://azgeology.azgs.arizona.edu/archived_issues/azgs.az.gov/arizona_ge...