Quaternary basaltic volcanic fields of the American Southwest


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    The southwestern United States contains numerous monogenetic basaltic
    volcanoes distributed in intraplate volcanic fields. We review, on a regional
    scale, our current understanding of the Quaternary basalts with a focus on
    aspects pertinent to hazard assessment, such as physical volcanology and
    geochronology, while also summarizing the several petrogenetic concep-
    tual models that have been proposed for the range of local tectonic settings
    in the region. We count 2229 volcanoes in 37 volcanic fields (including the
    Pinacate volcanic field, which is mostly in northern Sonora, Mexico). Volca-
    nic landforms are dominantly scoria cones and ramparts with attendant lava
    fields that have a spectrum of ‘a’ ā and blocky to p ā hoehoe morphologies,
    while a small percentage of the volcanoes are maars and tuff cones. Explo-
    sive eruption styles that were driven mainly by magmatic volatiles, where
    they have been studied in detail, included Hawaiian, Strombolian, violent
    Strombolian, and sub-Plinian activity. The latter two have resulted in sub-
    stantial fallout deposits that can be traced tens of kilometers from source
    vents. Phreatomagmatic styles have produced pyroclastic current (mainly
    pyroclastic surges), ballistic, and fallout deposits. These eruption styles pose
    hazards to humans when they occur in populated areas and to air travel and
    regional infrastructure even in sparsely populated areas. All but one of the
    major volcanic fields (fields that contain ~100 or more Quaternary volcanoes)
    together form a northwest-southeast–trending band, which we suggest may
    reflect an influence of plate-boundary-related shearing on melt segregation in
    the upper mantle along with other factors; this view is consistent with recent
    global positioning system (GPS) and structural geologic data indicating the
    influence of dextral motion along the North America-Pacific plate boundary
    deep inside the Southwest. Of the 2229 Quaternary volcanoes identified, ~548
    (25%) have been dated, and only ~15% have been dated with methods such
    as 40 Ar/ 39 Ar and cosmogenic surface exposure methods that are considered
    optimal for young basalts. Acknowledging the large uncertainty due to the
    poor geochronological data coverage, we use a simple Poisson model to pro-
    vide a first-order estimate of recurrence rates of monogenetic volcanoes on
    the scale of the region as a whole; recurrence rates using our compiled age
    data set range from 3.74 × 10 −4 yr −1 to 8.63 × 10 −4 yr −1 . These values are only
    based on dated and mapped volcanoes, respectively, and do not account for
    undated and buried volcanoes or other uncertainties in the volcano count.
    The time between monogenetic eruptions in the Southwest is similar to the
    repose times of some polygenetic volcanoes, which suggests that the regional
    hazard is potentially commensurate with the hazard from a reawakening
    stratovolcano such as those in the Cascade Range. Notable in our review is
    that only a few volcanoes have been the subject of physical volcanological
    characterization, interpretation, and detailed petrologic study that may elucidate factors such as magma generation, ascent (including time scales), and
    controls on eruption style.
    Original languageEnglish
    Article number2144-2171
    Pages (from-to)1-28
    Issue number6
    Early online date2 Nov 2021
    Publication statusPublished - 2 Nov 2021


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