Climbing Through Cascades Geology

Nonn demos his bona fides.

Synopsis of GSOC Past President Bo Nonn's 82nd Annual GSOC Banquet talk
March 12, 2017

GSOC Past President Bo Nonn delivered the 82nd Annual GSOC banquet speech on March 12 to a fascinated crowd at Ernesto’s Italian Restaurant in Beaverton. He has a unique perspective on the geology of the Cascade Mountains: he has witnessed it in person by climbing all 16 Cascade peaks more than once, and has received several certificates of achievement from the Mazamas, as well as being a climbing instructor with that organization.

Pacific Northwest Peaks

Nonn’s talk focused on the interaction of the geology and the climbing experiences of 19 Pacific Northwest peaks, 3 of which are not High Cascades peaks, but are interesting geologically (see map). He began the talk by reviewing the process which produces the Cascade Mountains—the subduction of the Pacific plate under the North American plate. After showing the basic slide of subduction in the Pacific Northwest, he discussed how the composition of the magma producing the volcanoes can change, depending on how much melting of the continental crust occurs, mixing with the basaltic magma coming up from the subducting oceanic plate below. This is why the Cascade volcanoes were built from a variety of lava types, depending on the environment and pathways of the magma at the time it was produced. 


Mt. Hood

Mt. Hood — Bergschrund (crevasse) up the Cloud Cap route. All the photos from this article are by Bo Nonn.

Nonn began his talk with Portland’s Guardian Peaks. First Cascade peak on the plate was of course Mt. Hood, the highest peak in Oregon and the closest to Portland. Nonn went through a few slides of the climb on the smooth south face route via Crater Rock to the top.  There are not a lot of rocks showing in the typical climb because it occurs when the snow is present. More slides showed why this is so—the underlying material is barely consolidated debris which is probably frozen in place. No wonder as it was produced in the Old Maid Eruptions about 230 years ago.

Mt. Hood — hot areas near Crater Rock

A notable except to the no exposed rocks rule occurs in two places—Hot Rocks and Devils Kitchen—where climbers have been known to be burned by the rocks. It’s well to remember that Mt. Hood is an active volcano and these hot spots and sulfurous fumes they produce are there to remind you of the fact. Nonn does not recommend packing egg salad sandwiches in your lunch bag for the climb. 

Mt Hood — vent at Cloud Cap.

Nonn went on to describe the other sides of Mt. Hood. At the summit ridge climbers look nearly straight down to the north along the headwall of Eliot glacier and the greatly eroded north side of the mountain. To the east is Cloud Cap and Cooper Spur and another route up the mountain, which was probably more popular back in the day when there was a fixed line to the top. Nonn also showed a few slides of the GSOC field trip to the Eliot Glacier moraine from a couple of years ago. He also showed slides of what he believes to be a volcanic vent at Cloud Cap at about 1.5 million years in age. The western side which we Portlanders are all familiar with contains rock exposures up to 1.6 million years in age and is quite eroded. 


Mt. Adams — top is overlapping craters.

Mt. Adams

Nonn next took the banquet attendees to Mt. Adams, which is the second largest peak in Washington state. It is actually the third mountain that has been built on the site, and is a compilation of several peaks. The false summit near the top is one of the summit cones, and the real summit is another. The southeast climb up Mazama glacier is really spectacular. On the way up Nonn photographed some glacial striations in pyroclastic breccia and mountain goats.

Mt. Adams — climbing the north side.

Mt. Adams — climbing the north side.

The base camp for the climb was located at toe of Mazama Glacier, in its terminal moraine. While trying to get a little shut-eye for the 1:00 am start, climbers are kept awake hearing large crashes in the nearby Klickitat Glacier Ice Fall. The next day the climb precedes straight up the glacier. Climbing the north side of Mt. Adams is very different. The climb goes up a rocky spur, and climbers continuously dodge rocks broken loose by their predecessors. It is not pleasant to climb. 


Mt. St. Helens

Mt. St. Helens — view to top in 1979

For the next peak Nonn tricked the audience by showing a slide and asking them if they knew where this mountain was. Of course it was a 1979 picture of Mt. St. Helens, which has changed quite a bit since then. Nonn climbed the mountain then and had some great pictures to show of the old mountain and Spirit Lake. In contrast to the lovely symmetrical cone of old, climbers now go up the south face on the lateral moraines of vanished glaciers to the truncated top of Monitor Ridge. Climbers are not allowed in the blast crater, and only 100 per day are permitted to climb to the top of the ridge.

Mt. St. Helens — Rhyolite erupting out of the dome vent in 2006 “whaleback” FormS

Nonn had some interesting slides of the changing appearance of the interior lava dome, as he has climbed the mountain multiple times since the 1980 eruption (1988 and 2006). Also, the walls of the crater provide a cross sectional view through the geology of Mt. St. Helens. 


Mt. Jefferson, Mt. Washington, Mt. Thielsen, and Three Fingered Jack

Mt. Jefferson

The GSOC audience were then treated to a brief stop on Mt. Jefferson, and Nonn started by showing a cross sectional slide of the stratovolcano, which has a huge basaltic pediment and a more silicic top cone. Nonn showed slides of the climb up the south ridge to the red saddle, skirting around the west face and up the north side of the summit ridge, which is eroded, steep and rocky. Jefferson is likely an extinct volcano, and has not erupted for thousands of years. 

Nonn next described three other extinct volcanoes that are quite similar in their craggy appearance—Mt. Washington, Three Finger Jack, and Mt. Thielsen near Diamond Lake. Despite similarities of appearance, each climbing experience is different.  Nonn described the summit of Three Finger Jack as “crumbly andesitic lavas and pyroclastics, barely held together by basalt dikes and sills.” The very top is shaky underfoot. Mt. Washington, however, is capped by the solid andesitic feeder vent that makes a good climbing substrate. Looking south from both Mt. Washington and Mt. Thielsen one sees volcanic features that are quite young, right next to these extinct peaks.  

Three Finger Jack, Mt. Washington, and Mt. Thielsen


Three Sisters

Youthful South Sister.

In contrast to these aged peaks, the Three Sisters are proper, young mountains. As in Mt. Jefferson, South Sister sits on a Pleistocene basaltic pediment. The main andesitic/dacitic main cone is surmounted with a basaltic/andesitic summit cone, which was fed by a different conduit. Bimodal later eruptions (rhyolite and basalt) festoon the flanks of the cone. Volcanic bombs are a common sight on the climb. Nonn treated the audience to some vistas looking southeast and southwest from the South Sister climb showing various rhyolitic eruptions in the near distance. Looking down from the summit one sees a beautiful little blue lake perched in a depression in the glacier. Nonn went down to it once to fill his canteen and discovered that it was solid ice.  

Middle Sister with climbing route marked.

Middle Sister has a youthful conic appearance on its western side but the eastern side has been heavily eroded by glaciers, leaving a steep cliff at the top. Nonn has climbed up the east side on Hayden Glacier to the saddle which delimits the edge of the glacial erosion and thence to the top. 

Climbing scary North Sister.

The North Sister has more glaciation than the others, as the lava supply got shut off about 100,000 years ago. Oddly enough there is a young shield volcano, Belknap crater, located directly to the north of North Sister. Collier Cone is very close to the mountain, and during the Little Ice Age, Collier Glacier butted up against the cone, but as the 20th century progressed the glacier retreated. GSOC Past President Ruth Keen took many of the glacial retreat photos of this area, archived by the Mazamas. North Sister is much more dangerous to climb than the other Cascade peaks due to erosion. Not only is the material very rubbly and loose, but there are some very steep areas that must be traversed.  


Mt. McLoughlin, Mt. Shasta, and Mt. Lassen

Mt. McLoughlin

Heading south from the Sisters area one encounters one of the most picturesque of the Cascade peaks, Mt. McLoughlin. Its almost perfect conic profile is marred by some erosion on the northeast corner, exposing a pyroclastic core overlain by andesite flows. 

Mt. Shasta — view from the freeway. Shastina is the peak on the right.

Mt. Shasta in northern California is the second most active Cascade peak, and the most massive. It is a compilation of four different cones, with Shastina on the south side the most recent. Shastina by itself would be the third tallest Cascade peak. Because it is so far south its glaciers have only one-tenth the area of its rival Mt. Rainier to the north. Shasta’s glaciers are also very responsive to snowpack. The north side is Nonn’s favorite climbing route. The top is very young with pool of sulfurous water at top.  

Lassen Peak

The Lassen Peak area, also in northern California, is active both volcanically and hydrothermally. The first peak to form in the area, referred to by Nonn as “Mt. Brokeoff,” formed about 600,000 years ago but has since been eroded by glaciers and hydrothermal action to a remnant ring. Lassen peak itself is a dacite cone, and the only Cascade peak over 10,000 feet that is not a composite cone. Lassen Peak’s last eruption was in 1915. Jagged black dacite lumps from the 1915 eruption can be seen near the top along the trail. The dacite was too pasty to flow down the mountain. Feldspar phenocrysts can be seen in the dacite rocks. Bumpass Hell is a nearby very active hydrothermal area. Colorful ashfall deposits are sprinkled about the numerous cinder cones in the park. Multimodal eruptions are also present—there are basalt eruptions as well as dacite.  


Mt. Rainier

Nonn’s favorite route is up the Emmons Glacier

Mt. Rainier in Washington State is the tallest of the Cascade peaks and the one posing the most danger to human life, as lahars from previous eruptions have travelled as far as present-day Tacoma. The Nisqually climbing route on the south side of the peak is the route taking by many climbers. Nonn prefers to climb the northeast side on the Emmons glacier. It is a very steep route but you get a view of the sunrise. Only about half of the climbs on Mt. Rainier are successful, as the mountain tends to make its own weather and its size (14,000+ elevation) makes for a very long climb. The structure of Mt. Rainier is a bit different from the Cascade peaks to the south, as it is built upon granodiorite basement rocks rather than a pre-cone stage basalt pediment. The lava conduits lead up to two overlapping summit craters. Lots of hydrothermal activity in the craters create ice caves, which in the past have sheltered climbers. The hydrothermal activity increases the hazards posed by the mountain, because the weakened rock near the summit can trigger landslides without an eruption or earthquake to set it off. 

Mt. Rainier — Hydrothermally altered rocks can fail, creating massive debris flows that can endanger nearby populations.

From Mt. Rainier onward, Nonn’s talk started to diverge along a couple of different tracks. This is in keeping with the geology in Washington. The origin of Cascade peaks in Oregon is fairly straight forward – they’re all volcanic and owe their origin to the subducting plate melting beneath, but not all the Cascade peaks to the north are even volcanoes. Nonn described mountains that were intrusive body remnants, mountains containing ophiolite sequences, and mountains formed by accreted terrain uplifting. All the peaks that Nonn himself has climbed. For this article we will finish with descriptions of three volcanic peaks north of Rainier that Nonn described in his talk. 


Glacier Peak, Mt. Baker, and Mt. Garibaldi

Glacier Peak.

Glacier Peak, just 70 miles from Seattle, is nevertheless almost completely hidden from the ground.  It is tucked in between high mountain ridges. Nevertheless it is one of the most active volcanoes in the state of Washington, and ash from its several eruptions since the Pleistocene are used as dating markers similar to Mt. Mazama or Mt. St. Helens ash. 

Mt. Baker — Active Sherman Crater

Majestic Mt. Baker is ice and snow covered, so it’s hard to see the andesite rock underneath. The glaciers are heavily crevassed and difficult to climb. Sherman Crater not far below the summit became active in 1975 and contained a pool of hydrogen sulfide in meltwater that has since drained to the relief of those keeping tabs on its hazards. 

Mt. Garibaldi in British Columbia

Mt. Garibaldi is a Cascade peak in Canada, tucked into some lesser peaks. It is underlain by Jurassic plutonic complexes with a dacite peak forming above. Nonn’s attempt to climb Garibaldi was not successful as the approach over the real Canadian ice field took too long. Instead, the weary hikers turned back down and headed into the setting sun. 


Our thanks again to Bo Nonn for sharing these wonderful memories. All the photos from this article were by Bo Nonn.