By Dave Tucker October 14, 2012
A cross-section through a volcano is exposed on a rock face in the headwaters of Deadhorse Creek at the junction of Chowder Ridge, Cougar Divide and Skyline Divide north of Mount Baker. The structure and rocks of this andesitic vent are briefly mentioned in two papers by Wes Hildreth and coauthors (see references below), where it is referred to by the geologic map code ‘acr’ (andesite of Chowder Ridge). No name was given this volcano in the papers, and it is virtually unknown to geologists. Every good volcano needs a name, so I’ll coin the name “Deadhorse Volcano”. I visited this geologic oddity for the first time on October 10, in the company of Keith Kemplin, Kiko Anderson, Kurt Parker, Jim Hestad and Ant Chapin.
The Deadhorse Volcano erupted through argillite (slightly metamorphosed marine shale) of the Early Cretaceous to Middle Jurassic (the span from 120-170 million years ago) Nooksack Formation. Erosion has cut a 400-foot-high cross section on a steep rock face. Most of the volcano’s rocks and structure have been removed, but enough remains to make some good guesses at its original appearance. The exposure is principally a funnel-shaped mass of volcanic breccia filling the volcano’s throat, at the point where the feeder dike reached the surface and began erupting pumice and lava. The contact of the vent with the surrounding argillite is knife sharp. Cross sections through this part of a small volcano are unusual, so this an intriguing geologic site to visit, and worthy of the 10.6 mile (17 km) round trip trek required to get to the best viewpoint.
Getting there: Hike the Skyline Divide Trail from the trailhead on Forest Road 37. This road branches off the Glacier Creek Road (Road 39) just east of the town of Glacier. The trailhead is 12.7 miles (20.3 km) up Road 37 at 4300’ (1310 m). Hike this extremely popular trail 1.9 miles (3 km) through the forest to a sudden meadow and views at 5800’ (1770 m) on the ridge crest. The trail continues south along the ridge, staying mostly on the crest but bypassing a couple of the higher bumps. The trail is dry once snow banks disappear, so always carry water. Take binoculars to get a good view of features of the Deadhorse Volcano.
Maps: The USGS Mount Baker 1:24,000 sheet shows topography. General geology is mapped on the 1:100,000 Mount Baker 30- by 60-minute quadrangle (Tabor and others, 2003; see reference below). This map does not show the Deadhorse volcano. The location is shown on Figure 11 of Hildreth and others (2003), marked as unit ‘acr’, and more generally on Figure 2 of Hildreth and others, 2004. The coordinates of the vent are 48° 49.674’N, 121° 50.810’W.
Geology guide: Rock outcrops along Skyline Divide are in the Nooksack Formation. These marine sedimentary beds dip shallowly to the west and are little deformed. Along the ridge crest you’ll see that these rocks are pervasively fractured by many close-set and parallel foliation planes. This foliation is caused by compression of the rock during thrust faulting and subduction of these seafloor rocks. The compression re-aligned the microscopic planar clay particles (clasts) that are the main component of these marine sedimentary rocks. The clay grains are now nearly perpendicular to the bedding in the rocks. Along Skyline Divide this foliation is steeply dipping and parallel to the north-south trend of the ridge so that the subdued rock exposures along the trail are very well diced and easily eroded.
Enjoy the views as you walk south along the ridge crest. There isn’t much up-close geology to see for quite a way, but the big picture of the northern Cascades surrounds you, dominated by Mount Baker and the Black Buttes to the south and Shuksan to the east. Cougar Divide is just across the deep valley of Deadhorse Creek to the east- read a hiking guide to the geology of that ridge here. About 3 miles (4.8 km) into the hike an obvious trail drops left into meadows below the ridge crest, but you keep right. Hike steeply up to a 6560’ high point 4 miles (6.4 km) from the trailhead- this is the summit of Skyline Divide. The boot- beaten trail beyond this point is spectacular. Sedimentary beds are now evident in the cliffs below the ridge crest. These are photographed on Plate 4A of Tabor and Haugerud (1999), which also includes a very brief description of Skyline Divide geology in Note 84, p. 94 (but there is no mention of the volcano you have come to see).
It is 5.3 miles (8.5 km) to a triangulated knob at 6395’ on the USGS topo map. Here is the best overall view of the Deadhorse Volcano, at the base of the steep face of Chowder Ridge only 300 yards away to the southeast. The interpretations below are entirely mine.
The most obvious rock in the cross-section is an upward widening, roughly funnel-shaped yellowish mass of volcanic breccia filling the throat of the small volcano (think cinder cone, rather than a stratocone like Mount Baker). The funnel, around 150 m wide at the top and only 10 or 20 meters wide at the bottom, was blasted through the uppermost few 10s of meters of bedrock. This cone-shaped crater formed at the top of a dike bringing magma to the surface. As the gaseous magma reached the surface, gases were no longer confined by the surrounding pressure of rock walls. They could decompress and expand outward, shattering the magma into blocks of pumice and tiny fragments of ash. The magma probably encountered water in the bedrock, and this could also expand to steam and help ream out a crater. Blown out of the magma at the mouth of the vent, pumice and ash fell back and filled the developing funnel-like crater at the top of the dike. Unfortunately, the dike is either eroded away or buried beneath the talus below the funnel. The breccia consists of small fragments of scoria and pumice, and much larger blocks of andesitic lava that are clearly visible from the viewpoint. Blocks of Nooksack sedimentary rock are also in the vent filling, torn from the walls of the growing crater and hurled into the eruption cloud. The yellow rock that dominates the funnel’s fill and that surrounds the larger fragments is called ‘palagonite’. It began as glassy volcanic ash and has been altered by water percolating through the mass, changing the structure of the glass to a clay-like material. (A 2002 paper by Stroncik and Schminke describes palagonite, its origins, and chemical evolution from glass.) With binocs, you can see the sharp vertical contact between the breccia and the Nooksack argillite on the east margin of the funnel. This marks the wall of argillite on the margin of the blasted crater; the argillite once completely surrounded the vent before erosion removed most of the vent and the rock to expose the cross-section.
A dark lava flow caps the vent-filling breccia; you can see its fractured structure in the two low triangular cliffs immediately atop the breccia with binoculars. This lava is mostly eroded away, and a sloping ledge of talus lies on top of it, beneath the high wall of flat-lying argillite extending to the top of the cliff.
Only a thin veneer of the volcano’s rocks remains, plastered against the Nooksack rock around the vent. The layered Nooksack argillite above the exposure could be confusing to you- this rock not actually on top of the vent, but is beyond the far side of the vent structure from your vantage point. The sharp wall of Nooksack on the east side of the funnel circles around to form the south margin of the vent. It must have originally curved northward toward you and eventually encircled the vent. The vent may have erupted on a steep slope eroded into the Nooksack, or it may have extended upward through argillite to at least the level of the rocks at the top of the big cliff.
The age of the Deadhorse volcano is unknown, other than it is almost certainly Pleistocene, like all of the rest of the rocks in this part of the Mount Baker volcanic field. Minerals in the volcanic rocks are too altered by hydrothemal activity to be dated. There is no preserved lava outside the vent, though several scraps of orphan andesitic lava flows are nearby on Cougar Divide and at the head of Thompson Creek. These are noted and mapped in Hildreth and others (2003). Chemistry of these flows is not a good match for the Deadhorse rocks, however. On the other hand, the Deadhorse volcanics are considerably altered by hydrothermally activity, so their chemistry has to be taken with a grain (or two) of salt. The best chemical match is the 878 ±18 ka (ka = thousand year) lava just west of the head of Thompson Creek (Hildreth’s unit atc), 2.5 miles (4 km) to the northwest of the Deadhorse vent. There are many andesitic dikes in the immediate vicinity. A swarm of them are at the head of Cougar Divide; others intrude argillite on Chowder Ridge on the way to Hadley Peak, and still more crop out in the basin at the head of Dobbs Creek (see the Cougar Divide field trip elsewhere on this website). Any or all of these may have reached the surface and erupted at a vent similar to the Deadhorse Volcano, but no vent structures are preserved at any of them.
You may choose to make the short cross-country trek to lay hands on the rock and maybe bring home a souvenir, but that requires steep traversing and climbing on loose scree and hard-packed dirt at the angle of repose. Stiff-soled boots are best for the trek, since non-rigid tennis shoes or trail shoes won’t bite in to the soil on the steep slope.There is always the threat of rock fall from the cliff above. The slope is north facing and holds snow until late in the season, so a trip in the fall is recommended to see the most of the exposure. Any snow remaining in the steep gullies near the exposure may be frozen in late fall, so an ice ax may be a good idea. The lowest corner of the crater wall contact on the east can be reached if you are willing to climb steeply on sliding scree. Note that the argillite along this contact is hard and brittle- it has been metamorphosed to a low-grade metamorphic rock called ‘hornfels‘ by the heat of the volcanic rock filling the crater.
If any readers have photos of dissected vent structures similar to this one, anywhere in the world, please post a comment on this page, or email me:
tuckerd at geol dot wwu dot edu.
Also, if you have alternatively interpretations about formation of this funnel-shaped vent, feel free to comment.
Hildreth, W., Fierstein, J. and Lanphere, M., 2003, Eruptive history and geochronology of the Mount Baker volcanic field, Washington: Geological Society of America Bulletin, v. 115, p. 729-764. Unit acr is mapped on Figure 11.
Hildreth, W., Lanphere, M., Champion, D.E. and Fierstein, J., 2004, Rhyodacites of Kulshan caldera, North Cascades of Washington: postcaldera lavas that span the Jaramillo: Journal of Volcanology and Geothermal Research, v. 130, p. 227-264. See particularly description of unit acr on p. 241.
Stroncik, N.A., and Schminke, H-U., 2002, Palagonite, a review: International Journal of Earth Science (Geol Rundsch) vol. 91, p. 680–697.
Tabor, R. and Haugerud, R., 1999, Geology of the North Cascades, A Mountain Mosaic. Seattle, The Mountaineers, 143 p.