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The Volcanoes of
Washington
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J. Figge 2008
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The Volcanoes of Washington
Science 118 North Seattle Community College
J. Figge, Instructor
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The Volcanoes of Washington
© John Figge 2008
Cover Photo: Mount Baker, from the north (Yellow Aster Meadows)
Inside Photo: Mount Adams, from the east.
The Volcanoes of
Washington
____________________
____________________
J. Figge 2008
The Volcanoes of Washington
Introduction:
The Volcanoes of Washington
Five great volcanoes rise along the western slopes of the
Cascade Range in Washington State, a chain of white-robed
sentinels which dominate the eastern skyline of the Puget
Sound region. These ice-mantled summits were held in great
awe by the Native Peoples, served as important landmarks to
early explorers, and guided the routes of early settlers as they
arrived from around the world.
From north to south, this chain of volcanic summits includes
Mount Baker to the east of Bellingham, Glacier Peak to the
east of Everett, Mt. Rainier to the east of Olympia, and Mt
St. Helens and Mt. Adams to the east of Kelso. These are the
Washington summits along a much longer chain of Cascade
volcanoes, extending from Mt. Garibaldi in British Columbia
to Lassen Peak in northern California.
Mt. Baker
Glacier Peak
Mt. Rainier
Mt. St. Helens
Mt. Adams
Figure 2:
State map showing the general location of the volcanoes of Washington. North is to the top of the map,
Pacific Ocean to the left.
These modern volcanoes are recent features on the landscape. All are less than a million years old, most
are largely younger than 100,000 years old, the youngest being less than 10,000 years old. These modern
volcanic cones erupted along the western slopes of the older Cascade Mountain Range, which started uplifting about 5 million years ago. Over the last two and a half million years, particularly on the northern end,
that range has been extensively sculpted by repeated glacial episodes. The modern volcanoes erupted on the
ridge-crests of this older mountain range, at elevations of 4 – 7,000 feet. The modern volcanoes are accumulations totaling 3,000 - 8,000 feet, rising as cone-shaped features above that older landscape.
By this course of development, and owing to the prevailing climatic conditions, the volcanoes of Washington present a truly unique montane environment. These are alpine-scale peaks cloaked in glaciers, finged
in luxurient alpine parklands, nourished by the maritime climate, rising above a more modest range of
glacially-sculpted summits ranging from 5-9,000 feet. This contrast distinguishes these peaks as among the
most spectacular and beautiful mountains in the world. By virtue of these qualities, they are now largely
preserved in wilderness areas and National Parks.
Figure 1 (Left)
Mt. Rainier, from the west
Image: National Park Service .
Table of Contents
Introduction: The Volcanoes of Washington...........................................1
Table of Contents....................................................................................3
On the Origin of the Cascade Volcanoes: Juan De Fuca’s Fire.............4
The Volcanoes of Washington:
Mount Baker, Koma Kulshan....................................................11
Glacier Peak, DaKobed............................................................21
Mount Rainier, Tahoma..........................................................29
Mount Adams, Klickitat...........................................................37
Mount St. Helens: Loo-wit.......................................................43
The Past and Future Cascade Volcanoes..............................................56
References............................................................................................58
A Few Last Words From Your Instructor................ .............................60
Figure 3: (Left)
Glacier Peak, from the east
Image: Walter Siegmund
3
Juan De Fuca
Plate
North American
Plate
Volcano
Plutons
Magma
Partial
Melting of
Rock
Juan De Fuca Ridge
Subduction Zone
On the Origin of the Cascade Volcanoes:
Juan De Fuca’s Fire
There are a variety of volcanic types around the world, but all share in the common characteristic that they
represent eruptions of magma (hereforeafter never again referred to as “liquid rock,” which is a contradiction in terms) onto the surface of the earth.
The magma which erupts to form the Cascade volcanoes is generated where a large section of oceanic rock
is being forced beneath the North American Continent. This “plate” of oceanic rock is about 15 km thick,
and is being generated along a system of fractures in the ocean floor known as the Juan De Fuca Ridge.
Along this ridge, magma is being forced to the surface, forming new oceanic plate rock. In this process the
(oceanic) Juan De Fuca Plate is being formed, and is being driven to the east as new rock is continuously
injected along the ridge.
Where the east-moving Juan De Fuca Plate meets with the west-moving North American (continental) plate,
the Juan De Fuca Plate is forced beneath the continent in a process known as “subduction”. As it descends,
the plate is heated by the internal heat of the Earth. Owing in large part to the huge volumes of (sea) water
carried with the descending plate, conditions develop at a depth of 100-150 km where certain (lighter) minerals in the descending plate start to melt. This magma accumulates above the zone of melting, and rises into
Figure 4: (Above)
Schematic diagram illustrating ocean plate production along the Juan De Fuca Ridge, subduction of the Juan De Fuca Plate
beneath the North American Continent, and subduction-generated magmatism supporting the Cascade Volcanoes. (Olympic
Peninsula omitted for clarity). .
4
British Columbia
Figure 5 (Right)
Map showing the North American, Juan De Fuca and Pacific
Plates, with their directions of
motion.
The Juan De Fuca Plate is
being forced beneath the North
American Plate in the process
of subduction. Magma derived
from the subduction process
rises into the continent, erupting on the surface as the Cascade Volcanoes (Red Triangles).
The Juan De Fuca, Explorer
and Gorda Ridges are not
labled, but appear in red.
Garibaldi
Explorer
Plate
Baker
Glacier
Juan De
Fuca Plate
Rainier
St.
Helens
Adams
Hood
Oregon
Idaho
Pacific
Plate
Gorda
Plate
Shasta
Lassen
North American
Plate
the overriding plate above. Some of this magma cools at depth to form plutonic (intrusive) rocks (plutons),
while some reaches the surface to erupt as volcanic (extrusive) forms. These surface rocks accumulate to
comprise the Cascade Volcanoes.
Thus the Cascade volcanoes are known as “continental arc” volcanoes, because they are volcanic arc (subduction generated) magmas which have erupted along the continental margin. Other types of volcanoes
include those which develop by the same process on colliding oceanic plates (Island-Arc volcanoes, a good
example being the Philippine Islands), those which arise along the mid-ocean ridges (Mid-Ocean Ridge
volcanoes), and those which rise above isolated “hot spots” of rising magma (Hawaii being the classic example).
Mid-ocean ridge volcanoes and hot-spot volcanoes derive their magma from deep earth sources. This is a
hot, thin, runny magma, rich in iron and magnesium. It cools to form a black volcanic rock called “basalt”.
When it erupts on the surface is spreads for long distances, forming a broad, low-angle pile known as a
“shield” volcano. Again, the flows seen on the islands of Hawaii are typical of this variety. They are thin,
hot, runny, black magmas which travel long distances and spread out over large areas.
By contrast, the process by which magma is produced in the subduction setting yields a different kind of
magma. This magma is cooler, has less iron and magnesium, and has more silica in the mix. The silica acts
as a thickening agent, producing magma that is much more viscous. In a continental-arc setting, more silica
is added to the magma as it rises through the surrounding (silica-rich, continental) rocks. The resulting rocks
5
Gabbro
Basalt
Figure 6 (Above)
Diorite
Andesite
Granodiorite
Dacite
Common volcano-forming rocks. Intrusive (plutonic) varieties shown above, extrusive (volcanic) varieties shown below. The
plutonic varieties feature large crytals, formed as they cooled slowly deep undergound. The volcanic varieties contains the same
minerals, but the crystals are very small in size - reflecting rapid cooling on the surface. Basalt is the common rock extruded
at mid-ocean ridges, at “hot spot” volcanoes, and in immature volcanic island settings. Andesite, and to a lesser degree dacite,
are common volcanic rocks in continental-arc settings. Much of the continents are constructed of diorite and granodiorite - type
rocks.
here are typically a volcanic variety known as andesite, and a plutonic variety known as diorite. These are
the primary rocks from which the Cascade Volcanoes are made. Andesite comes in a variety of colors, but
typical shades are gray, green-gray or red-gray. A slighly more silica-rich volcanic rock is dacite, and its
plutonic equivalent in granodiorite. These are also common rocks of the Cascade volcanoes.
Figure 7 (Right)
The Belknap Shield
Volcano, in east-central
Oregon. This is a typical
shield volcano, displaying the characteristic low
profile of such features.
Black basaltic lava covers
the area, from an eruption
about 1500 years ago.
Image:
US Geological Survey
6
Figure 8 (Right)
Eruption of basaltic magma,
on Mauna Loa, Hawaii. Note
the relatively quiescent form
of eruption, forming fountains of magma.
Image:
US Geological Survey
Figure 9 (Below)
Eruption of andesite magma
on Mt. St. Helens. Note the
relatively violent form of
this eruption, throwing vast
quantites of material from the
volcano.
Image:
US Geological Survey
Because andesite is a thick magma, flows do
not travel far from their source. As a result,
they build up into tall cone-shaped “stratovolcanoes.” The Cascade Volcanoes are typical
examples of this type. They are also known as
“composite volcanoes,” because their composition is a mix of lava flows, ash, pumice,
tephra, scoria, and other volcanic debris which
these cones erupt. Many of these varieties
reflect the effects of the huge quantities of
volcanic gasses which are dissolved within the
magma. In a basaltic (e.g. Hot Spot) volcano,
where the magma is thin and runny, those gasses can quietly bubble out of the liquid without much effect. This is similar to the effect
seen in an opened bottle of carbonated soda.
The resulting volcanoes, as typified by those
on Hawaii, host relatively quiescent eruptions.
In andesite volcanoes, like the Cascades, the
thick viscous lavas don’t allow those gasses
to escape as easily. As a result, eruptions of
andesite volcanoes tend to be explosive affairs.
The effect is similar to that which one would
get opening a well-shaken bottle of highlycarbonated toothpaste.
7
Figure 10 (Above)
Rapidly-eroding rock on Mount Rainier. Note the large blocks in some layers, and the generally poor degree to which things are
held together. These rocks are loosened by a host of adverse environmental conditions, and are altered by the effects of volcanic
gasses, which turn certain minerals into forms of clay. Image: From Pierce Co. CC (web photo).
Accordingly, the Cascade volcanoes are thick piles constructed out of layers of lava, frequently containing
large lava blocks, and alternating with layers of volcanic crud ranging in size from fine ash to big chunks of
rock ejecta. Many of these are pyroclastic layers, welded together as glowing avalanches of debris. Others are volcanically - entombed debris slopes, accumulated from disintegrating rocks above. The resulting
pile is often not a particularly durable structure. Glaciers easily carve into the softer layers, and rip out the
intervening lava beds. Water flows easily through various layers, mixing with volcanic gasses to form acids
which dissolve the rocks. Frost-wedging and thermal expansion take a toll on exposed rocks, as does the
incessant precipitation.
Volcanoes like this are often subject to large-scale structural failures, resulting in massive debris flows
(lahars) which fill the valleys around such mountains. In the end, volcanoes of this type are fairly transient
features on the landscape, quickly succumbing to the effects of the environment. Unless replenished by continuing activity, they fall apart on a scale of tens of thousands of years.
Figure 11 (Right)
Mount Adams, from the Goat Rocks Wilderness. Mt. Adams is the second-largest volcano in the state, at over 12,000 feet.
The view here is from the Goat Rocks region, the site of an ancestral volcano which existed prior to the modern cone of
Mt. Adams. The Cascade Volcanoes are all built upon the remains of older volcanic piles, at sites of repeated eruptive activity.
Image: Darryl Lloyd
8
9
Mount Baker
Koma Kulshan
10,778’
(3285m)
Rising just 30 miles east of the tidewaters on Bellingham Bay, Mount Baker is the dominant feature on
the eastern skyline from the Strait of Georgia to the Puget Sound. Owing to its northern latitude, Baker is
(pound per pound) the most heavily glaciated volcano in the chain. A dozen major glaciers stream from the
summit area, or are situated on its massive flanks. The northern face of the mountain has been carved into
an immense amphitheater, while the southern flanks have retained more of their classic conical profile. The
summit crater has been filled in by rock and ice, producing a flat summit area about ¼ mile square. This
gives the peak its distinctive flat-topped appearance (Kulshan – “shot at very point”).
Mount Baker is only the most recent in a long lineage of volcanoes in this area, stretching back some 36
million years to the inception of the modern Cascade Arc. The region east of Mt. Baker is dominated by
granite-type rocks of the Chilliwack Batholith, the roots of older Cascade-Arc volcanoes which erupted in
this area through about 7 million years ago. Between 5 and 7 million years ago, the north-south axis of the
Cascade Arc started to migrate westward, into the modern (northwest-trending) Mt. Baker – Garibaldi axis.
This reflects a steepening of the angle at which the Juan De Fuca Plate was being subducted.
Figure 12 (Left) Mount Baker, from Skyline Divide. View is to the south. Note the deep erosion of the north side of the peak.
Figure 13 (Above) Mount Baker, from Sauk Mountain. View is to the northwest. From this angle, the mountain preserves its
classic conical form. Peaks on the left skyline are the Black Buttes, the remnants of an earlier volcano.
11
Queen
Charlotte
Fault
Explorer
Plate
05 Ma - Present
Trace of the Cascade Arc
Vancouver
Island
36 - 07 Ma Trace
of the Cascade Arc
Garibaldi
Pemberton
Diorite
Nootka
Fault
Baker
PAcific
Plate
Chilliwack
Glacier
(Cloudy
Pass)
Juan De Fuca
Plate
Figure 14: (Above)
Map showing the migration of the Cascade Arc from the (36 - 07 Ma) Chilliwack - Pemberton axis to the modern
(05 Ma - present) Baker - Garibaldi axis. This happened as the angle of subduction increased after the Explorer
Plate broke off with the development of the Nootka Fault.
This progressive westward migration of the Cascade Arc
can be seen in the 4 Ma Lake Anne Stock just east of Mt.
Baker. These granite-type rocks are the plutonic “root” of
a volcano which erupted early in the uplift of the modern
Cascade Range. The volcanic edifice has long been lost
to erosion, but continuing uplift of the modern range has
exposed its plutonic roots. The cone of the modern Mt.
Baker Volcano has been built on this uplifted and eroded
region.
Figure 15 (Right)
The 4 Ma Lake Anne Stock, just east of Mt. Baker (Mt. Shuksan to
the rear). This stock fed a volcano which formed during the early
uplift history of the modern Cascade Range. With the uplift of the
range, the volcano has long since been eroded way. What remains
are the deeper plutonic rocks at its root. Mt. Baker is built on this
uplifted and eroded terrain.
12
Lake AnneVolcano
4 Million years ago
Figure 16 (Above)
The Kulshan Caldera, on the northeast side of Mt. Baker (seen in backround). See map below for location. This was a massive
caldera blasted out about 1.1 million years ago, probably in the middle of an ice age. Ash from this eruption has been found as far
as Lake Tapps, east of Tacoma.
The recent history of the Mt. Baker
area dates from about 1.1 million years
ago, when a 6-square mile area on what
is now the east side of the volcano
was blasted out in an extremely violent explosion which produced a huge
caldera (a crater. largely filled with
eruptive debris) here. This happened as
British Columbia
rising magma contacted large amounts
of sub-surface water, probably as the
area was mantled by a heavy icecap.
This is known as the Kulshan Caldera,
Kulshan
perhaps the earliest event in the history
Caldera
of the modern peak. More recent were
the eruptions of lava which formed the
Black Buttes, on the west side of the
modern cone, perhaps 200,000 years
ago. These are the remnants of an earlier volcano, rising to an elevation of
about 10,000 feet. As this vent closed,
activity moved east to its current locus.
Figure 17 (Right)
Map showing the location of Mt. Baker, the
Lake Anne Stock, and the Chilliwack Batholith. The Chilliwack Batholith represents the
plutonic “roots” of Cascade Volcanoes which
grew here between 36 and 7 million years ago.
13
Chilliwack
Batholith
Lake
Anne
Stock
Older Black Buttes Volcano
Figure 18 (Above)
Mount Baker from the north, showing the Black Buttes, the remnants of an older volcanic peak. Note that the rocks in the foregound are not volcanic species. Baker was constructed on a high ridge of about 7,000 feet,
There were likely earlier, perhaps smaller versions of Mt. Baker produced at this location, the rocks of
which lie buried under the modern cone. The modern cone has apparently been constructed over the last
50,000 years, given the degree of dissection by continental-scale ice. The south side has not been deeply
incised, and presents a youthful appearance. The rocks which make up Mt. Baker are a somewhat more
durable mix than is typical of our volcanoes, being in large part lava flows, with only a minor proportion of
pyroclastic debris. The summit crater became closed near the end of the construction of the modern cone,
and activity shifted to the current (Sherman) Crater on the southeast side.
Figure 19 (Right)
Mount Baker, from the
southeast. This view is up the
Rainbow and Park Glaciers.
Sherman Peak is the small
crest on the left skyline.
Image: John Roper
14
Figure 20 (Right)
Air view of Mt. Baker, from
the southeast. The main
summit is to the rear. The
summit in the foreground is
Sherman Peak. The crater
lies between the two summits, at the head of the
Boulder Glacier.
Sherman Peak
Image:
US Geological Survey
Talum Glacier
Boulder Glacier
The last major eruption from Sherman Crater was in 1870, but the crater was blowing small amounts of ash
as recently as the 1970’s. The peak has a long history of eruptive activity, as chronicled by native tribes and
early settlers. A sizable cinder cone erupted in the Schriebers Meadow area on the south flank of the mountain, sometime between 10,350 and 6,900 years ago. The largest recent eruption was about 8,700 years ago,
when a succession of pyroclastic, mud and lava flows formed the large fan-shaped fill in the Boulder Creek
Valley on the east side of the peak, descending all the way to Baker Lake. This may have been the event
which formed the modern Sherman Crater.
Figure 21 (Left)
Air view of Sherman Crater,
showing extensive staining of the ice by volcanic
emissions. Sherman Peak
is to the lower right, while
the actual summit is to the
rear. The glacier in the
foreground is the Easton
Glacier.
Image: W. WA University
15
Figure 22 (Above)
Volcanic hazards map for the Mt. Baker area.
Map: US Geological Survey
The major danger from Mt. Baker lies not from eruptive activity, but from mud and debris flows off its
flanks. On the south side of the mountain, lahars (mudflows) have swept as far as twenty miles down the
valleys of Rainbow, Sulpher Creek, Boulder Creek and the Park Creek. These drain into Baker Lake and
Lake Shannon, lakes impounded by hydroelectric dams on the Baker River. About 6,000 years ago, a huge
lahar (mudflow) surged down the Middle Fork Nooksack Valley on the west side of the peak, extending at
least eighteen miles. Older lahars have been also found on the North Fork Nooksack River, so no side of the
mountain is exempt from this threat.
16
Figure 23 (Above)
Mount Baker, from the Baker River. This is the side
of the mountain most vulnerable to lahar flows from
the crater area. The major point of concern is the two
large lakes below (see map, opposite page), which are
impounded by dams. The Baker River Dam below Lake
Shannon is a tall concrete arch dam, built not long after world war I. The town of Concrete lies just below it.
Figure 24 (Right)
Lahar deposits along the North Fork Nooksack River,
near Glacier, WA. Note large tree trunks carried in the
mix. This location is about 10 miles from the upper
slopes of Mt. Baker.
Image: US Geological Survey
17
Koma Kulshan
Kulshan was a handsome young brave of the Nooksack Tribe, who took two wives. One of those wives bore him three
children, while the other had none.The wife with his children became jealous of the other wife, feeling that Kulshan
should have the strongest affections for her, the mother of his children. Kulshan loved both of his wives equally, so he
said nothing.
The wife with his children decided to fool Kulshan by threatening to leave. She didn’t want to leave, but figured that
Kulshan would tell her that he loved her the most, and beg her to stay. But Kulshan was a proud man. When she
threatened to leave, he offered no argument. He didn’t want her to go, but he wasn’t going to beg her to stay. He said
nothing.
So she made a pack of all the good things around Kulshan- the flowers, seeds, roots and berries. She put these on her
back and headed south, certain that Kulshan would break down and call her back. But he didn’t call her back. She
kept traveling south until she found a high hill, and finally settled there. Around her she scattered all the seeds, flowers, berries, roots and the other nice things that she brought from Kulshan. Straining to look north to see her husband
and children, that woman grew into a tall mountain.That mountain is now Tahoma (Mt. Rainier).
The other wife stayed with Kulshan for many years, and eventually carried his child. She wanted to visit her mother
while the child was born, her mother who lived out in the Puget Sound. Kulshan summoned all the animals with claws
– bears, lions, marmots, beavers, mice and everything else that digs, and had them dig a channel to the Puget Sound so
that his wife could take a canoe to her mother.That deep channel is now the Nooksack River.
That wife went down the river in a big canoe, and when she came to the water, she left on each island a fish, berry,
or something else good to eat. So all the San Juan Islands between the Nooksack and her mother’s home had native
names for things to eat. She had her baby, but decided that she would stay with her mother rather than returning
home. At first she thought that she would stand up high in the water so to see Kulshan, but then decided that no, that
other wife chose to stand tall. Instead, she lay down so that all peoples could reach her head without climbing. She is
now Speiden Island.
Kulshan never married again, and lived out his life alone. He grew taller, so he could better see his wives and children.
Those children near Kulshan grew taller so that they could better see their father and mother.They are tall mountains
now, although none so tall as Kulshan.
Lummi Tribal Legend
18
Figure 25
Mount Baker from Austin Pass
19
Glacier Peak
DaKobed
10,451’
(3212m)
Viewed from the Puget Sound region, Glacier Peak is the least conspicuous of our Cascade Volcanoes. Although only about 300 feet lower than Mt. Baker, it is located much further to the east, well within the western ranges of the Cascades. It lies just a few miles off the Cascade Crest, some 60 miles east of the Puget
Sound tidewaters. While it can be seen from the lowlands, its silhouette is not the dominating presence
enjoyed by Baker or Rainier. From such a distance, it is easily lost amidst the number of prominent summits
in the north-central portion of the range.
Unique among its Cascade brethren, Glacier Peak has been preserved as a wilderness volcano, the centerpiece of the Glacier Peak Wilderness created in 1964. It is not accessible by motor vehicle, and can be
reached only on foot or by horseback. Indeed few roads even provide a view of this magnificent peak, so
relatively few people are familiar with it. It achieved national notoriety in the early 1960’s, when a huge
photograph of the mountain was displayed in New York’s Grand Central Station, part of the successful campaign to preserve the peak in its wilderness state.
Figure 26 (Left)
Glacier Peak from Image Lake
Figure 27 (Above)
Glacier Peak from the North
Image: John Roper
21
Glacier Peak is the most recent volcano in
a long legacy of volcanic cones which can
be traced back into the 22 Ma Cloudy Pass
Batholith, just northeast of the modern peak.
The modern cone erupted high on the eastern
side of Lime Ridge, at an elevation of perhaps
8,000 feet. This probably happened over the
last 100,000 years. Much of the peak was
built of dacite, a more felsic (silica rich) rock
Cloudy Pass
Batholith
Glacier Peak
Figure 28 (Above)
Glacier Peak, from the northwest. The mountain is
highly dissected on this side.
Figure 29 (Right)
Map showing the relationship between Glacier Peak
and the older (~22 Ma) Cloudy Pass Batholith. The
magmas of the Cloudy Pass Batholith once supported
a volcano here, long before the rise of the modern Cascade Range.
22
than is typical of the (andesite) Cascade volcanoes.
Moreover, it was an atypically fluid dacite magma,
flowing east into the Suiattle Valley. This growing accumulation of rock eventually forced the Suiattle River
into its circuitous route around the east side of the
peak. At a later date, flows overtopped the crest of lime
ridge, and extended west into the White Chuck Valley.
The modern cone only rises about 2,500 feet above the
crest of Lime Ridge.
The earlier, more fluid magmas of Glacier Peak were
replaced by a more viscous variety later in its history.
Dacite is typically a thick, pasty magma, capable of
holding large amounts of dissolved gasses. In the fairly
recent past, violent eruptions have wracked the mountain, sending huge clouds of ash to the east. The most
notable of these was about 13,000 years ago, erupting over 2.5 cubic miles of debris. This ash layer is an
important geochronographic marker unit across much
of the northwest. Not long after this event, a series
of massive pyroclastic flows swept down the White
Early Glacier Peak
1. Early Glacier Peak erupts on the northeast side
of Lime Ridge, in the Suiattle Valley
Peak Overtops Lime Ridge
2. Glacier Peak grows to overtop Lime Ridge, flowing west into the White Chuck Valley
Figure 30 (Above)
Glacier Peak from the northeast, views of the Chocolate and
Dusty Glaciers
Figure 31 (Right)
A series of panels illustrating the evolution of the Glacier Peak
region.
Modified from Tabor and Crowder, 1969
Modern Setting
3. Dacite domes and cinder cones erupt on the
flanks of the modern peak.
23
Figure 32 (Right)
Seattle
Map showing the distribution of ashfall from the
13,000 year-old eruption of Glacier Peak. Across the
area, this ash layer serves as an important geochronographic marker. The total area affected is somewhat
larger than indicated here. The ash is a unique buff
color, and is easily recognized in the field. It is an important chronographic marker because it was deposited at the end of the last ice ages.
Spokane
Portland
Missoula
Baker City
Boise
Chuck Valley, with thick lahars reaching all the way to modern-day Darrington. These flows changed the
course of local rivers. The Sauk and the Suiattle Rivers used to drain south and west via the Stilliguamish
River, a course subsequently blocked by debris flows. They now drain north to the Skagit River, down the
Sauk Valley.
Starting between 5,700 and 6,000 years ago the mountain started extruding a series of dacite domes on the
southern flank of the peak, domes which collapsed and sent pyroclastic avalanches down into the White
Chuck Valley. These flows buried large forest areas in volcanic debris. Between 5,000 and 5,500 years ago
huge pyroclastic flows swept down the east side of the mountain, depositing over a cubic mile of debris.
Large pyroclastic flows swept down the east and west slopes of the mountain between 1,700 and 1,800 years
ago, along with massive lahars which flowed into the valleys below. Some of these reached as far as twenty
miles from the peak. More pyroclastic and mudflows date from about 1,000 years ago, and from as recently
as 300 years ago.
Figure 33 (Right)
The White Chuck Cinder
Cone, in the upper White
Chuck Valley. This is a dacite cone, a relatively recent
development on the scene. In
large part, it is a pile of cinders (scoria) which erupted
from a central vent.
24
Despite its remote location, Glacier Peak is a hazard to modern residents in a much broader area. It’s propensity for violent explosive eruptions threaten large regions to the east with heavy ashfall. The massive
lahars which have swept down the valleys headed on this mountain have changed the course of rivers, and
have reached as far as modern communities. Large pyroclastic eruptions hold the potential for igniting huge
wildfires, or producing catastrophic floods on the surrounding rivers. It is the center of a dynamic, changing
landscape; one which will continue to develop with little regard for the modern residents.
The local Sauk-Suiattle Tribe knew this mountain as DaKobed – the “Great Parent.” It is a spectacularly
beautiful peak, a favorite among nature photographers. The thick layers of ash which it deposited across the
Figure 34 (Above)
Glacier Peak from the southeast. The protrusion on the left
skyline is the Disappointment Cleaver, the remains of a dacite
dome which was extruded late in the volcano’s history. Note
the extensive cover of glaciers.
Figure 35 (Right)
Glacier Peak from Cub Lake. The rocks here are part of the
Cloudy Pass Batholith.
25
Figure 36 (Above)
Map illustrating volcanogenic hazards in the Glacier Peak area
Map: US Geological Survey
26
surrounding landscape nurture luxuriant parklands of lush herbaceous meadows, providing a dramatic contrast with the brilliant ice-covered cone above. Our only wilderness volcano, it provides a glimpse on what
this amazing region looked like before modern man settled the area.
Figure 37 (Left)
The upper slopes of Glacier Peak, from the northwest. The large icefield is the
Kennedy Glacier. The left-hand skyline is known as “Frostbite Ridge.”
Figure 38 (Above)
Glacier Peak from the east, near Buck Creek Pass. The high country around Glacier
Peak hosts luxurient parkland meadows, a legacy of thick accumulations of
volcanic ash and tephra across the region. Suiatt;e valley in the foreground.
image: Walter Siegmund
27
Mount Rainier
Tahoma
14,410’
(4392m)
Rising 14, 410 feet in just 40 miles from the tidewaters on Commencement Bay, Mount Rainier is an absolutely dominating presence over the southern Puget Sound Lowlands. The local natives knew the mountain
as “Tahoma,” and revered it as their principal deity. It now stands as an icon of the modern northwest, our
most often-recognized landmark. It provides an absolutely magnificent backdrop for the metropolitan communities on Puget Sound.
Mount Rainier is the tallest and the oldest of the modern Cascade volcanoes. Like the others, its history can
be traced into the older volcanic and plutonic rocks in the area. Unlike regions to the north however, uplift
of the modern range has not eroded away all of the older volcanic cover here. . Accordingly, Rainier is built
on a thick pile of volcanic and plutonic rocks accumulated over the past 35 million years. These include the
35 – 25 Ma Ohanapacosh volcanics, the younger Stevens Ridge and Fifes Peak volcanics, and the ~18-20
Ma Bumping Lake and Tatoosh Plutons. These are the remains of a long succession of volcanoes which
have grown here, and which were eroded away with the passage of time.
Figure 39 (Left)
Mount Rainier from the north, view is up the White River
Figure 40 (Above)
Mount Rainier from the North, Little Tahoma on the left skyline, the Emmons Glacier to its right.
29
Recent volcanics
Snoqualmie
Batholith
Ellensburg Form.
C.R. Basalts
Miocene Plutons
Tertiary Volcanics
Eocene Sed. Rocks
Pre-Tertiary
Tatoosh
Pluton
Figure 41 (Right)
Rainier
Bumping Lake
Pluton
Map of the Mount
Rainier area, showing
Tertiary and Miocene
rocks of the Cascade
Arc, and their relationship to Mt. Rainier
The modern cone of Mt. Rainier started accumulating almost a
million years ago, erupting on a high plateau of older Cascade-arc
volcanic rock. The early eruptions may have been sporadic, and
interrupted by long periods of erosion. The periodic formation
of glaciers must have cut heavily into the accumulating pile, but
recurrent eruptions added new material on a regular basis. Its accumulation represented a continuing battle between volcanic eruptions and the erosive effects of periodic glacial episodes. Much of
the peak is likely constructed out of material accumulated over the
last 250,000 years.
The mountain reached its greatest extent about 75,000 years ago,
reaching an elevation of about 16,000 feet. At this stage it was
a fully mature feature, displaying the classic conical shape of a
composite stratovolcano. Despite the continuing effects of glacial
erosion, it maintained much of this stature through the end of the
last ice age, about 12,000 years ago.
Figure 42 (Right)
Mount Rainier, from above Snoqualmie Pass
30
Rainier is an andesite volcano, built of lava flows,
pyroclastic deposits, tephra layers and other volcanic
ejecta. It is not a particularly well-constructed edifice,
and periodically falls victim to the ravages of time and
the forces of gravity. A major collapse took place during a relatively minor eruption about 6,600 years ago,
where the entire west slope of the mountain slid down
into the White River Valley. This is known as the Greenwater Mudflow, and it filled the White River Valley to a
depth of several hundred feet. It triggered an avalanche
and an ensuing lahar on the Nisqually Glacier, burying
the Paradise Valley to a depth of 800 feet. These lahars
totaled about a fifth of a cubic mile of material. This was
the first in a succession of slope failures and mudflows
which have shaped the modern peak.
Figure 43 (Above)
Air view of Mt. Rainier. Note large depresssion in summit area,
caused by the Osceola Mudflow. It has since been re-filled to some
degree. Image: US Geological Survey
Figure 44 (Right)
The Nisqually Glacier, above Paradise. Note the abundance of rock
on the ice.
31
Figure 45 (Right)
Map showing the extent
of the recent Osceola and
Electron Mudflows. Note
the proximity of towns like
Enumclaw and Orting to
these flows. Map only shows
major areas of inundation.
Osceola flows continued
to Auburn and Kent, and
reached the shores of Puget
Sound.
Modified from USGS Image
Auburn
Puget
Sound
Osceola
Mudflow
5700 years ago
Tacoma
Enumclaw
Puyallup
Orting
Electron
Mudflow
600 years ago
Figure 46 (Below)
A volcano / lahar warning
sign, in the town of Orting.
Mt. Rainier
20 Km
Figure 47 (Left)
Deposits of the Osceola
Lahar, below the town of
Greenwater. The lahar here
is about 4 meters (12 feet)
thick, constrained by the
walls of the White River
Valley. Note the large boulders in the mix.
Image:
US Geological Survey
32
Figure 48 (Right)
The town of Orting, on the Puyallup River. The flat character of the
valley bottom is a reflection of the
extensive lahar deposits here.
Figure 49 (Below Right)
In the town of Orting. Developers
dig up buried stumps before building new homes. Person provides
scale. Orting boasts a lahar-warning system, and practices evacuation drills regularly.
The most massive lahar occurred about 5,700 years ago, known as
the Osceola Mudflow. This started with a catastrophic collapse of
the summit crater, as nearly a half a cubic mile of material swept
down the White River Valley. These flows buried over 125 square
miles of the Puget lowlands, including the modern-day sites of
Kent, Auburn, Sumner and Puyallup. They extended some 65
miles before reaching the tidewaters of Puget Sound. In the process, the mountain lost about 2,000 feet in elevation (see figure
43). These events have continued into historical times. The Electron Mudflow, just 600 years ago, buried the valley now occupied
by the thriving suburban community of Orting.
After the Osceola collapse left it disfigured, the mountain started
rebuilding its summit cone about 2,500 years ago. A series of eruptions involving pyroclastic flows and local lahars (mudflows) filled
the summit breach with about 500 feet of new material. This gave
the mountain its current form. While no eruptions have been recorded in recent times, reliable reports cite eruptions in the 1820’s,
40’s 50’s, 70’s and 90’s. There is current thermal activity in the
summit area, eroding a series of tunnels into the icecap.
Figure 50 (Left)
Mount Rainier, in the wintertime. View
is from the southwest. Little Tahoma
on the left skyline.
33
34
Figure 51 (Left)
Rainier area hazards map, US Geological Survey
Figure 52 (Above)
Mount Rainier, Southeast side. Note well-defined layers of volcanic rock
Image: Wayniak
The modern setting of Mt. Rainier is both spectacular and beautiful. Its flanks are carpeted in luxuriant subalpine parklands, while its footings rest in deep forests and river canyons. The summit hosts the most extensive glacial system in the US, outside of Alaska, with 28 active ice streams perched on its sides. It is a very
popular destination for recreational and tourist activities, and is preserved in a small national park.
Owing to its proximity to major population centers, a large-scale eruption of Mt. Rainier would be a catastrophic event. The primary danger would be from massive mudflows (lahars) generated by melting glaciers,
mudflows which would sweep down the west-side valleys now occupied by towns and cities. More disconcertingly, even minor eruptive events are capable of generating huge mudflows, as repeatedly evidenced
in the geologic record. For these reasons, Mt. Rainier is considered the most dangerous volcano in North
America.
35
Mount Adams
Klickitat
12,276’
(3742m)
At over 12,000 feet, Mt. Adams is the second-highest volcano in Washington. It is not however, a well-recognized feature. It is not commonly visible from the Puget Lowlands, the better view being from the east
side on the Yakama Indian Reservation. It is a massive and imposing peak, clearly constructed of three overlapping cones along a northwest-southeast axis.
The mountain owes its name to an interesting coincidence in history, dating from the late 1830’s, as part of
Hall J. Kelley’s plan to name the major peaks of the Cascade Range after the American Presidents. Kelly
intended the name for Mt. St. Helens, but on his map he inadvertently located the title well to the east of
that peak. By a curious coincidence, there was a large nameless volcano in that area, to which the name was
subsequently applied. The Yakama Indians knew the mountain as Klickitat, who with his brother Wy’east
(Mt. Hood) fought for the favor of Loo-Wit (Mt. St. Helens). Half of the mountain now resides within the
Yakama Reservation. The other half is in a small wilderness area.
Figure 53 (Left)
Air view of Mt. Adams. Located in the southern Cascades of Washington, this is a region of high rolling hills which accentuate
the impact of this large volcanic massif.. Image: US Geological Survey
Figure 54 (Above)
Mount Adams from the south. From this perspective one can clearly see the mountain as a series of overlapping cones.
Image: John Roper
37
Mt. Rainier
Map Area
Figure 55 (Right)
Randle
Map showing the location of
Mt. Adams, and Mt. St. Helens, along with Mt. Rainier
and Mt. Hood.
The map also shows the
location of the older Goat
Rocks volcanic center, and
the three more recent volcanic centers in the area. These
include the Simcoe Mountains Volcanic Field (SMVF),
the younger Mount Adams
Volcanic Field (MAVF) and
the younger Indian Henry
Volcanic Field (IHVF).
Goat
Rocks
Mt. Adams
Mt. St. Helens
SMVF
MAVF
IHVF
Goldendale
The Dalles
Portland
Mt. Hood
Like the other volcanoes of Cascades, Mt. Adams is built on the remains of a long legacy of volcanic forms
built in this area over the last 35 million years. North of the modern cone, the volcanics of the Goat Rocks
area are the remains of a fairly large volcano which rose here in mid to late Pleistocene time. The rocks beneath Mt. Adams show a history of eruptions dating from 275-200,000 years ago, and between 150,000 and
100,000 years ago. This later episode includes the eruption of the Goat Butte volcanics on the south flank of
the modern peak. These older accumulations were heavily eroded by repeated episodes of glaciation.
Figure 56 (Right)
The Goat Rocks region,
the site of an older volcano
which has since fallen victim
to erosion. View is to the
north, Mt. Rainier in the
distance.
See also: Figure 11
38
The modern cone of Mt. Adams was likely constructed during the last ice age, between 25,000 and 12,000
years ago. In contrast to St. Helens and Mt. Rainier, it was built largely of andesite flows, with relatively
little tephra or pyroclastic debris. Some of these flows were quite thick, on the order of 200 feet. The peak
evolved through the successive eruption of a series of overlapping cones, accumulating to an extent not significantly in excess of that seen today. It is a massive mountain with a base covering over 250 square miles,
and a volume of over 18 cubic miles of material. A large number of smaller cones and vents surround the
peak, and the region is covered with deceptively fresh-looking lava flows from these features. Most are over
Figure 57 (Above)
Mt. Adams from the east,
from a locale north of
Goldendale.
Figure 58 (Right)
Mt. Adams from the south.
Note fresh landslide debris
in center of image.
Image:
US Geological Survey
39
10,000 years old. The youngest flows are the Muddy Creek flows, erupted between 2500 and 3500 years
ago.
Owing to its rather durable structure, Adams has not been the site of repeated mudflows or other collapse
features. While a system of local glaciers has cut deeply into the peak, it still retains its original outlines as a
volcanic pile. Some of those glaciers (particularly, the Klickitat) have carved spectacular exposures into the
lava flows which comprise the peak. The mountain has not been verifiably active in modern times, although
gasses are being emitted from the summit area. Heavy deposits of sulpher at the main vent were the basis for
a short-lived mining operation, which ran pack trains to the summit. In its time, it was the highest working
mine in North America.
Figure 59 (Above)
Mount Adams from the south.
Note extensive lava fields
(Indian Henry Volcanic
Field) in foreground.
Image:
US Geological Survey
Figure 60 (Left)
Mount Adams from space.
Note the NW - SE axis of
the peaks which form the
mountain.
Image: NASA
40
Given its lack of recent eruptive history, and a history not marked by extensive lahar events, Mt. Adams is
considered the least dangerous of the Cascade Volcanoes in Washington. It is also located well away from
major population centers, in a sparsely-settled region. It is a truly unique province, featuring the combination of a high volcanic peak rising above the more subdued hills of the southern Cascade Range. The high
country around the peak features extensive parklands of grass and heather, and an abundance of wildflowers
in season. Perhaps for a lack of familiarity, it is not as popular a locale for recreation as one might expect for
such a spectacular setting. In compensation, it typically offers a degree of solitude not found in the nearby
National Parks and recreation areas.
Figure 61 (Above)
Mount Adams from the west .
Image:
US Geological Survey
Figure 62 (Right)
Mount Adams from the northwest. The large glacier in the
center is the Adams Glacier.
Prominent north ridge on the
left side.
41
Mount St. Helens
Loo-Wit
7,200’
(2195m)
Prior to the 1980 eruption which destroyed much of the peak, Mt. St. Helens was a truly beautiful mountain, known as the “American Fujiyama” for its perfect symmetry. From an elevation of 9677 feet, 11 glaciers descended from its summit, including the large Forsyth and Wishbone Glaciers on the north side. The
smallest and westernmost of the Cascade Volcanoes, it was a popular locale for recreation and tourism.
The perfect conical form of Mt. St. Helens was a clue to the relative youth of this mountain. All of this peak
was constructed after the last ice ages of 20-12,000 years ago, most of it having been built over the last 5000
years. The magmas of St. Helens are largely andesites and dacites, and frequently produce explosive eruptions. St. Helens has a legacy of voluminous ash eruptions. The peak is constructed of layers of lava flows
interspersed with layers of ash, tephra, scoria and other debris. It is not a particularly durable structure.
Like the rest of the Cascade Volcanoes, St. Helens grew atop a base of older volcanic and plutonic rocks.
The oldest rocks at the base of the modern peak date from about 50,000 years ago, with periodic eruptions
Figure 63 (Left)
Mount St. Helens, prior to the 1980 eruption. Note the symmetry of the peak. Mt. Adams in the background.
Figure 64 (Above)
Mt. St. Helens, after the 1980 eruption. (2003 photo).
43
over the next 45,000 years. The rocks of the modern peak began accumulating 4,500 years ago, starting with
a massive eruption which ejected some 2.5 cubic miles of ash. Eruptions 1,500 – 3,000 years ago accumulated a significant pile of material, as did eruptions from 1,700 to 2,400 years ago. Much of the modern cone
was built of material erupted between 500 and 350 years ago. This 150-year period of activity ended with
the emplacement of a large dacite dome on the summit, in about 1647. Cumulatively, this distinguishes Mt.
St. Helens as the most active volcano in North America over the last 4,500 years. There were a variety of
eruptions recorded between 1800 and 1857, including a particularly spectacular event in 1842 and 1843. The
mountain has never left any question as to its volcanic origins.
Figure 65 (Above)
Crater from the initial eruption of Mt. St. Helens. Note
surface cracks, reflecting the
swelling of the summit area.
Image:
US Geological Survey
Figure 66 (Right)
Mt. St. Helens, from the
south, before the eruption.
The Shoestring Glacier
descended from the summit
on this side.
Image:
US Geological Survey
46
Figure 67 (Right)
Photograph of the landslide
on the north side of the
mountain, moments before
the eruption. This was the
largest landslide ever witnessed by man.
The remarkable images on
this page were taken by Keith
and Dorothy Stoffel, a pair
of geologists who were flying over the mountain in an
airplane when the eruption
happened.
The modern cone of Mt. St. Helens was doomed when a large body of magma started rising under the
mountain in the late 1970’s. It was enough magma that it would have likely resulted in the destruction of the
summit dome even under the most benign of circumstances. As that magma rose into the peak in early 1980,
it precipitated minor eruptions accompanying large-scale deformation on the north flank of the mountain.
Structural weaknesses on that side permitted the mountain to “bulge” as it inflated with magma from below.
This bulge eventually accumulated to some 400 feet of displacement. This set up a situation which was
literally primed for cataclysm. Geologists were just beginning to recognize the gravity of the situation when,
all of a sudden, it happened.
Figure 68 (Right)
Photograph showing the
initial explosion in the
eruption of May 20. This
happened as the landslide
exposed magma within
the peak.
Image: Keith and Dorothy
Stoffel.
47
Image 69 (Left)
The eruption of Mt. St.
Helens, showing the plume
which rose to over 60,000
feet.
Image:
US Geological Survey
Image 70 (Right)
Ash plume, drifting northeast
from the mountain. This ash
wreaked havoc on Eastern
Washington.
Image: Charles Rosenfeld
At 8:32 on Sunday May 18, a magnitude 5.1 earthquake centered under the bulge initiated the largest landslide ever witnessed, as the entire north side of the mountain slid down towards picturesque Spirit Lake.
This uncovered the magma, which erupted explosively as the confining pressure was released. A massive
pyroclastic cloud swept down the mountainsides at speeds of up to 670 miles an hour, a churning hurricane
of hot boulders, glowing rock and coarse ash which swept across the landscape with devastating consequence. Blasting out of the north side of the mountain, it absolutely flattened 230 square miles of forested
landscape, felling tens of thousands of trees in a fan-shaped arc extending some 19 miles north from the
summit. The magnitude of the blast was amplified as hot magma was blown into Spirit Lake, creating a
violent explosion as the waters were vaporized to steam. As the eruption progressed, wave after wave of
pyroclastic flows swept the north side of the mountain, searing the region and leaving debris scars across the
landscape.
Image 71 (Right)
A team of geomorphologists surveys the scene after
the eruption. As it turned
out, some life managed to
survive, underground.
Image:
US Geological Survey
49
Figure 72 (Above)
Downed timber on the flanks of the mountain. Hundreds of square miles of timber were flattened in this event.
Image: US Geological Survey
A huge mushroom – shaped column of ash rose above the peak, to an elevation of almost 12 miles. Spreading east, this ash rained across eastern Washington, all the way to Montana. Yakima was blanketed in four
to five inches of the heavy ash, decreasing to about a half inch in Spokane. Transportation systems largely
ground to a halt, as the high-silica ash is highly damaging to machinery. In some places, it took a month to
clear the heavy accumulations away.
When the mountain erupted, the eleven glaciers which mantled the peak were very quickly converted to
steam and absolutely massive quantities of very hot water. This water mixed with rock, ash and debris of
all forms to create huge lahars which swept down the valleys heading on the mountain. These vast flows of
mud, including boulders, trees, and everything else stripped from the landscape, overfilled the valleys in a
seething flow of debris which coursed from the mountains. The North and South Forks of the Toutle River
carried immense quantities of debris to the Cowlitz River, to where it finally emptied into the Columbia
River some 17 miles to the south. Twenty-four hours later, the mud was still over 90 degrees F where the
Toutle river is crossed by Highway 99.
50
Figure 73 (Right)
A geologist inspects the
moonscape left in the wake
of the eruption. This location is at the head of the
Toutle Valley.
Image:
US Geological Survey
Figure 74 (Right)
Mudflow on the Toutle River.
This is some 40 miles from
the peak. The mudflows
stripped large amounts of
timber from the sides of the
river.
Image: Charles Rosenfeld
Figure 75 (Right)
Mt. St. Helens, after the
eruption. This location is
just below Spirit Lake.
Image:
US Geological Survey
51
Figure 76 (Left)
Air view of the Upper Toutle
Valley mudflow, not long
after the eruption. This massive flow completely filled
the valley, extending all the
way to the Columbia River.
This black-and-white image
may appear stark, but it was
a scene totally devoid of
any color. Gray ash covered
everything.
Image:
Charles Rosenfeld
Image 77 (Right)
Toutle River mudflow, several years later. The ash has
been washed off the trees,
but the scale of devastation
is still evident.
Image:
US Geological Survey
52
The events of this eruption profoundly changed the landscape around Mt. St. Helens. The peak had lost
some 1314 feet in elevation, with a gaping crater a mile wide now facing out the north side of the mountain.
The area to the north was a smouldering moonscape with a radically altered topography, a scene of incredible devastation. Hundreds of square miles of timber had been flattened in the blast, and the valleys radiating from the mountain were filled with thick accumulations of mud and debris swept from the slopes above.
The entire region was covered in a layer of fine volcanic ash, while a phenomenal 650 million cubic yards
Figure 78 (Above)
The crater of Mt. St. Helens,
showing the growing lava
dome. This is an early photo.
Since that date the lava dome
has grown considerably.
Image:
US Geological Survey
Figure 79 (Right)
View of Mt. St. Helens from
space. Note prominent crater,
opening to the north.
Image: NASA
53
Figure 80 (Above)
Volcanic hazards map for the Mt. St. Helens area
Image: US Geological Survey
of mud had been discharged into the rivers. Over a thousand people lost their homes, and at least 57 people
died from the effects.
Mount St. Helens continued to erupt periodically over the next few months, including some spectacular explosive events. By the end of the year, activity had shifted to an active dome-building phase, a phase which
essentially continued for the next twenty five years. Dome-building involved the construction of large dacite
domes in the crater, the first reaching a height of some 876 feet, completed by 1986. The second dome accumulated to some 1400 feet in height before activity ceased in early 2008. If this general pattern of activity
was to continue, the crater would be filled over the course of the next 200 years.
Mount St. Helens erupted in 1980, which was just after the modern theories of plate tectonics were broadly
accepted by American researchers. As a result, our knowledge about volcanoes was truly in its infancy.
Although armed with only the most general knowledge about the workings of these features, it was the first
time that an eruption occurred under detailed monitoring conditions. In the end, the mountain has yielded
a wealth of information on the workings of volcanoes, perhaps more than had been accumulated in all the
previous past. Mount St. Helens remains one of our most important natural laboratories on the processes of
volcanism, and will continue to serve in this capacity well into the future.
54
Loo-Wit
Long ago,Tyhee Saghalie, chief of all the gods, had two sons by the
names of Klickitat andWy’east.These brothers went down the Columbia
River looking for a place to settle, and became enamoured with the area
around The Dalles. They argued over who would live on what ground,
so Saghalie settled the matter by firing two arrows into the air, landing
on opposite sides of the river. Klickitat settled on the north side,Wy’east
settled on the south side. Between them, Saghalie built the Tanmahawis,
the Bridge of the Gods, so that their family could visit them.
In time, both brothers became attracted to a young maiden by the name
of Loo-wit, who could not bring herself to choose between them. Eventually, the brothers had a violent fight over her, a fight that shook the
landscape, felled forests, buried towns, and devastated the area. In the
fight,Wy’east hit Klickitat over the head with a club, permanently flattening it. In the end, their fight was so violent that it caused the Bridge
of the Gods to collapse into the river.
When Saghalie heard of the fight he came and put an end to it. Furious that they had destroyed the bridge, and fed up with their fighting,
he turned them into Mountains. Wy’east is now Mt. Hood, Klickitat is
now (flat-headed) Mt. Adams, and Loo-wit became Mt. St. Helens. The
remains of the Bridge of the Gods form the rapids at The Dalles, on the
Columbia River.
55
The Past and Future Cascade Volcanoes
The Last Volcanoes of Washington
The modern Cascade volcanoes are only the most recent features in a long lineage of volcanoes which have
risen and fallen along the coast here for the last 36 million years. Several hundred such volcanoes have
likely existed over this period, under a wide range of climatic conditions. Prior to 5 million years ago, those
volcanoes erupted on a broad coastal plain, typically rising to elevations of 3-5,000 feet. Volcanoes of this
type are not particularly durable constructions, and are eroded away relatively quickly in the absence of
continued volcanism.
As the modern Cascade and Olympic Mountains were uplifted over the past 5 million years, the accumulated volcanic cover was in part eroded away. In the Cascades north of Snoqualmie Pass, almost all the volcanic cover was eroded away, revealing the deeper plutonic “roots” of those older volcanoes. The modern Cascade Volcanoes have been constructed on top of this uplifted mountain range. All are built on the remains of
older volcanic edifices, rocks which often date back as much as a million years. Where older plutonic rocks
are exposed, that history can often be traced back tens of millions of years.
The modern volcanic peaks of Washington will continue to experience growth and destruction over the
next several thousand years, events which will certainly alter their appearance to some degree. The current
patterns of global warming, if unchecked, will result in the loss of most of their glacier cover over the next
century, and timberlines will advance on the mountains. Future generations of humans may never again see
these magnificent summits cloaked in ice. If they don’t practice better management of their ecosystems,
there simply won’t be many generations of humans into the future.
Figure 81 (Above) Mount Baker
Figure 82 (Above left) Mount Rainer, Seattle in the foreground. Seattle Times Photo
On a larger scale however, the continental icecaps will almost certainly once again form within the next
– 40,000 years, advancing on the region as the next ice age. It is almost a certainty that our modern civilizations will not be around to experience this event. That ice, along with subsequent glaciations, will periodically carve at the local volcanic peaks for at least the next several million years, and perhaps longer. As
volcanic vents close, the modern peaks will eventually erode away over the course of time. New volcanoes
will erupt in their place, building new mountains above the old. Given relatively constant rates of plate consumption, the rates of volcanic activity should remain fairly constant over the next few million years.
Given the westward progress of North America, it can be estimated that the continent will override the Juan
De Fuca Ridge in something like 20 million years, resulting in the extinction of the Cascade Arc. Before
that happens however, the Juan De Fuca Plate will probably continue to break up into a collection of smaller
plates. As this happens, the location of the Cascade Arc may migrate locally, and may also locally lapse into
extinction. Curious events typically surround the final demise of oceanic plates, so we might expect some
interesting developments over its final 15 million years or so. When the Juan De Fuca Plate is finally overridden, the Cascade Arc will be extinguished, and the existing volcanoes will succumb to erosion over the
ensuing years.
The modern Cascade Volcanoes are a product of the Cascade Arc, which has been established here for the
last 36 million years. Prior to that date, there has been a relatively continuous succession of continental arc
complexes which have developed across the state over the last 300 million years. These volcanic chains
have developed as oceanic plates to the west have been subducted beneath the North American continent.
With the demise of the Juan De Fuca Plate some 20 million years in the future, that setting will be lost for
the first time in 300 million years. It is unlikely that such a setting will be re-established for hundreds of
millions of years into the future. The last of the Cascade Volcanoes will be the last volcanoes here for a very
long time.
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A Few Selected References:
The CascadeVolcanoes: Books and Articles on Their Geology
Doukas, M.P. and Swanson, D.A. 1987 Mount St. Helens, Washington, with emphasis on 1980-1985 eruptive activity
as viewed from Windy Ridge Geological Society of America Centennial Field Guide – Cordilleran Section p 333-338
Dragovitch, J.D. McKay, D.T., Dethier, D.P. and Beget, J.E. 2000 Holocene Glacier Peak Lahar Deposits in the
Lower Skagit Valley, Washington. Washington Geology V 28 #1/2 p 19-21
Green, N.L. Armstrong, R.L. Harakal, R.E, Souther, J.G. and Read, P.B. 1988 The Eruptive history and K-Ar geochronology of the late Cenozoic Garibaldi volcanic belt, Western British Columbia Geological Society of America
Bulletin 100 p 563-579
Hammond, P. 1987 Lone Butte and Crazy Hills: Subglacial volcanic complexes, Cascade Range, Washington Geological Society of America Centennial Field Guide – Cordilleran Section p 339 – 344
*Harris, Stephen L. (1988) Fire Mountains of the West: The Cascade and Mono Lake Volcanoes Mountain Press
Publishing Co Missoula, 1988. A revised edition of Fire and Ice, the Mountaineers Publishing Co, Seattle 1976.
Hildreth, W. 1996 Kulshan Caldera: A Quaternary subglacial caldera in the North Cascades, Washington Geological
Society of America Bulletin 108 p 786-793
Hildreth, W. and Lanphere, M. 1994 Potassium-argon geochronology of a basalt-andesite-dacite arc system: The
Mount Adams Volcanic Field, Cascade Range of southern Washington Geological Society of America Bulletin 106 p
1413-1429
Lipman, P.W. and Mullineaux D.R. 1981 The 1980 Eruption of Mt. St. Helens, WA US Geological Survey Provessional Paper 1250 844p
Pringle, P.T. (ed) 1994 Mount Rainier, A Decade Volcano GSA Field Trip (in) Swanson, D.A. and Haugerud. R.A.
(1994) Geologic Field Trips in the Pacific Northwest V 2 Published by the UW Department of Geological Sciences in
conjunction with the Geological Society of America - Annual Meeting, Seattle WA 1994
*Pringle, P.T. (1993) Roadside Geology of Mt. St. Helens National Volcanic Monument Wa. Dept. of Natural Resources Information Circular 88
*Rosenfeld, C. and Cooke, R. 1982 Earthfire: The Eruption of Mt. St. Helens MIT Press
Tabor, R.W. and Crowder, D.F. (1969) On Batholiths and Volcanoes: Intrusion and Eruption of Late Cenozoic Magmas in the Glacier Peak Area, North Cascades, WA USGS Professional Paper 604
*Volcano: The Eruption of Mt. St. Helens (1980) Combined staffs of the Daily News (Longview, WA) and the Journal-American (Bellevue, WA) Longview Publishing Co. Longview, Madrona Publishers, Seattle.
* These are popular-interest books
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Websites: The CascadeVolcanoes
USGS Cascade Volcano Observatory
http://vulcan.wr.usgs.gov/
USGS Publications: Risks from Volcanoes:
http:/pubs.usgs.gov/fs/1997/fs165-97
USGS Publications: Glacier Peak
http://pubs.usgs.gov/fs/2000/fs058-001
Volcanocams:
http://www.fs.us/gpnf/volcanocams/msh
Volcanoworld Department of Geosciences, OSU
http://volcano.und/nodak.edu
Michigan Technological University
http://www.go.mtu.edu/volcanoes
Washington Department of Natural Resources:
http://www.dnr.wa.gov/researchscience/topics/geologicalhazardsmapping/pages/
volcanoes.aspx
http://www.dnr.wa.gov/publications/ger_field_trip_cascades_volcanoes.pdf
Websites: The Regional Geology of Washington
Washington State Museum of Natural History and Culture (Burke Museum)
http://www.washington.edu/burkemuseum/geo_history_wa
The CascadeVolcanoes: General Interest Books
Beckey, F. (1973-76) Cascade Alpine Guide (v 1-3) The Mountaineers Press, Seattle
Miles, John C. (1984) Koma Kulshan: The Story of Mt. Baker The Mountaineers Press, Seattle.
Molenaar, Dee, (1971) The Challenge of Rainier The Mountaineers Press, Seattle
Rusk, C.E. (1978) Tales of A Western Mountaineer The Mountaineers Press, Seattle
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A Few Last Words From Your Instructor:
The modern Cascade Volcanoes are the most recent features on this truly spectacular geologic landscape which
we call the Pacific Northwest. While they are a dominating presence on our eastern skyline, their only really
unique quality is that they are part of our modern geologic setting. They are only the most recent in a long history
of Cascade-arc volcanoes which have been growing here
for the last 36 million years, the most recent in hundreds
which have developed over this period.
Moreover, the Cascade Arc is only the most recent chain
of volcanoes which has marked the northwestern coast of
North America over time. This region has hosted a succession of at least four major volcanic regimes over the
last 200 million years, some far more extensive than the
modern Cascade episode.
Perhaps even more significantly, this long succession of
volcanic-arc regimes is only one part of a much broader
tapestry of geologic history which has produced this
very unique landscape. This is a region which has been
assembled from volcanic island chains and scraps of
Pacific seafloor, deformed into a series of mountain
belts, capped by a succession of volcanic chains, buried
in astonishing thicknesses of sediment, capped by voluminous eruptions of flood basalts, carved by repeated
episodes of glaciation, scoured by the most cataclysmic
floods known in geologic history, and only most recently,
as home to the modern Cascade volcanoes.
Your instructor. A veteran of nearly forty years of climbing in these ranges, and some three decades devoted
to their study, he enjoys a unique familiarity with these
mountains.
The Pacific Northwest is home to some of the most remarkable geology to be found anywhere on the
planet. No region can claim to a greater variety of rock types, or features them in more spectacular settings.
More significantly, no region affords such a remarkable venue on the truly collosal forces which drive the
dynamics of our planet, or such graphic illustrations on the variety of geologic processes which they support. There is simply no better place on the planet to see how the Earth works. There is certainly no better
place on Earth to learn and experience geology.
Most students who are taking this course are not planning to major in the sciences, and have other plans
for their immediate future. My intention is not to dissuade anyone from following their passions. My only
point is that, if you are planning on living in this area, you should know that you are living in the midst of
some of the most incredible geology in the world. You should know that the modern landscape that surrounds you is the product of a truly amazing course of geologic history, one that stretches back hundreds of
millions of years. You should recognize that you occupy a unique point in time and space in that course of
history, and in the ongoing geologic processes which will continue to shape this region into the future.
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Eldorado Peak, North Cascades
The rocks here are the deep plutonic “roots” of a volcano which grew in this area some 90 million years ago.
Image: Eric Andersen