USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Roles of magma and groundwater in the phreatic eruptions at Inyo Craters,
Long Valley Caldera, California
Mastin, L.G., 1997,
The roles of magma and groundwater in the phreatic eruptions
at Inyo Craters, Long Valley Caldera, California:
Bulletin of Volcanology 53: 579-596.
Abstract
The Inyo Craters (North Inyo Crater and South Inyo Crater), and a third crater,
Summit Crater, are the largest of more than a dozen 650- to 550-yr-BP phreatic craters
that lie in a 1-km-square area at the south end of the Inyo Volcanic Chain, on the west
side of the Long Valley Caldera in eastern California. The three craters are aligned
within a 1-km-long N-S system of fissures and normal faults, and coincide in age with
aligned magmatic vents farther north in the Inyo Volcanic Chain, suggesting that they
were all produced by intrusion of one or more dikes. To study the sequence and
mechanisms of the eruptions, the deposits were mapped, sampled, and compared with
subsurface stratigraphy obtained from the core of a slant hole drilled directly below the
center of South Inyo Crater from the southwest.
The deposits from the two Inyo Craters are fine-grained (median diameter less
than 1 mm), are several meters thick at the crater walls, and cover at most a few km2 of
ground surface. Stratigraphic relationships between the Inyo Craters and Summit Crater
indicate that the eruptions proceeded from north to south, overlapped slightly in time,
and produced indistinctly plane-parallel bedded, poorly-sorted deposits, containing debris
derived primarily from within 450 m of the surface. Debris from the deepest identifiable
unit (whose top is at 450 m depth) is present at the very base of both Inyo Craters
deposits, suggesting that the eruptive vents were open and tapping debris from at least
that depth, probably along preexisting fractures, even at their inception. According to
ballistic studies, the greatest velocity of ejected blocks was on the order of 100 m/s. All
eruptions, particularly the least powerful, selectively removed debris from the finest-
grained, most easily eroded subsurface units. Although juvenile fragments have been
previously identified in these deposits, they are confined primarily to the grain size
fractrion smaller than 0.25 mm dia. and probably did not constitute more than several
percent of the deposit. It is therefore suggested that these juvenile fragments were not
the main source of heat for the eruptions, and that the eruptions were caused either by (1)
heating of water by fragmented magma that was not ejected before the eruption shut off;
(2) slow heating (over months to years) of groundwater in a confined aquifer by
unfragmented magma, followed by a later event (seismic faulting or intrusions) that
breached the confinement; or (3) breach of a pre-existing confined geothermal aquifer.
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05/28/04, Lyn Topinka