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ESS 305
Mt. St. Helens National Volcanic Monument
Spring 2015 16 May
The major volcanic peaks of the Cascades are short-lived features-- they are rapidly
constructed by lava flows and the accumulation of pyroclastic debris, and they are quickly torn
down by erosion and later eruptions. Several such episodes of growth and destruction may occur
in the life of one of these volcanoes before the underlying igneous activity shifts to a different
outlet. In the case of Mt. St. Helens, the youngest of the major Cascade volcanoes, its first
formative eruptions began only 40,000 years ago, yet it is sitting in the midst of an area that has
been volcanic for the last 40 million years. The earlier volcanoes have vanished, but their
associated intrusive and extrusive products form the base upon which St. Helens sits.
St. Helens itself has erupted in a wide range of manners in its brief existence, producing
everything from runny lavas which flowed gently across the landscape, to thick, sluggish
accumulations and intermittent violent explosions. Its recent activity, beginning in 1980, has
demonstrated how these volcanoes can quickly tear themselves down and start to rebuild.
Prior to 1980, the symmetrical Mt. St. Helens edifice was home to a pristine, year-round
recreation and vacation area. As early as March, 15, 1980, a swarm of small earthquakes began.
This continued through April, and was accompanied by small steam eruptions. All the while,
rising magma in the north side of the volcano pushed its way upward, creating a noticeable bulge
which grew at 5-6 feet per day through April and May. By May 18th, this bulge had pushed the
north flank over 400 feet outwards. Prior to the major eruption, over 10,000 earthquakes were
recorded since mid-March. Finally, at 8:32 on Sunday morning, May 18, 1980, a magnitude 5.1
earthquake caused the north face of the mountain to slide away, initiating a catastrophic eruption.
The release of overlaying material on the north side caused a 'lateral blast'. Pyroclastic flows and
old flank rock raced towards a USGS outpost at Johnston Ridge at near super-sonic speeds.
Today, you will see the remnants of that eruption, and the amazing recovery of the surrounding
area over the past three decades.
For today's trip, we will examine evidence from three periods: First, the pre-St. Helens
rocks, which will provide glimpses into the long history of volcanism and related activity in this
area; second, the rocks of St. Helens itself, of which a representative sampling were displaced
into our field area by the catastrophic 1980 eruption; and third, the materials and landforms
shaped during and after that catastrophic eruption.
The overall directions for this trip are simple: drive south on I-5 to Exit 49, which is the
Castle Rock/S.R. 504 exit, then head east along 504.
Create a notebook entry for each stop, taking care to consider and briefly answer the
questions listed below as part of your entry. Observe the outcrop, then make your interpretation
of what is revealed there.
ITINERARY
While approaching Exit 49 from the north, we will cross the Toutle River bridge; note the
large mounds of material piled up along the west side of the freeway to the north and south of the
bridge. This material, carried downstream after the May 18, 1980 eruption, was dredged from
the Toutle and Cowlitz rivers in an effort to keep it from entering the Columbia. Even so, the
sediment which did reach the Columbia was sufficient to completely block navigation for two
weeks and disrupt shipping to and from Portland for three months. Farther up the Toutle we will
encounter a sediment retention structure which was built to contain even more of this debris.
STOP #1: Silver Lake Visitor Center
Take Exit 49 from Interstate 5 South and turn left onto the overpass. Continue for five
miles east along 504 to the visitor center on the right.
Use this stop to develop an overview not only of the May 18, 1980, eruption, but also of
previous eruptions and eruptions from similar volcanoes in the Cascades and worldwide.
Note that Silver Lake was created by the damming of Outlet Creek by a volcanic
mudflow (lahar) from St. Helens approximately 2500 years ago, and that it sits upon even older
lahar flows. The landslide and mudflow deposits from 1980 which fill the valley floor closer to
the volcano are merely from the most recent in a series of similar events which have occurred
here.
STOP #2: Lahar Deposits
Continue for five miles on 504, passing Toutle Lake High School and a post office on
your left. At the fork, take a right on S. Toutle Road, and continue for a mile. While approaching
the river on the left, watch for a gravel turnout on the left side of the road. Park there and look
for the houses half-buried in lahar deposits. Return to 504.
STOP #3: Sediment Retention Structure (SRS)
Continue eleven miles east; pull off to the right before the Toutle River bridge and park.
A short walk into the woods leads to a panoramic view.
Finished in 1989, the function of the SRS, as the name suggests, is to trap some of the
enormous quantity of sediment now carried by the Toutle River, which increased by slightly over
100x after 1980. What are the potential short-term and long-term effects of this effort?
STOP #4: Hoffstadt Viewpoint
Continue east for six miles; pull off and look out over the valley.
Look up the river valley to observe how the flatness of the valley floor changes. What
you observe in the distance is the limit of the debris avalanche from the May 18, 1980, eruption.
How does the topography of the floor change?
STOP #5: Oligocene columnar flows
Continue one mile east. Stop at the cliffside outcrop.
What is this rock? Given the nominal composition of Mt. St. Helens, is this the expected
rock for this area?
What geologic structures are present here and how do they form?
STOP #6: Tertiary green dikes
Continue 4.5 miles east. Stop at the cliffside outcrop.
The dike material was once basaltic andesite; clearly it has been altered. This is an
example of hydrothermal alteration. What is the green material now?
The rock into which the dikes intruded was "cooked" as a result. Describe its texture.
STOP #7: Altered Tuff
Continue 4.5 miles east. Stop at the cliffside outcrop.
This roadcut exposes more of the bedrock of this area, in this case what was formerly a
pumice-rich tuff (a layer of volcanic ash). What does this imply about the composition and
eruptive style of this deposit? What is exposed above it?
STOP #8: Castle Lake Viewpoint
Continue 3.5 miles east. Stop to look out over the valley.
What are the hummocky features in the valley below? Is there evidence for more than
one debris avalanche during the May 18, 1980, eruption? What is the light-colored material near
the breach in Mt. St. Helens, if visible?
STOP #9: Tertiary fault
Continue 1.5 miles east. Stop at the cliffside outcrop.
What is one explanation for the presence of a fault here?
STOP #10: Coldwater Ridge
Continue 1.5 miles east. Descend two miles to the bridge over Coldwater Creek and park
at the trailhead.
Walk along the trail past hummocks of former Mt. St. Helens flank rock. In this material
you can observe the remnants of previous eruptions which built the pre-1980 cone. Describe
some of the rock types present.
Find a breadcrust bomb. What evidence suggests that this was still molten when it was
ejected during the eruption?
STOP #11: Johnston Ridge Observatory
Continue to the end of the road. Enjoy the exhibits and the view, if any. On a good day,
steam from the lava dome is visible. Look for evidence of the structure of St. Helens on its
flanks and inside the crater. In the surrounding hills, note the pattern of destruction in the blast
zone, as highlighted by the differing modes of preservation of trees in the area.
Return to Seattle.
Suggested Readings:
Decker R and Decker B (1981), Volcanoes; W. H. Freeman and Company, New York, 243 pp.
An introduction to all kinds of volcanoes. Very accessible, 5th edition just published.
Harris, SL (1988), Fire Mountains of the West: The Cascade and Mono Lake Volcanoes;
Mountain Press, Missoula, 379 pp. Harris' updated sequel to Fire and Ice, with a long section on
the 1980 eruption of St. Helens, of course. A lot of solid information about each volcano, though
a little repetitious. Needs a guide to localities and much better introductory sections.
Lipman PW and Mullineaux DR (1981), The 1980 eruptions of Mount St. Helens, Washington;
USGS Prof. Paper 1250, 844 pp. Monster compilation of mostly technical articles published
shortly after the eruption. Very good resource.
Lipman PW, Chapin CE and Dungan MA (1989), Cenozoic Volcanism in the Western United
States; Amer. Geophys. Union, Washington, D. C., 730 pp. This is a volume of selected reprints
from the Journal of Geophysical Research, all technical papers, but a huge coverage and a handy
reference.
Pringle PT (1993), Roadside Geology of Mount St. Helens National Volcanic Monument and
Vicinity; WDGER Info. Circ. 88, 120 pp. The best field guide and geologic introduction to Mt.
St. Helens, with special emphasis on the 1980 eruption.
Ream L (1983), Northwest Volcanoes: A Roadside Geologic Guide; BJ Books, Renton, 124 pp.
This slim, overlooked volume is actually quite good, gives a broad overview of many of our
volcanoes.