Ubehebe Craters, northern Death Valley
Mosaic of Ubehebe Crater from parking lot
Google Earth image viewed towards the south of Ubehebe Craters.
Abstract from Crowe and Fisher
Ubehebe craters, Death Valley, California, include over a dozen maar volcanoes formed primarily
by phreatic eruptions of trachybasalt through a thick and permeable fanglomeratic sequence on
the north slope of Tin Mountain.
Tuff derived from Ubehebe Crater, the largest crater in the area, is characteristically thinly
bedded or laminated and was deposited by airfall and base-surge processes. Thick-bedded
deposits showing evidence of mass flow occur where base surges were concentrated within, and
followed gullies which had been carved into the fanglomerate prior to eruption.
Cross-bedded sequences were deposited by base surges that moved radially outward from
Ubehebe Crater. They occur in the form of relatively small and large dunelike structures with
spacing and morphologic features similar to antidunes and migration patterns somewhat similar to
climbing ripples. The largest dunes in the area are composite structures that preserve a sequence
of bed forms deposited in the high flow regime. Deposition apparently began in the antidune
phase of the upper flow regime, progressing in time through sinuous lamination to plane beds as
flow power decreased. Laminations are well developed and bed forms are preserved at each level
within the composite structures because of a high rate of deposition and high sediment cohesion
during flow of the base surges.
Abstract from Sasnett et al., 2012
Phreatomagmatic eruptions occur when rising magma encounters groundwater and/or
surface water, causing a steam explosion and the ejection of country rock and pyroclastic
material. The predominance of this type of activity at the Ubehebe volcanic field in northern
Death Valley, California, is enigmatic owing to the extremely arid climate of the region. A
novel application of 10Be surface exposure dating is presented to determine the timing of
phreatomagmatic eruptions at Ubehebe Crater and to test the idea that volcanism may
relate to a wetter than present hydro-climate. Twelve of the fifteen ages obtained lie
between 0.8 and 2.1 ka, while three samples give older, mid-Holocene ages. The cluster
between 0.8 and 2.1 ka is interpreted as encompassing the interval of volcanic activity
during which Ubehebe Crater was formed. The remaining older ages are inferred to date
eruptions at the older neighboring craters. The main and most recent period of activity
encompasses the Medieval Warm Period, an interval of prolonged drought in the American
southwest, as well as slightly wetter conditions prior to the Medieval Warm Period.
Phreatomagmatic activity under varied hydrologic conditions casts doubt on the idea that
eruptive timing relates to a wetter hydro-climate. Instead, the presence of a relatively
shallow modern water table suggests that sufficient groundwater was generally available
for phreatomagmatic eruptions at the Ubehebe site, in spite of prevailing arid conditions.
This and the youth of the most recent activity suggest that the Ubehebe volcanic field may
constitute a more significant hazard than generally appreciated.
Questions
What are the physical boundary conditions that controlled the development of the craters at
Ubehebe? What is required for them to form?
How many meters of ash were deposited during the eruption of the main crater? Why is
this material so well bedded and banded?
What type of rocks are exposed on most of the crater walls (beneath the ash)? Under what
conditions were they in when the eruptions formed?
Do you see evidence for strain in any of the rocks underlying the ash? If so, what is this
strain formed by?
What is a base surge? What would this eruption have looked like if you have been watching
it from 10 miles away? What would you have seen and heard as it occurred?
How much energy do you think is required to excavate a crater of this size? Would it be
made by: a conventional bomb, an atomic bomb the size used on Hiroshima, a larger 100
kiloton atomic bomb, a large (many megaton) hydrogen bomb?