Growth of the 2004-2006 lava-dome complex at Mount St. Helens, Washington

The eruption of Mount St. Helens from 2004 to 2006
has comprised extrusion of solid lava spines whose growth
patterns were shaped by a large space south of the 1980-86
dome that was occupied by the unique combination of glacial
ice, concealed subglacial slopes, the crater walls, and relics
of previous spines. The eruption beginning September 2004
can be divided (as of April 2006) into five phases: (1) predome deformation and phreatic activity, (2) initial extrusion
of spines, (3) recumbent spine growth and repeated breakup,
(4) southward extrusion across previous dome debris, and (5)
normal faulting of the phase 4 dome to form a depression, a
shift to westward extrusion and overthrusting of earlier phase
5 products. Overall, steady spine extrusion gradually slowed
from 6 m³/s in November 2004 to 0.6 m³/s in February 2006.
Thermal camera data show that phase 1 activity included
low-temperature thermal features, such as fumaroles, fractures, and ground warming related to rapid uplift, as well as
deformation in the south moat of the crater. The relatively cold
(<160°C) phreatic eruptions of early October heralded activity
at a subglacial vent situated along the south-sloping margin of
the 1980–86 dome. Thermal infrared imagery, documenting
increased heat flow, presaged phase 2 extrusion of the October
11–15, 2004, lava spine. The thermal images of the extruding
spine revealed a hot basal margin and highest temperatures of
600–730°C.
During phase 3, a recumbent whaleback-shaped spine
with a low-temperature shroud of fault gouge and a hot,
U-shaped basal margin extruded. This spine pushed southward
along the bed of the glacier until it encountered the south wall
of the 1980 crater, whereupon it broke up, decoupled, and
regrew. Continued southward growth of the recumbent spine pushed cold deformed rock, hot dome rubble, and glacier
ice eastward at a rate of 2 m/d. In April 2005, breakup of the
whaleback and growth of a lava spine across previous dome
rubble heralded phase 4 spine thrusting over previous spine
remnants. During phase 4, the active spine pushed southward with an increasingly vertical component and increasing
incidence of large rockfalls. In late July, the spine decoupled
from its source, the vent reorganized, and a new spine began
to grow westward at right angles to the previous growth direction, defining phase 5. Dome migration again plowed glacier
ice out of the way at a rate of about 2 m/d, this time westward. In early October, the spine buckled near the vent and
thrust over the previous one. A massive spine monolith had
been constructed by December 2005, and growth of spines
with increasingly steep slopes characterized activity through
April 2006.
The chief near-surface controls on spine extrusion during
2004-6 have been vent location, relict topographic surfaces
from the 1980s, and spine remnants emplaced previously
during the present eruption. In contrast, glacier ice has had
minimal influence on spine growth. Ice as thick as 150 m has
prevented formation of marginal angle-of-repose talus fans
but has not provided sufficient resistance to stop spine growth
or slow it appreciably. Spines initially emerged along a relict
south-facing slope as steep as 40° on the 1980s dome. The
open space of the moat between that dome and the crater walls
permitted initial southward migration of recumbent spines.
An initial spine impinged on the opposing slopes of the crater
and stopped; in contrast, recumbent whaleback spines of phase
3 impinged on opposing walls of the crater at oblique angles
and rotated eastward before breaking up. Once spine remnants
occupied all available open space to the south, spines thrust
over previous remnants. Finally, with south and east portions of the moat filled, spine growth proceeded westward.
Although Crater Glacier had only a small influence on the
growing spines, spine growth affected the glacier dramatically,
initially dividing it into two arms and then bulldozing it hundreds of meters, first east and then west, and heaping it more
than 100 m higher than its original altitude.