The Hopi Buttes volcanic field consists of ~300 late Miocene volcanic centers within ~ 1800 km2 of the field, located in northeastern Arizona (see Figure 1).  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 (See Figure 2).  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. 

Figure 2.
Figure 3.  Figure 4.

The Hopi Buttes volcanic field is one of the few places in the world in which an entire suite of maar crater deposits, tephra aprons, and underlying diatremes is exposed. My research involves describing marginal deposits of the maar craters from the north-central portion of the volcanic field.

Located mostly on Navajo Nation land, the Hopi Buttes occur on the southwestern Colorado Plateau.  Throughout Cenozoic time, the Colorado Plateau has remained a relatively stable tectonic feature.  Lithospheric boundaries of Proterozoic provinces probably lie beneath the Colorado Plateau and possibly beneath the Hopi Buttes volcanic field.  The Hopi Buttes is one of a few volumetrically significant areas of late Tertiary volcanic activity on the Colorado Plateau that produced basanites, nephelinites, and monchiquites (all three are silica undersaturated with high alkali values basalts).

Bidahochi Formation
The Mio-Pliocene Bidahochi Formation (16 – 4 Ma), which includes the Hopi Buttes rocks, covers ~16,000 km2 of the southern part of the Colorado Plateau.  The formation is informally divided into three members; lower, middle, and upper. 

Phreatomagmatism | Phreatomagmatism is any subaerial volcanic activity in which steam explosively results from the interaction between magma or lava and ground water or surface water, including seawater, meteoric water, hydrothermal water, or lake water.  These interactions are called molten fuel-coolant interactions (MFCIs).  The explosive interaction of magma with water occurs due to rapid heating of the coolant (water, impure water, or liquefied sediments) by the fuel (magma).  These eruptions produce maar volcanoes, sensu lato, during monogenetic or single eruptions.  Maars are the second most common volcanic landform after cinder cones.  Maar volcanoes include three types of land forms: maars, narrowly defined, tuff rings, and tuff cones.  Maar (sensu stricto) – steep-sided crater cut below the pre-eruptive surface, often filled with a small lake, surrounded by low-lying outward-dipping ejecta layers (tephra ring) that rapidly decrease in thickness away from the vent rim.  Tuff rings – craters at or below the pre-eruptive surface with both inward- and outward-dipping beds of ejecta.  Tuff cone – small crater above the pre-eruptive surface and surrounded by a steeply-dipping pyroclastic apron.  The conduits that feed maar volcanoes are composed of accidental wall rocks and fragmented magmatic material and are known as diatremes.

The basin (Hopi Lake, from Williams, 1936) into which the volcanoes erupted, was an intermittently inundated playa with fine-grained sediments.  The central area of the basin was commonly covered by water, while the marginal areas were rarely inundated.  Marginal areas of the basin contained sand flats and mud flats that were frequently dry.  The basin was watered by ephemeral streams carrying fine-grained sands and coarse-grained tephra.  Perennial channels only occurred during occasional flooding of the basin.  Evidence for this relatively dry environment includes a lack of soft-sediment deformation of mudrock beneath thick tuff beds and desiccation cracks that occur in several horizons throughout the Hopi Buttes.

My M.S. Research
Working with Dr. Michael Ort, I am describing the marginal deposits of maar craters in order to determine the timing and origin of deformation and how this relates to eruptive processes.  My project involves mapping volcanic rock and vents over 60-km2 of the north-central portion (within the First Flat Mesa 7.5” quadrangle) of the volcanic field. 

Figure 5.

This will provide the location of vents and show which vents provide the best exposure of marginal and proximal deposits, as well as crater infill.  Determination of vent locations will also help to produce a relative chronology of deposition and major eruptive events within different vents in the area (See Figure 5).  Studying proximal and marginal deposits should provide insight into vent processes during eruptions while distal deposits, on the other hand, undergo processes that may remove or filter indicators of vent processes.  Concentrating on the margins of maars and the crater infill will show how water and/or sediments interacted with magma to produce explosive eruptions and how conditions at the conduit-surface interface (i.e., vent) changed throughout the eruption. 

These changes may be able to be inferred by changes in facies laterally and vertically at the margins of the maar.  I will measure stratigraphic sections inside and outside of the craters in order to describe deformational structures (faults, folds, liquefaction of sediments) and depositional structures.  These sections will be correlated and provide magma-water efficiency for the eruption throughout the measured sections.  The relative amount of water to magma will be determined through scanning electron microscope (SEM) analysis of ash particle shapes.  Molten field-coolant interactions (MFCI) experiments produce ash particles with shapes similar to naturally produced ash, and the shapes can provide insight into the relative amount of water to magma within the eruption.  The goal of my study is provide insight into vent processes and how these processes change throughout an eruption.  This is important to understand due to the explosive and violent nature of these types of eruptions and their widespread occurrence.

selected, recommended readings

Dallegge, T.A., Ort, M.H., and McIntosh, W.C., 2003, Mio-Pliocene chronostratigraphy, basin morphology and paleodrainage relations derived from the Bidahochi Formation, Hopi and Navajo Nations, northeastern Arizona: The Mountain Geologist, v. 40, p. 55-82.

Hack, J.T., 1942, Sedimentation and volcanism in the Hopi Buttes, Arizona: Geological Society of America Bulletin, v. 53, p. 335-372.

Ort, M.H., Dallegge, T.A., Vazquez, J.A., and White, J.D.L., 1998, Volcanism and sedimentation in the Mio-Pliocene Bidahochi Formation, Navajo Nation, northeastern Arizona In Duebendorfer, E.M., ed., Geologic excursions in northern and central Arizona: GSA, Field Trip Guidebook, Rocky Mountain Section Meeting, Northern Arizona University, Flagstaff, Arizona, p. 35-57.

Repenning, C.A., and Irwin, J.H., 1954, Bidahochi Formation of Arizona and New Mexico: American Association of Petroleum Geologists Bulletin, v. 38, p. 1821-1826.

White, J.D.L., 1989, Basic elements of maar-crater deposits in the Hopi Buttes volcanic field, northeastern Arizona, USA: Journal of Geology, v. 97, p. 117-125.

White, J.D.L., 1990, Depositional architecture of a maar-pitted playa: Sedimentation in the Hopi Buttes volcanic field, northeastern Arizona, U.S.A.: Sedimentary Geology, v. 67, p. 55-84.

White, J.D.L., 1991, Maar-diatreme phreatomagmatism at Hopi Buttes, Navajo Nation (Arizona), USA: Bulletin of Volcanology, v. 53, p. 239-258.

Williams, H., 1936, Pliocene volcanoes of the Navajo-Hopi country: Bulletin of the Geological Society of America, v. 47, p. 111-172.