Download 10. St. Helens

Field Trip Guide To The
Mt. St. Helens
National Volcanic Monument
Science 118: TheVolcanoes of Washington
North Seattle Community College
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This book is prepared exclusively for the use of students enrolled in Science 118:TheVolcanoes ofWashington, at North Seattle Community College, to whom it is issued as part of that body of academic coursework.
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Much of the content of this booklet, including a number of the illustrations, are taken from the Roadside
Geology of Mt. St. Helens National Volcanic Monument and Vicinity by Patrick
Pringle. This was published (without copyright restrictions) as the Washington State Department of Natural Resources Division of Geology and Earth Resources Information Circular 88 (1993).This guidebook
is available at local bookstores, including at the Johnston Ridge Observatory. For those interested in more
information on this subject, this book is highly recommended.
Field Trip Guide to the
Mt. St. Helens National Volcanic Monument
Science 118:
North Seattle Community College
Field Trip Guide To The
Mt. St. Helens National Volcanic Monument
Science 118: North Seattle Community College
Introduction
When Mount St. Helens erupted in
1980, it killed 57 people. It damaged
over 300 buildings, destroying nearly
half of them. It devastated over 150
square miles (400 square kilometers)
of woodlands, flattening over three billion board feet of timber. It unleashed
massive mudflows which extended
some 45 miles (70 km) to the Columbia river, delivering some thirty feet
(10m) of sediment to the river floor
over a distance of about four miles.
It erupted a vast ash cloud which rose
to some 60,000 feet (20 km), and then
rained down across Eastern Washington. There, farmers and ranchers suffered almost a quarter-billion dollars in
damage.
For all the death, destruction, and expense that the eruption visited on the people of Washington, it was a seminal event in the study of volcanoes. Occurring just a few years into our adoption of the modern theories of plate tectonics, it was the first continental-arc volcano to be
studied in detail under this new paradigm. The knowledge that has been gained from the study
of this mountain over the last quarter-century has immeasurably enhanced our understanding
of these phenomena. In the end, that knowledge has probably saved thousands of lives around
the world. For all that was lost in the eruption, a great deal has been gained.
Today, Mt. St. Helens serves as a geologic laboratory on the processes of volcanism. The
mountain is monitored by the US Geological Survey through the Cascade Volcano Observatory (CVO) in Portland, by the University of Washington, and has been the site of on-going
studies by dozens of agencies and institutions over the past twenty-five years. Now preserved
as the Mt. St. Helens National Volcanic Monument, it has been developed to provide for the
education of the public on this unique geological setting. That development has included the
construction of new roadways, and several interpretive centers. Not surprisingly, it is the most
popular geology field trip in the state of Washington.
1
Evolution of the Pre-St. Helens Landscape
Like the rest of the modern Cascade volcanoes, Mount St. Helens is built on older volcanic
and plutonic rocks of the 36 Ma – present Cascade Arc. Locally, it also overlies older volcanic
rocks (the ~40 Ma Goble Volcanics) erupted during Eocene time. Owing to only a modest
degree of uplift associated with the recent rise of the modern Cascade Range, a great volume
of volcanic rocks are preserved in the St. Helens area. The most voluminous of these are the
volcanic and volcaniclastic rocks of the 36 – 17 Ma Ohanapacosh Formation, which may total
as much as 30,000 feet in thickness. In the area south of Snoqualmie Pass, these rocks cover
much of the western slope of the modern Cascades.
The rocks of the Ohanapacosh Formation, along with other local volcanic units of that age,
accumulated prior to the uplift of the modern Cascade Mountain Range. They grew to form a
modest volcanic plateau, rising gently above a lowland province marked by west - northwesttrending hills and valleys. Uplift of the modern Cascade Mountains Range commenced about
5 million years ago, tilting these rocks and promoting a modest measure of erosion. At this
latitude, total uplift over this period has probably been less than 1 km. To the northeast of Mt.
St. Helens, the 3.2 – 1.0 Ma Goat Rocks volcanic center is the remains of a series of volcanoes which grew during that uplift.
Scattered younger volcanics in the area include the Indian Heaven Volcanic Field to the east,
and the Goat Mountain Plug Dome, which date from about 700,000 years ago. More recently,
Figure 2
(Above) Volcanic rocks of the Ohanapacosh Formation, near Mt. Rainier
2
Mt. Rainier
Chehalis
Cascade Volcanic
Rocks
Glacial
Sediments
Goble Volcanics
Castle
Rock
Mt. St. Helens
Mt. Adams
Figure 3 (Above) Bedrock geology of the Mt. St. Helens area. Much of the area shown as “Cascade
Volcanic Rocks” consists of the Ohanapacosh Formation. Dark areas are plutonic intrusions. Dark line
heading east from Castle Rock is the field trip route up Highway 504, to the Johnston Ridge Observatory.
the Marble Mountain Shield Volcano, to the south of the modern peak, erupted sometime
before 160,000 years ago.
These rocks illustrate the rather unusual range of magmas which have erupted here over the
last million years. The Marble Mountain Volcano is largely a basaltic edifice, while the Indian
Heaven Field varies from basalt to andesite. By contrast, the Goat Mountain Plug ranges from
andesite to dacite in composition. Basaltic rocks are relatively rare in continental-arc settings
(note that the basaltic Goble Volcanics are not volcanic-arc rocks), where andesite and dacite
are the more common varieties.
The circumstances behind this unique range of magmas is a subject of some debate. A leading
interpretation is that the basaltic rocks formed as magmas underwent fractionation during a
prolonged residence at depth. Heavier, more mafic magma accumulated on the bottom of the
magma chamber, and was erupted late in the eruptive cycle. In some cases however, basaltic
magmas were apparently not preceded by more felsic (andesite, dacite) types. Another interpretation is that some of the basaltic eruptions developed as more mafic magmas rose rapidly
from depth along fault zones. In this interpretation, there magmas did not accumulate in a
deep magma “chamber”, as is typical of most volcanic settings. Most of these magmas appear
to have erupted along extensive NE - trending fracture zones.
3
Accumulation of the Modern Pre-1980 Peak of
Mt. St. Helens
The modern cone of Mt. St. Helens started to accumulate about 50,000 years ago, and grew
through a series of four eruptive phases. These include the 50 -36 ka Ape Canyon eruptions,
the 20-18 ka Cougar phase, the 13-10 ka Swift Creek episode, and the 3900 – current Spirit
Lake phase. These rocks accumulated on a plateau of older volcanic rock rising to an elevation of about 3500 feet. This plateau had been sculpted by several episodes of glaciation
before the modern cone started to accumulate.
The Ape Canyon eruptions were typical of St. Helens’s character – a series of explosive
eruptions accumulating thick deposits of dacite pumice and tephra. These were followed by
a quiescent period of some 14,000 years, broken by the 2,000 year period of activity known
as the (20 – 18 Ka) Cougar phase. This episode included andesite flows and dacite tephra, but
also featured some large mudflows (lahars). These events took place during the height of the
Figure 4 (Above) Mount St. Helens, before the 1980 eruption. Spirit Lake in the foreground. Note
the distinctive conical shape of the peak, a reflection of its relative youth.
Image: National Forest Service
4
Years B.P.
Eruptive Stage
Years B.P.
Eruptive Period
Goat Rocks
Spirit
Lake
500
10,000
Swift
Creek
?
20,000
?
Pre-1980
Summit dome
Sugar Bowl
Dome
1,000
Sugar Bowl
Cougar
Small lateral
Explosion
2,000
30,000
Castle
Creek
Pine
Creek
?
3,000
40,000
50,000
Goat Rocks Dome
Kalama
Ape
Canyon
?
Dacite
Cave Basalt
Erupted
Domes
constructed
during this time
crop out in the
crater
Smith
Creek
4,000
Andesite and Dacite
Basalt
Figure 5 (Above) Chart showing the eruptive history of the Mount St. Helens area, over the last
50,000 years. The recent (<4800 B.P.) Spirit Lake Episode is detailed on the right side. Adapted from
Pringle, 1993.
last climax of alpine glaciation in the Cascade Mountains. It was followed by a quiet period
of some 4,000 years, a period characterized by the most recent episode of continental glaciation. Magma again erupted in the (13 – 10 ka) Swift Creek Episode. This episode featured
voluminous dacite tephra deposits, in eruptions marked by extensive pyroclastic flows.
These earlier eruptions accumulated a volcanic edifice of modest proportions, rising to an
elevation of something like 5000 feet. Rocks above this elevation are the product of the recent
Spirit Lake Episode, which dates from about 3900 years ago, and continues into the present.
These were events witnessed by native peoples in the area. The Spirit Lake Episode consists
of a half-dozen eruptive phases, including the 1900 -1300 BC Smith Creek phase, the 900
– 500 BC Pine Creek Phase, the 200 BC – 400 AD Castle Creek phase, the younger Sugar
Bowl phase, the (AD) 1480 – 1700 Kalama Phase, and the 1800 – 1857 Goat Rocks Phase.
5
Dogs Head
Dome
Sugar Bowl
Dome
Goat Rocks
Dome
Loowit
Forsyth Glacier
Glacier
Wishbone
Glacier
Figure 6 (Above) Mount St. Helens, from the north, prior to the 1980 eruption. Note location of the
Goat Rocks, Dogs Head and Sugar Bowl Domes. Image: US Geological Survey
The modern (1980 - ) eruptive period is considered part of this nearly 4-millenia long episode. This course of activity makes Mt. St. Helens the most active volcano in the continguous
United States over the last 4000 years.
The 1900 – 1300 BC Smith Creek phase was a particularly explosive set of events, with one
eruption producing more than 13 times the ashfall deposited in the 1980 eruption. The 900
– 500 BC Pine Creek phase was a more intermittent pattern of eruption, characterized by
extensive pyroclastic flows and large lahar deposits. The Castle Creek phase from 200 BC
to 400 AD displays a classic succession from basalt to andesite to dacite, including the 100
AD Ape Cave basalt flows and the Dog’s Head dacite dome. These accumulations raised the
elevation of the peak to nearly its early 1980 height.
Eruptions over the last 500 years have added modest amounts to the mass of the mountain.
The 1480 – 1700 AD Kalama phase started out with a dacite eruption producing over three
times the volume of tephra as seen in the 1980 eruption, followed by an explosive andesite
eruption in 1500. From the 1650’s through the late 1600’s the silica content of the magma
continued to rise, producing dacite domes and plugs. The (1800 – 1857) Goat Rocks eruptive
period similarly shows this trend, starting with the Floating Island andesite flow in 1800, a
rising silica trend in eruptions dated 1842 and 1857, culminating with the emplacement of the
Goat Rocks dacite domes.
6
Figure 7 (Above) Eruption of the Goat Rocks Dome on the north side of the mountain, painted by
Canadian artist Paul Kane in 1847. Photo from the Royal Ontario Museum.
Eruptions of the Goat Rocks period were witnessed not only by native people, but by the early
settlers to the region. Native peoples gave early trappers and explorers reports of the 1800
eruption, while early settlers and missionaries gave first-hand reports of eruptions starting in
1842. The eruption in 1857 was followed by a period of one hundred and twenty three years
of quiescence, during which there are no reliable reports of activity.
The Pre-1980 Peak of Mount St. Helens
Owing to the post-glacial accumulation of the rocks on its upper slopes, the pre-1980 cone of
Mt. St. Helens displayed a classic conical shape – earning it the appellation of the “American
Fujiyama” for its resemblance to Mt. Fuji in Japan. At an elevation of just 9677 feet, it did not
host an extensive glacial cover, but its proximity to the coast and the Columbia River afforded
enough snowfall to support a vigorous local system. The largest bodies of ice were the broad
Forsyth and Nelson Glaciers on the north side, but the only glacier to deeply incise the peak
was the narrow Shoestring Glacier on the southeast side. The slopes of the mountain were
generally rather featureless, one prominent point being the “Dog’s Head”, - a dacite dome
on the northeast side emplaced at about 100 AD. In pre-eruption times, the Spirit Lake Road
extended to a picnic area at about 4300 feet on this side of the mountain.
Owing to the relative abundance of volcanoes in this state, Mt. St. Helens was part of the Gifford-Pinchot National Forest, managed primarily for timber production and recreation. Had
this peak erupted in Vermont, it would have been preserved as our first National Park. The
lower slopes of the peak were extensively logged, in a patchwork of public and private prop7
erty. It was a popular area for hunting, but less so for hiking. Recreational development on
the mountain centered on facilities at Spirit Lake, including those run by the infamous Harry
Truman – a cantankerous old alcoholic whom the press exploited for “local color” in the days
ahead of the eruption. Harry died when the mountain finally erupted, burying him at his lodge
by Spirit Lake.
The first studies on Mt. St. Helens date from the late 1930’s, but little substantive research
was done until the late 1970’s, when we started to understand the fundamentals of volcanism.
Mt. St. Helens was an early object of interest, as its classic form clearly reflected recent activity. In 1978 the mountain was studied by the USGS as a preliminary hazards-evaluation project, which confirmed the potential for explosive eruptions and extensive lahar flows. Given
the historical frequency of eruptions, it was considered the most likely volcano to erupt in the
foreseeable future.
Figure 8 (Above) Mount St. Helens, prior to the 1980 eruption. The conical symmetry of the peak is
evident from this perspective. Note the glacier cover, and erosion into soft volcanic sedimetns around
the base of the peak. Mount Adams in the distance.
Image: US Geological Survey
8
Figure 9 (Above) The initial summit crater, formed on March 27th.
Image: US Geological Survey
The 1980 Eruption of Mt. ST. Helens
The modern eruptive period was initiated in late March of 1980, as earthquakes signaled
the movement of magma under the mountain. On March 27 a minor eruption of steam and
ash created a small summit crater, and event which attracted considerable attention. Minor
eruptions continued over the next seven weeks, but without any trend towards increasing
frequency or magnitude. Over this period, the more disconcerting development was the swelling of a large “bulge” on the north side of the mountain, evident as it fractured the glacier ice
with its growth. Sophisticated laser-rangefinder survey equipment (cutting-edge technology
at that time) was brought on site in Mid-April, and over the next few weeks it confirmed that
this bulge was growing at a rate of about 5 feet (1.5m) per day. With this finding, additional
monitoring equipment was brought in for further studies.
With this alarming development, Governor Dixy-Lee Ray ordered that the immediate area
around the mountain evacuated, and restricted access to the surrounding region. This was not
a popular move with local residents, some of whom had to be escorted out by the State Patrol.
The local timber industry was particularly critical of the decision, and lobbied heavily for a
less-restrictive plan. Grave concerns were voiced for the economic impact of the decision on
the small timber-dependent towns of Randle, Packwood and Toutle, and there was a strong
local sentiment that the government was over-reacting to the situation. Among scientists,
few felt that the measures were unreasonable. The increasing concern was as to whether they
would be adequate. By mid-May, there was a general consensus that an eruption of some form
was imminent.
9
Summit Dome produced
in Kalama episode
A
On May 18, just after 8:30 in the morning, a
magnitude 5.1 earthquake set off a chain of
events resulting in a catastrophic eruption of
the mountain. Centered underneath the expanding “bulge” on the north side of the peak,
it caused the bulge to collapse in a series of
three massive slide blocks, heading down the
mountain toward Spirit Lake. This debris avalanche, the largest landslide in recorded history, swept down to Spirit Lake and over the
top of Johnson Ridge behind it, into the Coldwater Creek drainage. Some of the debris also
swept westward from the mountain, down the
North Fork of the Toutle River. Consisting of
over a half a cubic mile of material, it reached
speeds of 70 – 150 miles per hour . When it
finally came to rest, it buried the upper Toutle
Valley to an average depth of 150 feet, locally to as much as 600 feet.
Profile of “Bulge” by
May 18
Goat Rocks
Dome
May 18th, before
eruption B
Magma intrusion which
pushed out the bulge Catastrophic explosion
as landslide removes the
cap over the magma
C
The massive landslide on the north side of the
mountain removed the overburden which had
contained the gas-charged magma chamber
under pressure. As that confining pressure
was released, the magma exploded violently.
As the north side of the peak slid away, this
7-megaton explosion was channeled laterally
to the north, erupting out of the massive amphitheater now marking that side of the mountain. The blast took the form of an immense
pyroclastic cloud of boulders, rocks, tephra
and ash, exploding across the landscape at
over 650 miles an hour (300 m/s), searing it
at temperatures of over 600 F (320C). Close
to the mountain, it stripped every scrap of soil
and vegetation from its flanks. Flowing into
Spirit Lake, it caused a massive secondary
explosion as the water flashed to steam. As it
entered timberland, the blast wave absolutely
flattened forests for a distance of some 12
miles (18 km) in a 180 – degree arc north of
the peak. In all, about 230 square miles (600
14 Seconds
Second slide block
of avalanche
Blast expands
rapidly
D
21 Seconds
Incipient third slide
block of avalanche
Debris avalanche
slides downslope
Vertical eruption
E
column develops
After 1 Minute
Figure 10 (Left) Sequence of eruptive events.
Adapted from Pringle, 1993
Pyroclastic flows issue
from crater
10
Figure 11 (Right)
The initial eruption of
Mt. St. Helens, in a series of photographs taken by Keith Ronnholm,
of Remote Measurement
Systems, Inc. The two
photographs are taken
about 17 seconds apart.
This is estimated to
have been a 7-megaton
explosion.
square kilometers) were severely damaged in the blast. All of this happened within the first
minute of the eruption.
After the initial blast, a vertical column of ash and debris rose from the crater, reaching an
elevation of 12 miles (20 km), developing into an enormous mushroom cloud some 45 miles
(75 km) across. This ash drifted eastward to wreak havoc across Eastern Washington. From
the base of this column, pyroclastic avalanches periodically streamed down the side of the
peak. By noon, these glowing avalanches of debris were flowing out of the crater at 50 – 80
miles an hour, and sweeping across the north side of the mountain. At temperatures of about
1300 F (700C) they blanketed the slope in pumice, ash and rocks of various sizes. In places,
these accumulated to over 100 feet (30 m) in thickness. These pyroclastic flows continued for
almost six hours before they started to taper off.
In addition to the massive debris slide which had flowed into the North Fork of the Toutle
River, the initial blast melted the glaciers on the side of the peak, initiating lahars which
swept down into the surrounding valleys. A large debris flow surged down the South Fork of
11
the Toutle River, carrying
away logs, boulders, heavy
trucks and buildings, wiping out roads and bridges
as it rolled down the valley.
On the east side of the
mountain, lahars flowed
down the Muddy River,
wiping out major bridges.
On the south side, debris
flowed down the Pine
Creek and Swift Creek
valleys, to reach the Swift
Reservoir below.
The largest lahar developed
from the thick debrisavalanche deposit which
had flowed into the North
Fork of the Toutle River in
the initial landslide. This
was thoroughly saturated
with water, formed as the
ice from the Loowit and
Forsyth Glaciers were
melted in the blast. Later
in the day, vast amounts of
water started to drain from
this pile, forming the largest and most destructive
Figures 12, 13, 14 (Right)
Top: A modern view of Mt. St.
Helens
Middle: Flattened timber, in
the blast zone. Image from the
US Geological Survey
Bottom: Trucks, timber, and
other mudflow debris. Image from the US Geological
Survey.
12
Figure 15 (Right)
A geologist inspects
the moonscape of
Mt. St. Helens, in the
Toutle Valley. Image
from the US Geological Survey
lahar of the event. This massive mudflow coursed down the North Fork of the Toutle River, at
speeds of up to 27 miles an hour (12 m/s). It formed a churning mass of boulders, timber, and
other debris in a steaming caudron of ash-gray mud, rolling down the valley and flattening
everything in its path The mudflow eventually reached all the way to the Columbia River, a
distance of some 45 miles (70 km), where it deposited over four million cubic yards of sediment into the river channel. Along the way it destroyed roads, bridges, homes and businesses,
and threatened to take out the Interstate 5 Bridge over the river. It left the valley filled with a
thick deposit of debris, burying the older landscape.
The massive eruption tapered off towards the end of the day. In the end, the mountain had lost
more than 1300 feet (396 m) in height, and about 0.6 cubic miles (2.5 cubic kilometers) of its
volume. The crater it left behind is about a mile and a half in diameter, and over 2,000 feet
deep. Some 57 people died in the eruption, and about 200 homes were destroyed or seriously
damaged. Over 200 miles (320 km) of roads and 15 miles (24 km) of railway were destroyed,
including over 40 bridges. Ashfall across Eastern Washington lead to massive economic losses
in the agricultural sector, and damaged transportation and public utility systems. In places, it
took months to finally clean up the ever-present gray dust.
Figure 16 (Right)
Biologists inspect the
scorched landscape after the eruption. This is
on the outskirts of the
blast zone, where some
trees were stripped, but
left standing.
Image: USGS
13
Figure 17 (Above) The lava dome, growing within the crater. This is an older photo. Since that date,
the size of the dome has grown substantially. Image from the US Geological Survey.
In the six months after the initial eruption, there were about 5 explosive eruptions on the
mountain, sending pyroclastic flows down the north side of the peak. These accompanied the
extrusion of magma domes, but subsequent eruptions destroyed earlier versions accumulated
before October of that year. From October 1980 through October 1986 some 17 eruptions
were recorded, as the lava dome grew steadily. By October of 1986 it had reached an elevation of 876 feet above the crater floor. After reaching this point, activity fell into a relatively
quiescent period from 1987 to 2004.
Starting in October of 2004, dome-building resumed with the extrusion of a large “whaleback” form, measuring 1500 feet by 500 feet in area. Growth of this dacite feature continued
up until early 2008. It brought the accumulated post-eruption pile to a total of 1400 feet above
the crater floor. At this rate of accumulation, it will only take about two centuries to rebuild
the cone of Mt. St. Helens to its former elevation.
Perhaps the most remarkable aspect over the years since the 1980 eruption has been the
rapid recovery of the affected region. Although most life was lost in the eruption, plants have
rapidly colonized the lower regions, and large animals have moved in to fill open niches in the
evolving ecosystem. Within fifty years, a common person will not even recognize the scale of
devastation which was experienced in the lower valleys. Plants are also returning to the upper
slopes, including coniferous species which will form the basis for future forests. Even above
timberline, where the results of the eruption were so intense, pioneer species of plants are
becoming established. It has been a rate of recovery that is much faster than most would have
predicted after the initial eruption. The region is preserved as a natural biological laboratory
for studying the post—eruption recovery.
14
Field Trip Guide and Road Log
Via State Route 504, The Spirit Lake Memorial Highway
(Toutle River Valley Approach to Johnson Ridge)
Part 1: Seattle to Castle Rock, Via Interstate 5
All distances are given in miles from North Seattle.
19.5
Bridge crossing the Duwamish (Green) River. The lahars of the Osceola mudflow
from Mt. Rainier reached this far.
31.5
Federal Way: A non-descript unincorporated section of urban sprawl between
Tacoma and Seattle. Unremarkable except for the fact that my friend Lisa lives
there.
38.0
Fife Perhaps the most notorious speed-trap in the state.
39.0
Commencement Bay. The Puyallup river, rising on Mt. Rainier, flows into
the Puget Sound here. . The lahar of the Osceola Mudflow, some 5700 years ago,
reached the Sound at this point.
40.5
Tacoma: Founded in 1875, the “City of Destiny” boasts a population of
about 194,000. The large blue structure by the freeway is the “Tacomadome”,
the worlds largest free-standing wooden-roofed multi-purpose arena. It is a
popular venue for recreational vehicle shows, religious revivals, and monster truck
rallies.
45.0
Fort Lewis: At 87,000 acres and over 19,000 in popularion, Fort Lewis is one of our
nation’s largest military bases. Founded in 1917, it is the most requested duty site in
the Army system.
55.5
Bridge crossing the Nisqually River. The Nisqually river rises from the Nisqually
Glacier on Mt. Rainier. The Nisqually Delta, to the west, is one of the most important
estuaries in the Puget Sound basin. It is preserved as a wildlife area.
67.5
Olympia: The State Capitol, founded in 1851. It has a population of about 42,500
Olympians.
15
90.5
Centralia: Centralia was founded in 1850, as a stagecoach stop half-way between
Seattle and Portland. Later, it was an important fuel and water stop for the railroad
between these two points. The town is known historically as the site of the
Centralia Massacre in 1919, a conflict between the local American Legion
Post and the International Workers of the World, a labor union. Six died in a gun
battle here, with others wounded, and several were later imprisoned .
It is the largest city in the area, with a population of about 14,750. Until recently, the
large coal-fired power plant here was fueled by local coal, which is relatively dirty
to burn. In its place, coal is now imported from Wyoming.
94.5
Chehalis: Founded in 1873, population 7,057. Located along the Chehalis River, it
has long been subject to flooding. The December 2007 flood was particularly
disasterous in this area. In the hills to the west, as much as 19 inches of rain fell in a
24-hour period. Levees along the river failed, flooding the region. The town is still
recovering from that event.
Bridge crossing the Cowlitz River
121
123.5
Bridge crossing the Toutle River
Just past the bridge, notice the large piles of sediment heaped along side the Toutle
River. These were lahar deposits, dredged from the river.
Castle Rock. Incorporated in 1883, it has a current population of about 2130.
Section 2 Castle Rock to The Johnston Ridge Observatory
Mileage given as per markers along State Route 504, from exit 49 on Interstate 5
Heading east from Castle Rock, the highway crosses though Pleistocene and Holocene deposits for the first two miles. These include local lahar deposits, alpine
glacial tills, and deposits left by floodwaters of the catastrophic Lake Missoula
floods. This is a landscape of very recent construction. Beyond the 2-mile point, the
roadway cuts through Late Eocene volcanic rocks of the Goble Volcanics (part of the
Crescent Suite) and younger sedimentary rocks of the (Miocene) Wilkes Formation.
3.0
Quarry on the north side of the road is in the Goble Volcanics. The rocks
are basaltic andesites, quarried for road-bed material.
5.3
Mt. St. Helens Visitors Center
This is the first of two interpretative centers operated by the U.S. Forest Service, a
part of the Department of Agriculture. The center features interpretative displays, and
a short movie presentation scheduled on the half-hour.
The center sits on the shores of Silver Lake, a shallow body of water. It was formed about
2500 years ago when a series of large lahars descended the Toutle Valley and dammed Outlet
16
Castle
Rock
N.F. Toutle
River
Silver Lake
Spirit
Lake
S. Fork
Toutle River
Crater area
Blast Zone
Lahar Flow
Debris-Avalanche Flow
Pyroclastic Flows
Road
Swift Resevoir
Figure 18 (Above) Map of the Mt. St. Helens area, showing the field trip route. Map adapted from
Pringle, 1993.
Creek. These date from the Pine Creek eruptive period. The level of the modern lake is controlled by a dam.
Traveling up the valley, the highway ascends the Toutle Valley. Much of the route is marked
by ancient landslide debris, older deposits from Mt. St. Helens, and outcrops of the Goble
Volcanics.
11.0
Coal Banks Bridge
This bridge is about 30 miles below the volcano, and was destroyed by the lahar
flows in 1980. In the bluffs above you can see lahar deposits from the above-noted
Pine Creek eruptive period. These were immense lahars, one with a maximum dis
charge of nearly 9 million cubic feet per second (roughly that of the Amazon River at
flood stage).
The highway continues up the valley, occasionally passing outcrops of the Goble Volcanics
along the north side of the road.
17
Elk Rock
Mt. St. Helens
Terminus of debris
- avalanche deposit
Remains of
early retention
dam
Figure 19 Panorama from Hoffstadt Viewpoint.
Adapted from Pringle, 1993
21.3
Sediment Retention Structure
The road to the right just before the Toutle River Bridge leads to an overlook above
the Sediment Retention Structure, which was completed in 1989. It was built
by the US Army Corps of Engineers. The structure was built after a number of
smaller diversion features were built upstream, and which failed at an early date.
The structure is essentially a dam, designed to slow the water speed and allow fine
sediment to settle out. It is now full, and no longer really serves a purpose except to
hold back accumulated sediment.
27.0
Hoffstadt Viewpoint
This site, about 15 miles northwest of the peak, provides a panorama on the lower
Toutle Valley. From here you can see the valley floor paved in lahar deposits, and
at the head of the valley, you can see the terminus of the avalanche – debris deposit
from which the large lahar of May 18 1980 emanated. Just below the debris-flow
terminus you can make out the linear trace of an early sediment-retention structure
which was built immediately after the eruption. It was topped by a lahar in 1982, and
was damaged in subsequent floods. Because of the failure of this structure, the larger
dam was built downstream in 1989.
28.0
Columnar jointing in Oligocene basalt flows on the north side of the road. These
features result from the contraction of the rock as it cools.
29.9
High bridge spanning Hoffstadt Creek, some 370 feet below. On the far
side, the road enters the blast zone of the 1980 eruption. This becomes evident
on the ridge above. Here, the only trees spared were the very largest, or those pro
18
Toutle Mountain Range
North Fork
Toutle River
(Braided Stream)
Old Spirit
Lake Road
Pre-1980 Lahar Deposits
from Mt. St. Helens
`
tected on lee slopes. While the forest is starting to re-grow, you can see the landscape
littered with the trunks of fallen trees.
32.3
The road here cuts across the promontory of Elk Rock for the next mile. The rocks
here are dominantly volcanic breccias of Tertiary age, cut by light-green dikes. The
rocks have been recrystallized to some degree.
33.3
North Fork Viewpoint.
From here you can look down on the expanse of the North Fork Toutle River and see
the hummocks left by the 1980 debris-avalanche flow.
37.2
Elk Rock Viewpoint
Here, the road enters the Mt. St. Helens National Volcanic Monument. Please do not
collect samples beyond this point. Upstream, the effects of the blast from the 1980
eruption become increasingly evident. Looking down on the debris-flow field, you
will also notice that it acquires more relief above this point. This happened as the
flow was partially restrained by a constriction in the valley floor. Some 30 miles to
the east, one can see the summit of Mt. Adams on a clear day.
37.8
Elk Pass, elevation 3800 feet (1159 m)
39.0
The roadway goes through a notch cut in Tertiary rock. Much of this rock
has been altered to bright greens and pinks. The green rocks are an altered
pumiceous tuff.
40.7
Castle Lake Viewpoint
This viewpoint provides an excellent vista on the volcano, the debris-flow avalanche
19
Sugar Bowl
Dome
Lava Dome
Windy Ridge
Pumice
Plain
North Fork Toutle River
Figure 20 (Above) The modern peak of Mt. St. Helens. This photo was taken some 20 years after the
eruption. Note how the Toutle River has cut into the sediments deposited in the eruption, forming terrances along its banks.
deposit, and other features. Clearly visible are the hummocks of the debris-flow avalanche.
Also visible are terraces eroded out of recent and older material by the Toutle River, which
has been meandering across the floor of the valley since the 1980 eruption.
43.5
Coldwater Ridge Visitor Center
This location provides a fine view on Coldwater Lake and the Toutle debris flow.
The lake was formed when the debris flow (1980) dammed Coldwater Creek. A delta
has been building into the lake from the outfall pipe installed to drain Spirit Lake
above in 1985. After the eruption, there was great concern that these lakes might
breach their restraining levees, emptying catastrophically into the valley below.
45.4
Crater Rocks Trail
This short trail explores the hummocks of the debris-flow avalanche. Visible are
large chunks of the pre-1980 cone of St. Helens, along with material generated in the
eruption.
The road ascends the valley of South Coldwater Creek, which was partially filled by the debris-flow avalanche. Before the eruption, this was a U-shaped glacial valley. The valley floor
is now more than 240 feet higher that the old surface on its west end, and 80 feet above the
old surface on the east end.
20
50.3
Johnston Ridge Observatory
This is the premier site for observing the effects of the 1980 eruption. The
interpretative center offers a number of displays, and a movie presentation on the
eruption.
There is also a bookstore, and restroom facilities.
Pringle’s (1993) description of the events of the eruption are spectacularly illustrated here.
The following is taken from his guidebook:
(1)
As material from the first slide block of the debris avalanche was topping the saddle
between Johnston and Harrys Ridges at speeds greater than 60 mph (27 m/s), the second and
third slide blocks, which were penetrated in part by the blast and mixed with and propelled by
blast material, caught up with the first slide block and passed it. This mixed material formed a
hummocky deposit of shattered pieces and block of Mt St. Helens in a gravelly sand matrix.
(2)
The debris-avalanche slide blocks dug as deep as 6.5 ft (2 m) into the pre-1980
soil. The avalanche left a distinct trimline on the valley walls of South Coldwater Creek as
it flowed downvalley. Deposits of this phase of the debris avalanche are also hummocky and
commonly contain tree trunks and limbs, clots of eroded soil picked up in transit, and some
lenses of blast material.
(3)
The slide-block material was followed closely by the main surge from the blast,
which moved at speeds greater than 560 mph (286 m/s). The blast left a sandy, rubbly deposit
containing abundant blast dacite, remnants of the cryptodome that had intruded the mountain
and pushed out the bulge. This flow deposit formed a veneer on the surfaces of some of the
hummocks and was plastered on the valley walls.
(4)
Within minutes after the blast swept across Johnston Ridge and into the South Coldwater Creek Valley, the hot, freshly deposited material flowed to the valley bottom as a secondary pyroclastic flow, covering many of the deposits mentioned in (3) above with a layer of
sand and rocks 3.5 – 7 ft (1-2m) thick. The surface of this redeposited blast material is fairly
flat and is banked up against the valley walls and fills the swales between the hummocks of
the underlying debris avalanche.
(5)
All of the above deposits were then covered by a layer of accretionary lapilli that fell
from the great mushroom cloud and, in areas nearer the volcano, by ash fallout from the ongoing pyroclastic flows as well.
(6)
Beginning at about 9:00 AM and increasing in intensity until about noon, pyroclastic
flows swept down the north face of Mt. St. Helens and filled low spots between the freshly
deposited hummocks in the debris-avalanche deposit to depths greater than 100 feet (30 m).
Temperatures in the pyroclastic flow deposits were 780 F (415 C) two weeks after the eruption. Pits as much as 200 feet (60m) across are scattered across the surface of the Pumice
Plain. These rootless explosion craters were created when ground water came in contact with
hot pyroclastic deposits, causing steam-driven explosions. The deposits remained hot (over
100C) for several years after the eruption.
21
Johnston Ridge Observatory
To US 12 and Randle
North Fork Toutle River
Debris Avalanche
Pumice Plain
South Fork Toutle River
Mt. St. Helens
Kalama
River
Marble Mtn.
Volcanic
Hazard
Zones
Lewis River
Pyroclastics, Lava
Pyroclastic surge
lahars, floods
Figure 21 (Above) Mt. St. Helens, showing the extent of pyroclastic flows, surges, and lahars. Note the
location of the Johnston Ridge Observatory.