Quaternary basaltic volcanic fields of the American Southwest

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.