Phreatomagmatic vs magmatic eruptive styles in maar-diatremes: a case study at Twin Peaks, Hopi Buttes volcanic field, Navajo Nation, Arizona

The Hopi Buttes volcanic field (HBVF) is located on the Colorado Plateau, Northern Arizona. In this Miocene volcanic field, the erosion level increases southward, allowing the study of maar-diatreme volcanoes from top (posteruptive crater infill and ejecta ring) to bottom (lower diatreme). The Twin Peaks volcanic complex consists mostly of two hills (North Peak and South Peak) with thick lavas at their summits and pyroclastic rocks underneath. In the HBVF, such volcanic remnants have received little scientific attention so far, despite their relative abundance. Our field observations allow us to interpret the North and South Peaks as remnants of two maar-diatreme volcanoes which evolved into lava lakes filling the craters. Within the complex, we distinguish four volcanic units (from unit 1 at the bottom to unit 4 at the top). On the basis of the field description of the deposits and the componentry measurements, we suggest that unit 1 is phreatomagmatic, unit 2 is phreato-strombolian (with mixed phreatomagmatic and strombolian characteristics), unit 3a is phreato-hawaiian (with mixed phreatomagmatic and hawaiian characteristics), unit 3b is hawaiian (formed by lava fountains) and unit 4 consists of lava lakes filling the maar craters. There is therefore a progressive evolution from a purely phreatomagmatic eruptive style, which excavated the craters and diatremes and partly filled them, to magmatic explosive to nonexplosive eruptive styles, which filled the maar craters up to the pre-eruptive surface. We discuss traditional criteria used to distinguish phreatomagmatic from magmatic eruptive styles in ultramafic to mafic maar-diatreme volcanoes.