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A slab fragment wedged under Tokyo and its tectonic and seismic implications
The Philippine Sea and Pacific plate slabs both subduct beneath Tokyo, and so their configuration and seismic potential have been subject to intensive study. Previous work suggests that the Philippine Sea slab extends up to 100 km northwest of Tokyo and subducts to a depth of 90 km beneath the Kanto basin, where it is folded against the underlying Pacific slab. Here we evaluate seismic data in thr
Authors
Shinji Toda, Ross Stein, Stephen H. Kirby, S.B. Bozkurt
Timing of degassing and plagioclase growth in lavas erupted from Mount St. Helens, 2004-2005, from 210Po-210Pb-226Ra disequilibria
Disequilibrium between 210Po, 210Pb, and 226Ra was
measured on rocks and plagioclase mineral separates erupted
during the first year of the ongoing eruption of Mount St.
Helens. The purpose of this study was to monitor the volatile
fluxing and crystal growth that occurred in the weeks, years,
and decades leading up to eruption. Whole-rock samples were
leached in dilute HCl to remove 210Po pr
Authors
Mark K. Reagan, Kari M. Cooper, John S. Pallister, Carl R. Thornber, Matthew Wortel
Constraints and conundrums resulting from ground-deformation measurements made during the 2004-2005 dome-building eruption of Mount St. Helens, Washington
A prolonged period of dome growth at Mount St. Helens
starting in September-October 2004 provides an opportunity
to study how the volcano deforms before, during, and after an
eruption by using modern instruments and techniques, such as
global positioning system (GPS) receivers and interferometric
synthetic aperture radar (InSAR), together with more traditional ones, including tiltmeters, tria
Authors
Daniel Dzurisin, Michael Lisowski, Michael P. Poland, David R. Sherrod, Richard G. LaHusen
238U-230Th-226Ra Disequilibria in Dacite and Plagioclase from the 2004–2005 Eruption of Mount St. Helens
Uranium-series disequilibria in whole-rock samples and
mineral separates provide unique insights into the time scales
and processes of magma mixing, storage, and crystallization.
We present 238U-
230Th-226Ra data for whole-rock dacite and
gouge samples and for plagioclase separated from two dacite
samples, all erupted from Mount St. Helens between October
2004 and April 2005. We also present
Authors
Kari M. Cooper, Carrie T. Donnelly
Trace element and Pb isotope composition of plagioclase from dome samples from the 2004-2005 eruption of Mount St. Helens, Washington
We report the results of in-situ laser ablation ICP–MS
analyses of anorthite content, trace-element (Li, Ti, Sr, Ba, La,
Pr, Ce, Nd, Eu, Pb) concentrations, and Pb-isotope compositions
in plagioclase from eight dome-dacite samples collected from
the 2004-5 eruption of Mount St. Helens and, for comparison,
from three dome samples from 1981-85. For 2004-5 samples,
plagioclase phenocrysts range
Authors
Adam J. R. Kent, Michael C. Rowe, Carl R. Thornber, John S. Pallister
Constraints on the size, overpressure, and volatile content of the Mount St. Helens magma system from geodetic and dome-growth measurements during the 2004-2006+ eruption
During the ongoing eruption at Mount St. Helens, Washington, lava has extruded continuously at a rate that decreased
from ~7-9 m3
/s in October 2004 to 1-2 m3
/s by December
2005. The volume loss in the magma reservoir estimated from
the geodetic data, 1.6-2.7×10
7
m3
, is only a few tens of percent
of the 7.5×10
7
m3
volume that had erupted by the end of 2005.
Authors
Larry G. Mastin, Evelyn Roeloffs, Nick M. Beeler, James E. Quick
Plagioclase populations and zoning in dacite of the 2004-2005 Mount St. Helens eruption: Constraints for magma origin and dynamics
We investigated plagioclase phenocrysts in dacite of the
2004-5 eruption of Mount St. Helens to gain insights into the
magmatic processes of the current eruption, which is characterized by prolonged, nearly solid-state extrusion, low gas
emission, and shallow seismicity. In addition, we investigated
plagioclase of 1980-86 dacite.
Light and Nomarski microscopy were used to texturally
character
Authors
Martin J. Streck, Cindy A. Broderick, Carl R. Thronber, Michael A. Clynne, John S. Pallister
Effects of lava-dome growth on the crater glacier of Mount St. Helens, Washington
The process of lava-dome emplacement through a glacier
was observed for the first time as the 2004-6 eruption of
Mount St. Helens proceeded. The glacier that had grown in the
crater since the cataclysmic 1980 eruption was split in two by
the new lava dome. The two parts of the glacier were successively squeezed against the crater wall. Photography, photogrammetry, and geodetic measurements doc
Authors
Joseph S. Walder, Steve P. Schilling, James W. Vallance, Richard G. LaHusen
Dynamics of seismogenic volcanic extrusion resisted by a solid surface plug, Mount St. Helens, 2004-2005
The 2004-5 eruption of Mount St. Helens exhibited
sustained, near-equilibrium behavior characterized by nearly
steady extrusion of a solid dacite plug and nearly periodic
occurrence of shallow earthquakes. Diverse data support the
hypothesis that these earthquakes resulted from stick-slip
motion along the margins of the plug as it was forced incrementally upward by ascending, solidifying, gas
Authors
Richard M. Iverson
Extrusion rate of the Mount St. Helens lava dome estimated from terrestrial imagery, November 2004-December 2005
Oblique, terrestrial imagery from a single, fixed-position
camera was used to estimate linear extrusion rates during
sustained exogenous growth of the Mount St. Helens lava
dome from November 2004 through December 2005. During
that 14-month period, extrusion rates declined logarithmically
from about 8-10 m/d to about 2 m/d. The overall ebbing of
effusive output was punctuated, however, by ep
Authors
Jon J. Major, Cole G. Kingsbury, Michael P. Poland, Richard G. LaHusen
Evolving magma storage conditions beneath Mount St. Helens inferred from chemical variations in melt inclusions from the 1980-1986 and current (2004-2006) eruptions
Major element, trace element, and volatile concentrations in 187 glassy melt inclusions and 25 groundmass glasses from the 1980-86 eruption of Mount St. Helens are presented, together with 103 analyses of touching FE-Ti oxide pairs from the same samples. These data are used to evaluate the temporal evolution of the magmatic plumbing system beneath the volcano during 1980-86 and so provide a frame
Authors
Jon Blundy, Katharine V. Cashman, Kim Berlo
Frictional properties of the Mount St. Helens gouge
Frictional properties of gouge bounding the solid dacite
plug that extruded at Mount St. Helens during 2004 and 2005
may have caused stick-slip upward motion of the plug and
associated seismicity. Laboratory experiments were performed
with a ring-shear device to test the dependence of the peak
and steady-state frictional strength of the gouge on shearing rate and hold time. A remolded gouge s
Authors
Peter L. Moore, Neal R. Iverson, Richard M. Iverson