*all but the later three photos in this paper were taken by me…

 

Introduction

 

Driving east on Highway 84 out of Portland, Oregon, the Columbia River Gorge rises in steep, vertical basaltic walls all around.  To the north, Washington slopes down into the river, where large slabs of rock tilt like ramps rising out of the Columbia, launching your eyes north.  To the south, Oregon juts steeply upwards, shrouded in a thick coniferous blanket of green and draped in the mist of plunging waterfalls.  In the center, rolls the Columbia River, one of only two rivers to dissect the Cascade volcanic range, and the largest river in the west.  With all of this, it is easy to miss what you can only glimpse through a small pass in the ramping cliffs of Washington. 

 

Tucked away in southern Washington, and accessible only via a tiny dead-end county road leading to the small community of Trout Lake, Mount Adams remains hidden to most.  The geologic history of this mountain is not necessarily unique or dynamic but does have its special qualities.  Mount Adams is not an eruptive mountain like we would think of with regards to statovolcanoes; that is, it is not “explosively” eruptive.  Built out of layer upon layer of andesite and the occasional dacite flows, Mount Adams is quiescent giant.  Both its bulk and height make it one of the most impressive Cascade peaks.  Yet it receives little attention, both for its remoteness and its volcanic laziness.  There are aspects of this mountain though, fiercer than any surge of magma.    

 

 

Present Day Setting

 

Mount Adams is presently the second largest Cascade volcano, having an approximate mass of 350 km3, and is the third highest peak of Cascade volcanic chain rising to 3,751 meters (Hammond, 39).  Mount Adams is in many ways the "forgotten giant" of the High Cascades.  Located in southwest Washington just 35 miles east of Mount Saint Helens, Mount Adams rises out of a rolling volcanic region rich in scoria cones and smeared by Quaternary basaltic lava-flows.  The peak itself is massive and broad and stands about 2500 meters above the surrounding landscape (Hopkins, 1).  Only a few small communities are located near the mountain, the closest, Trout Lake, lies to the southwest about 15 miles from the summit.  Evergreen coniferous forests take up the remainder of the area around Mount Adams.  Politically, this forested region is divided into three principle allotments: the Mount Adams Wilderness Area, the Gifford Pinchot National Forest, and the Yakima Indian Reservation.

 

The immediate area surrounding the cone of Mount Adams is host to three major drainages.  The Cispus, White Salmon, and Lewis Rivers all flow from tributaries originating on the west and south slope of Mount Adams, and the Klickitat River drains out to the east across the Yakima Indian Reservation (Hopkins, 3).  All of these drainages "...lie in prominent valleys, 300 to 750 meters deep..." and are fed by glaciers mantling the broad cone of Adams (Hopkins, 3).

 

The community of Trout Lake is located in a rather large, flat and wide valley called the "Trout Lake lowland" (Vallance, 5).  The valley ranges in width from 2 to 5 kilometers, is situated in the White Salmon River drainage, and is an important topographic feature of the region when discussing lahars, as we shall see later in this paper.  

 

Geologic History

 

The landscape surrounding and comprising Mount Adams is a geologic scrap-yard spanning over half-a-million years of High Cascade volcanic and glacial history.  Even the cone itself is a hybrid of at least two main-vents that are unrelated along with other smaller vents, and the slopes and flanks of Mount Adams are composed of a series of episodic lava flows and somewhere around 30 different scoria-cones ranging in composition and age (Harris, 195-196).  But before we delve into the present too quickly, we need to step back half-a-million years and have a look at the foundation of the beast.

 

Mount Adams is part of the High Cascade volcanic chain, which runs from Northern California to the south, up through Oregon and Washington, and finally to one last volcano across the border in southern British Columbia.  The High Cascades first sparked-up around 4 million years ago and built a large platform of basalts and tuffs upon which the higher peaks of our present day stratovolcanoes are built (Orr, 145).  Even today, the High Cascades are predominantly composed of basaltic lavas, which make up 85% of the total rock volume (Orr, 145).  The mountains that we see today are only the most recent addition to a long history of composite volcanic peaks, which rise out of the High Cascade chain, and Mount Adams is no exception to this.  The oldest rocks in the area surrounding Mount Adams are dated at around 520,000 years old (Vallance, 12).  These rocks are the remnants of an ancient and very extinct volcano that once dominated the region around where Mount Adams now stands.  This extinct volcano is referred to as the Hellroaring volcano and was "...centered about 5 km southeast of the modern summit [of Mount Adams]" (Vallance, 12).

 

Mount Adams first came to life around 460,000 years ago, but the vast majority of the rocks making up the present day cone range in age from 25,000 to 10,000 years old (Harris, 193).  Mount Adams is somewhat unique in that it is composed "...almost exclusively of lava flows" (Harris, 193).  The geology of the summit of Mount Adams is also rather distinct.  Unlike many of its fellow Cascade kin, Mount Adams' cone is not the product of a single large volcanic vent but rather consists of several different vents of various ages.  This feature is what gives the cone its broad and symmetric appearance and can be easily missed, as most people only see Mount Adams from the area around Trout Lake to the south.  From this angle, the peak is steep and narrow, as are most of the other Cascade peaks.  The best view of the mountains broad peak is from the west. 

 

Viewed from the west, however, the main cone has a broad, irregular profile that consists of a central summit closely flanked on the northwest and on the south by two subordinate summits.  The multiple summits are strongly suggestive of a compound structure, produced by lateral shifting of the vent over short distances...The separate summits, of course, could have formed by partial destruction of a single, larger summit, known as Pinnacle, may have formed this way.  (Hopkins, 61)

 

 

 

                                         

               

 

 

 

 

 

 

 

 

 

    

 South slope of Mount Adams as viewed                        West slope of Mount Adams, as viewed from

                      from Trout Lake.                                                 Forest Service road 23.

 

The summit of Mount Adams is nearly a mile wide north to south, and the true summit of the mountain is called the Pinnacle, which lies on the northwest end of the cone (far left of the upper right photo) (Harris, 195).  Suksdorf Ridge descends from the Pinnacle to the southern "false summit" of the mountain and is "the only unglaciated high part of the volcano" due to the fact that it is composed of numerous recent lava flows (Harris, 195).  According to Hopkins, the vent of the southern summit is unrelated to the central vent, which formed the Suksdorf Ridge flows, and the northern Pinnacle vents (63).  Many other smaller vents and scoria-cones abound on the flanks and throughout the area surrounding Mount Adams.  Harris notes that in just the past 10,000 years Mount Adams "...has erupted at least seven different times, mostly from vents below the summit, but at elevations above 6,500 feet" (194).  A large, blocky andesite flow called the A.G. Aiken Lava Bed erupted around 3,500 years ago from the south flank of Mount Adams and flowed approximately four and half miles south of its eruptive fissure (Harris, 195).

 

                            

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A.G. Aiken Lava Flow...blocky andesite!       Mount Adams south face from near toe of

                       A.G. Aiken Lava flow.

 

 

Composition of the rocks which make up the various peaks of the Mount Adams cone are all..."breccia, scoria, and lava of porphyritic andesite to basaltic andesite" (Vallance, 12).  Vallance also states that their mineralogy consists of "...phenocrystic hypersthene, augite, and plagioclase (ranging from andesite to labradorite)..." and also that "...the andesites have porphyryritic, pilotaxitic or hyalopilitic textures"  (15).  Pyroclastic flows are virtually absent from the geologic story of Mount Adams, and play no part whatsoever in its most recent volcanic episodes.  This has strong implications when looking at lahars and how they originate on the slopes of this steep mountain. 

 

 

The Glaciers with attitude

 

Glaciers play a major role in both the appearance and form of Mount Adams and in association with lahars.  Although there is no evidence that lahars on Mount Adams are the result of rapid glacial melt, glaciers do play a role, via their physical and chemical weathering on the volcanic ranks, which comprise the upper reaches of the mountain.  This process will be discussed in detail here in a few. 

 

Mount Adams is host to ten glaciers, five of which originate and precipitate from the summit area, while the other five find homes lower on the flanks of the mountain in ancient Pleistocene glacial cirques and trenches. (Harris, 191).  Glacial moraine deposits/drifts surrounding the mountain are from three different periods of Pleistocene glaciations (Hopkins, 102), and according to Harris, Mount Adams was covered by 90% ice during late Pleistocene time (191).  Of the six major lahars studied on Mount Adams, all were chiefly spawned by the White Salmon and Avalanche Glaciers located on the southern slope of the mountain (Vallance, 26).  These two glaciers now occupy a cirque, which is the suspected source area of Mount Adams largest mudflow, the Trout Lake Mudflow.  Approximately 64 million m3 of material was ripped loose from the area around the cirque to form the Trout Lake Mudflow and another 15 million m3 of material was loosed to form the Salt Creek Lahar (Vallance, 40).  This combined with several other smaller episodes gives a total volume of 85 million m3 of material suspected to have come down from the southern slope of Mount Adams to form lahars and debris flows (Vallance, 41).  It is on this historically unstable flank of the mountain that two glaciers are still actively eroding away at and hydrating the steep rocky walls.          

 

 

 

 

 

 

 

 

 

 

 

 

 

View of Mount Adams from the south.  Note White Salmon Glacier below the sharp peak to the left, Mount Adams summit in the center, and Pikers Peak below it to the right.  Avalanche Glacier sits just below the rock wall beneath Mount Adams summit. 

 

 

 

 

 

 

 

 

 

 

 

 

    

 

 

 

Mount Adams summit in the center top, and                     White Salmon Glacier to the left.  Note the large

Avalanche Glacier located at the bottom left                       cirque and over-steepend rock wall above it.

of the picture.

 

 

 

Lahars

 

"Lahars are the most common and most hazardous volcanic process at Mount Adams." Vallance

 

The major lahars that have occurred on Mount Adams can all be classified as secondary lahars.  Although Mount Adams does have a recent history of volcanic activity, none of the major lahars studied on the mountain can be positively linked to eruptive episodes (Vallance, 41).  Instead, lahars on Mount Adams are the product of slope failure due to secondary mineral alteration of volcanic rocks, primarily andesite.  Alteration products include "...alunite, kaolinite, opal, and smectite" (Vallance, 14).  Permeable layers of brecciated andesite and andesite scoria allow for "...the upwelling of hydrothermal gases and the subsequent downward flow of acidic hydrothermal fluids" (Vallance, 13).  This process has been at work on Mount Adams for some time.  Vallance notes that "exposures of solfatarized rock" cover an area of 4.3 km2, and 1.5 km2 of the rock presently exposed on the flank of the mountain (13).  This "extensively altered" rock, covers "…at least 25 percent of the surface of the central cone" (Vallance, 14).  These altered rock masses can absorb large amounts of water due to an increase in porosity, and when over-steepened by physical weathering, i.e. glacial scouring, or by overburdening of snow, they can give way forming lahars as the saturated, altered rocks disintegrate.  This has happened at least five times on Mount Adams southern face inundating the White River drainage system.

 

 

 

 

 

 

 

 

 

 

 

 

Hydrothermally altered rock near the southwest                   Hydrothermally altered rock. Note chlorite vein

base of Mount Adams.  This is chlorite alteration,                in faulted area.

as opposed to sulfur alteration near the summit.

Alteration products are still clay-type minerals

with similar properties …increased porosity,

 lowered permeability

 

 

Late Pleistocene/Early Holocene Lahars

 

The oldest lahars studied and dated on Mount Adams form a group of two different units.  These two units are rather distinct from one another although poorly preserved.  Outcrops of them are exposed along the Cascade Creek drainage system on the southwest side of the mountain.  These lahars are underlain by glacial till of the Evans Creek Glaciation period, so their minimum age is at least younger than the maximum of that glacial episode (Vallance, 26).  The first lahar of the two is distinct, in that it is somewhat well sorted and contains a high number of rounded clasts (Vallance, 24).  The later of the two lahars is unique in that it contains clasts that are "...nearly all (96 percent) massive, unaltered andesite from Mount Adams" (Vallance, 25).  This fact may indicate that the cause for this lahar deposit was in fact primary, owing itself to some sort of eruptive episode.  This has not been correlated though, and evidence of this beyond speculation is non-existent.  Lahar deposits predating these two earliest ones have yet to be identified.  Pleistocene glacial advance and retreat may have erased any such evidence if it did exist.

 

The Trout Lake Mudflow...the big one

 

By far the largest of the lahars recorded in the Mount Adams area, the Trout Lake Mudflow was first described by Hopkins in 1976.  This lahar covers a massive area, some "15 km2 of the White Salmon River Valley near Trout Lake" alone, and carried with it 64 million km3 of material from the cirque located where the White Salmon and Avalanche Glaciers now reside (Vallance, 26-40).  The age of this flow is estimated around 6,000 years old and is the result of slope failure due to alteration of the cones bedrock to secondary minerals as described earlier.  The main composition of the lahar consists of "...strongly solfatarized and hydrothermally altered rock" (Hopkins, 133).  Samples of the lahar deposits taken from two different sites yielded average clast sizes of: clay-sized fraction 3 to 8 percent, silt-sized 5 to 12 percent, sand-sized 25 to 44 percent, and gravel-sized 38 to 62 percent (Vallance, 31).  Vallance also noted that "...[an] abundance of incorporated wood fragments indicates that much of the Trout Lake lowland was heavily forested at the time of the mudflow" (30).  These fragments include whole tree trunks, limbs, and other various fragments.  In one instance, Vallance noticed the presence of "overridden stumps in the original growth position" (30). 

 

Evidence of the Trout Lake Mudflow is still conspicuous in places today.  The farthest known reaches of the lahar are located some 60 km down the White Salmon drainage from the mountain (Vallance, 26).  The Trout Lake Lowland valley, home to the small community of Trout Lake, is covered in a thick veneer of this lahar deposit.  Vallance states, "…the lack of conspicuous internal stratification in the Trout Lake Mudflow suggests that it was deposited as a single unit" (30).  This single massive, catastrophic episode deposited a mass of material, which now covers the Trout Lake Lowland valley floor (Hopkins, 132).  This unit is up to 12 km in length, 2 km wide, and 20 meters thick in places (Vallance, 28).  It may not seem apparent when you first enter the broad, flat valley, but on closer, inspection the surface of the valley floor is dotted with large boulders severs meters in diameter.  This is near where the Salmon River emerges from the base of Mount Adams and where the lahar had its greatest velocity and accumulated mass.  The lahar also damned off Trout Lake Creek forming what is now Trout Lake (Vallance, 29).  A large area surrounding the lake is composed of "interbedded alluvial sediments", which would indicate that the lake was once much larger than it is presently (Vallance, 29). 

 

 


           

 

 

 

 

 

 

Wide view of the upper Salmon River Valley.  Mount Adams in the center obscured by vegetation...oops.  This valley is covered by up to 20 meters of lahar deposits from the 6000 year old Trout Lake Mudflow.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 Salmon River Valley, note large rock            Large rock in Salmon River Valley,                         Very large rock out in field. 

               out in field.                                            obviously not put there by cows.               

                                                                                                                                                

            

 

The Nameless Lahar

 

Cascade Creek is one of the many streams feeding off of Mount Adams southwest slope.  Salt Creek (which will be discussed in the next section), feeds into Cascade Creek, which runs southwest and feeds into the White Salmon River.  Upper Cascade Creek is the most westerly creek to stem from the White Salmon and Avalanche Glaciers, and is the site of an unnamed lahar studied by Vallance and age correlated by overlying and underlying Tephra deposits to be between 2,500 and 500 years old (33). This deposit was estimated by Vallance to have a volume of 4.2 million cubic meters with an average thickness of around 3 meters (33).  It runs for 4 kilometers with the headwaters of Cascade Creek dissecting it completely in places.  Its overall width is thought to be up to several hundred meters.  Compositionally, this lahar deposit varies from the standard Mount Adams lahar make-up of hydrothermally altered rocks.  Vallance suggests that this deposit possibly has its origins in glacial till (34).   

 

 

One for the road…Lahar at Salt Creek

 

The lahar at Salt Creek is the second largest lahar in the geologic record-books for Mount Adams.  Salt Creek itself feeds directly from the base of Avalanche Glacier, which sits directly below White Salmon Glacier.  Salt Creek Lahar deposits terminate 30 km from the source, average a thickness of 2 meters, and contain 1.5-meter boulders along with cobbles, pebbles, sand, silt etc (Vallance, 35).  This lahar has a maximum age of 500 years using an underlying tephra deposit (Vallance, 34).  Vallance went on to analyze woody debris entrained within the lahar.

 

Tree stumps on the surface of the lahar exhibit up to 195 tree rings.  At one locality on the lahar surface which was logged in 1957, tree stumps were observed having 175 and 177 growth rings.  If the trees reestablished quickly, the lahar probably occurred in the later half of the 18th century.  (34)

 

The Salt Creek Lahar has a number of distinguishing characteristics also.  One of these features is the presence of flow-directional mounds, some as tall as 2 meters and up to 5 meters wide (Vallance, 35).  The mounds also contain "cores of breccia and fragmental andesite" (Vallance, 35).  A similar feature is also exhibited at Mount Saint Helens in the landslide deposit within the hummocks.  The cause for the Salt Creek Lahar is identical to that of the Trout Lake Mudflow—hydrothermally altered rocks that are oversteepened or overburdened and fail when saturated, dissolving into a lahar.  Vallance notes that…

 

A cliff approximately the same place as the west-facing cliff that now heads Avalanche Glacier was the source of the Salt Creek lahar.  The cliff exposes the most strongly altered rock visible on Mount Adams.  A slab, approximately 100 m high, 200 m thick and 0.75 km long, collapsing from this cliff today, would equal the volume of debris in the Salt Creek lahar. (38)

 

The Salt Creek Lahar is most recent large lahar to occur on Mount Adams.  Other smaller lahars and debris avalanches have been recorded in recent times and will be discussed in the following section.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Lahar deposit near the junction of                                                                                                                                                                                    

Cascade Creek and the White Salmon-

River, just above Trout Lake.                            Lahar Deposit…note black shoe        Lahar deposit near the headwaters

                                                                                for scale.                                                of the White Salmon River on Mount

                                                                                                                                                Adams west-southwest slope.

 

 

Historic events and all the rest…

 

In May of 1921 a debris avalanche occurred on the southwest slope of Mount Adams originating from the same cliff face as the previously discussed lahars (Vallance, 38).  4 million meters cubed of material was deposited near the base of timberline on the southwest face of the mountain. 

 

Glacial outburst lahars have been recorded on Mount Adams as well in the past 100 years.  In 1981, a lahar resulted from a spell of "hot, dry weather" and "cut a path 10 to 100 meters wide nearly 4 km down stream" (Vallance, 39).  That same year, the north slope of Mount Adams experienced a glacial outburst lahar as well.        

 

In 1997, two separate debris avalanches occurred on Mount Adams.  Detailed information on these two events is available on the USGS website, but for this report, I will just mention their occurrence as it is relevant to the occurrence of lahars on Mount Adams. The first of the avalanches occurred on August 31st, and the second on October 20th.  From the images I copied from the USGS, you can see that the August debris avalanches did not extend as far as the larger 1921 debris avalanche, as the toe of the flow appears to have been deposited on top of previous avalanche deposits and glacial till.  Some large blocks are visible within these recent deposits, especially near the toe of the October avalanche.  The August avalanche occurred in the same area as the previously discussed major lahars—the area of the White Salmon and Avalanche Glaciers.  The second, October, debris avalanche occurred in an area that is infrequently traveled by people—the east side of the mountain in the Klickitat Glacier Cirque.  This area is within the boundaries of the Yakima Indian Reservation and so is off-limits to most of the public.  In fact, the picture below of the October debris-flow is the first photo I have been able to find of Mount Adams east face.  According to the USGS,

 

 The Oct. 20 avalanche appears unrelated, except in the broadest fashion, to a similar-sized avalanche that occurred on the western flank of Mount Adams about seven weeks earlier (August 31, 1997). Both avalanches originated in areas composed of rocks evidently weakened by intense hydrothermal alteration. Both avalanches may have been triggered, in part, by wet subsurface conditions associated with late-season thawing of exceptionally heavy snowpack in conjunction with early-season storms. Both avalanches contained a large percentage of ice, although the August 31 avalanche appears to have BEGUN as an ice avalanche, whereas the October 20 avalanche clearly began as a rock avalanche that subsequently scoured and entrained glacial ice. Neither avalanche was triggered by earthquake or volcanic activity. (USGS)  

 

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

August 31st, 1997 lahar on Mount Adams southwest slope.  Deposits extend down                         Avalanche originated in the  

across Avalanche Glacier.  This was one of the largest landslide in the Cascades                           Avalanche Glacier Cirque.

in historic times.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

October, 1997 Debris Avalanche on Adams east                        Toe of the October avalanche temporarily dammed off

slope.  Klickitat Glacier Cirque.                                                   the headwaters of Big Muddy Creek forming the small            

             pond seen below.  Note the large blocks clustered near  

              the toe of the flow!!!

 

 

The southwest slope of Mount Adams is by far the most active part of the mountain with regards to lahars and debris avalanches.  There is evidence though of lahar activity on the north and east slopes of Mount Adams.  The only reference to these older deposits I was able to attain are from a geologic map released in 1987 by the Washington Department of Natural Resources, Open File Report #87-5.  The copy of the map I received was sadly lacking in the color department…it was black and white.  This was at first a source of great dismay for me, but I later recovered from this realizing that I could color it the way I wanted.  I simplified this process, by designating five different colors and patterns to the map—ignoring (leaving white) anything older than Quaternary for the purposes of simplicity.  There is a key located at the top of the map, which is included with this paper.  Solid-blue designates either landslide or lahar deposits.  You can see from the distribution of these deposits that the southwest slope and surrounding area of Mount Adams hosts the majority of the blue that is within proximity to the summit.  Other blue areas, such as those to the northeast of the mountain, are too far away and too isolated to probably be associated with lahars originating from the cone.  Instead these isolated and distanced areas are most likely true landslides due to local/regional slope failure.  The north face of Mount Adams does have a few lahar deposits mapped and marked in blue.  They are labeled Qaav on the map and are proportionally smaller than those on the southwest slope.  They are also situated more or less directly below large deposits of glacial till (lime-green on the map).  The northwest slope of Mount Adams is rather steep, especially the upper cone, which is host to Adams Glacier—a steep, precipitous glacier that dives for 3000 feet and then runs out for another 2000 feet before terminating at a large glacial moraine.  This region is thick with glacial moraine deposits, which could act as a loose-sediment source for landslides and lahars if saturated enough and either overburdened or oversteepened. 

 

 

Conclusion: That’s Not All Folks

 

For the relatively few communities that live in the vicinity of Mount Adams, the real threat comes not from what most people might assume—large eruptions spewing hot ash and pumice, blanketing the surrounding land.  This is the image of Mount Saint Helens and not of Mount Adams.  Adams has its own history of terror, and in many ways, it is a much more worrisome habit which Adams bears.  While eruptions can be devastating, they are also predictable, or at least foreseeable.  Large slabs of rock ripping loose in the middle of the night are not foreseeable, and it is this very habit of Mount Adams that should be a source of concern for those living near the mountain.  Although Mount Adams has erupted seven times in the past 10,000 years, all of these events have been relatively small/isolated, non-explosive, and short-lived events.  Eruptive events do have the potential to spur lahar activity on the mountain, but again, most if not all eruptive events are somewhat foreseeable—foreshadowed by increasing tectonic activity.  Masses of weakened, hydrothermally altered rock still reside on the upper slopes of Mount Adams peak today.  Sulfur gasses are still being emitted, and snows still melt adding fresh water to the equation—meaning that rocks are still being altered at present.  Weathering, both chemical and mechanical, is at work on the slopes of Adams today, and it is only a matter of time before these soft, saturated rocks rip loose without warning.  The scale to which, and the time when this occurs are the only things presently left to question.