USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Observations of the Eruptions of July 22 and August 7, 1980, at Mount St. Helens, Washington
--
Hoblitt, Richard P., 1986,
Observations of the Eruptions of July 22 and August 7, 1980, at Mount St. Helens, Washington:
U.S. Geological Survey Professional Paper 1335, 44p.
Abstract
The explosive eruptions of July 22 and August 7, 1980, at Mount
St. Helens, Wash., both included multiple eruptive pulses. The beginnings of three of the pulses-two on July 22 and one on August
7-were witnessed and photographed. Each of these three began with
a fountain of gases and pyroclasts that collapsed around the vent and
generated a pyroclastic density flow. Significant vertical-eruption
columns developed only after the density flows were generated. This
behavior is attributable to either an increase in the gas content of the
eruption jet or a decrease in vent radius with time. An increase in the
gas content may have occurred as the vent was cleared (by expulsion
of a plug of pyroclasts) or as the eruption began to tap deeper, gas-rich magma after first expelling the upper, gas-depleted part of the
magma body. An effective decrease of the vent radius with time may
have occurred as the eruption originated from progressively deeper
levels in the vent. All of these processes-vent clearing; tapping of
deeper, gas-rich magma; and effective decrease in vent
radius-probably operated to some extent. A "relief-valve"
mechanism is proposed here to account for the occurrence of multiple
eruptive pulses. This mechanism requires that the conduit above the
magma body be filled with a bed of pyroclasts, and that the vesiculation rate in the magma body be inadequate to sustain continuous
eruption. During a repose interval, vesiculation of the magma body
would cause gas to flow upward through the bed of pyroclasts. If the
rate at which the magma produced gas exceeded the rate at which gas
escaped to the atmosphere, the vertical pressure difference across the
bed of pyroclastic debris would increase, as would the gas-flow rate.
Eventually a gas-flow rate would be achieved that would suddenly
diminish the ability of the bed to maintain a pressure difference between the magma body and the atmosphere. The bed of pyroclasts
would then be expelled (that is, the relief valve would open) and an
eruption would commence. During the eruption, gas would be lost
faster than it could be replaced by vesiculation, so the gas-flow rate in
the conduit would decrease. Eventually the gas-flow rate would
decrease to a value that would be inadequate to expel pyroclasts, so
the conduit would again become choked with pyroclasts (that is, the
relief valve would close). Another period of repose would commence.
The eruption/repose sequence would be repeated until gas-production
rates were inadequate to reopen the valve, either because the depth of
the pyroclast bed had become too great, the volatile content of the
magma had become too low, or the magma had been expended.
A timed sequence of photographs of a pyroclastic density flow on
August 7 indicates that, in general, the velocity of the flow front was
determined by the underlying topography. Observations and details
of the velocity/topography relationship suggest that both pyroclastic
flows and pyroclastic surges fonned. The following mechanism is consistent with the data. During initial fountain collapse and when the
flow passed over steep, irregular terrain, a highly inflated suspension
of gases and pyroclasts fonned. In this suspension, the pyroclasts
underwent rapid differential settling according to size and density; a
relatively low-concentration, fine-grained upper phase fonned over a
relatively high-concentration coarse-grained phase. The low-particle-concentration phase (the pyroclastic surge) was subject to lower internal friction than the basal high-concentration phase (the pyroclastic
flow), and so accelerated away from it. The surge advanced until it
had deposited so much of its solid fraction that its net density became
less than that of the ambient air. At this point it rose convectively off
the ground, quickly decelerated, and was overtaken by the
pyroclastic flow.
The behavior of the flow of August 7 suggests that a pyroclastic
density flow probably expands through the ingestion of air wherever
it passes over surfaces whose relief is a significant fraction of the flow
thickness. Thus, a pyroclastic flow may spawn one or more pyroclast
surges at locations remote from the source volcano. The ingestion of
air by a pyroclastic surge would increase the time that particles would
be held in suspension and, thus, extend the lifetime and length of the
pyroclastic surge.
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02/02/06, Lyn Topinka