Geologic Field Guide for Mount St. Helens, Washington

By: Lance Wilson and Rachael Rutter University of Alabama Goals

During our trip we will be making stops in the areas along the eastern and southern flanks of Mount St Helens. Hopefully, if weather allows, we will be able to view lahars, ancient lava flows, Ape Cave lava tube, tephra deposits, pyroclastic debris flows, and of course the crater. Our mission will be to provide a complete and in-depth field guide for whoever is interested in traveling to Mount St. Helens and would like to acquire knowledgeable information on the surrounding geology.

  1. Our goals for stopping in this area will be to investigate and record the geologic features surrounding a recently erupted composite volcano.
  2. To examine and attempt to identify ages of ancient tephra deposits, and to gain an understanding of the eruptive history through stratigraphic relationships.
  3. We will discuss the flow mechanics of a lahar by studying the remnants of past flows.
  4. To study the transformation of the surrounding landscape before, during and after the eruption of 1980.

Field Trip Stops: Day 10

  • Stop 10-1: Harmony Trail (46°16’17.54″N, 122° 6’37.90″W)
  • Stop 10-2: Smith Creek Viewpoint (46°15’8.82″N, 122° 7’7.01″W)
  • Stop 10-3: Windy Ridge Viewpoint (46°15’0.24″N, 122° 8’13.62″W)
  • Stop 10-4: Lahar Viewpoint and Lava Canyon Trail (46°10’0.43″N, 122° 5’4.54″W)
  • Stop 10-5: Ape Cave (46° 6’29.37″N, 122°12’41.63″W)

Road Log for Outcrop Locations

Figure 1:Map of field trip in the area of Mount St. Helens, WA

We will start out on Forest Road 25 to the northeast of Mount St. Helens. We will head south on this road until the Forest Road 26 branch. This road will take us south toward the volcano until it ends at Forest Road 99. Heading west on FR 99 will take us to our first three stops: Harmony Trail, Smith Creek Viewpoint, and Windy Ridge Viewpoint. To get to our next stop we will drive back the way came heading east on FR 99 until it intersects FR 25. Again, we will be heading south on FR 25 until it turns into Hwy 90, then going west. We will then take Forest Road 83 north toward the southern flank of volcano and the rest of our stops. Lahar Viewpoint will be on this road and Ape Cave Lava Tube is shortly off this road on Forest Road 8303.

Randle, WA to Windy Ridge Viewpoint
Turn onto Nat For Dev Rd 25 from Highway 12
About 10 mins go 5.9 mi
total 8.8 mi
Continue onto Forest Rd 26
About 5 mins go 1.9 mi
total 10.8 mi
Continue onto Nat For Dev Rd 26
About 37 mins go 15.0 mi
total 25.8 mi
Turn right at Nat For Dev Rd 99
This road may be seasonally closed About 15 mins go 6.0 mi total 31.8 mi
Turn right to stay on Nat For Dev Rd 99
This road may be seasonally closed
Windy Ridge Viewpoint will be on the right
About 3 mins go 1.0 mi
total 32.8 mi
(about 1 hour and 15 min)
Windy Ridge Viewpoint to Ape Cave Travel on Nat For Dev Rd 99 This road may be seasonally closed About 26 mins go 10.5 mi total 11.5 mi
Turn right to stay on Nat For Dev Rd 99 About 11 mins go 4.5 mi total 16.0 mi
Turn right at Nat For Dev Rd 25 About 3 mins go 1.7 mi total 17.7 mi
Turn left to stay on Nat For Dev Rd 25 About 12 mins go 6.8 mi total 24.5 mi
Turn right to stay on Nat For Dev Rd 25 About 12 mins go 6.9 mi total 31.4 mi
Turn right to stay on Nat for Dev Rd 25 About 16 mins go 8.9 mi total 40.4 mi
Continue onto Hwy 25 About 1 min go 0.8 mi total 41.1 mi
Continue onto Hwy 90 About 5 mins go 2.7 mi total 43.8 mi
Hwy 90 turns slightly left and becomes Nat for Dev Rd 90 About 16 mins go 9.2 mi total 53.0 mi
Turn right at Nat for Dev Rd 83 About 4 mins go 1.7 mi total 54.7 mi
Turn left at Nat for Dev Rd 8303 and Ape Cave Parking will be shortly on the right. About 2 mins go 1.0 mi total 55.7 mi (about 1 hour and 52 min)
Ape Cave to Lahar Viewpoint and Lava Canyon
Head southeast on Nat for Dev Rd 8303 toward Nat for Dev Rd 83 About 2 mins go 1.0 mi total 1.0 mi
Turn left at Nat for Dev Rd 83 About 10 mins go 4.2 mi total 5.2 mi
Continue straight onto Nat for Dev Rd 83 and Parking for Lahar Viewpoint and Lava Canyon/Waterfalls will be on the right. About 13 mins go 5.4 mi total 10.6 mi (about 26 min)
Lahar Viewpoint to Interstate 5 (that travels to Portland, OR) Continue straight onto Nat for Dev Rd 83 About 27 mins go 11.2 mi total 11.3 mi
Turn right at Rd No 90 About 5 mins go 2.6 mi total 14.0 mi
Rd No 90 turns slightly left and becomes Rd 90 About 2 mins go 0.8 mi total 14.8 mi
Continue onto Lewis River Rd until Interstate 5 About 52 mins go 31.5 mi total 46.3 mi (about 1.5 hours )
Total trip: 145 miles – about 5 hours
Geologic Map

Figure 2: Geologic Map of Mount St. Helens. This data is based on1:100,000 scale mapping done by T.J. Walsh and others of the Washington Division of Geology and Earth Resources.

Regional Geology Mount St Helens is part of the volcanic arc known as the High Cascades volcanoes. The Cascades range extends from Canada south to Northern California. These composite volcanoes exist as a result of subduction of the Juan de Fuca plate beneath the North American plate which produces an arc of large composite volcanoes. The Cascades characteristically erupt andesitic lavas, and typically have large pyroclastic flows. Mount St. Helens first erupted 40,000 to 50,000 years ago. Before the 1980 eruption the volcano stayed relatively inactive for 2500 years. Mount St. Helens has had four main eruptive stages which have typically erupted dacite interrupted by lengthy dormant stages (Alt and Hyndman, 1987). Large volumes of tephra deposits give clues to the ages of past eruptions. The most recent stage is the Spirit Lake eruptive stage (3,900 yrs B.P to present), which, itself, has had seven eruptive periods marking different eruptive styles. The first in this stage is the Smith Creek (set Y), noted because the volume of tephra left from its Plinian style eruptions record the largest eruption discovered from Mount St. Helens, which we will be able to view on the eastern flank of the volcano. The third period, Castle Creek (2,200 to 16,000 yr B.P.), is also of importance because of its peculiar alterations from high to low silica content. Basalt, andesite, and dacite were produced during this time. On our journey we will encounter the Cave Basalt eruption, on the southeastern region of Mount St. Helens, which produced non-characteristic pahoehoe flows and formed one of the longest lava tubes in the world. Since the Castle Creek eruptions, Mount St. Helens has reverted back to a more andesitic and dacitic composition (Pringle, 2002). Mount St. Helens has currently been the most active volcano in the Cascades range, with its last eruption occurring on May 18, 1980. In the spring of 1980 Mount St. Helens began showing external signals of activity within the magma chamber, producing a large bulge on the northern flank of the mountain. On May 18, the bulge collapsed by means of three massive slide blocks, initiating the largest recorded landslide in history. The huge debris avalanche propagated through Spirit Lake and into the Coldwater Creek drainage basin and destroyed a large portion of the Gifford-Pinchot National Forest. While in the field we will be able to see evidence of the landslide such as hummocky deposits in the northern side of the volcano, large displaced boulders, and the collapsed northern slope of the volcano. The landslide triggered a lateral blast followed by a series of devastating pyroclastic flows, lahars, and ash fallout (Pringle, 2002).
Stop 1: Harmony Trail –( 46°16’17.54″N, 122° 6’37.90″W)
This 3 mile round trip trail will take us down to the eastern edge of Spirit Lake along the wall of a cirque of Evans Creek age (22,000-11,000 years B.P) (Pringle, 2002). The shoreline of Spirit Lake was severely damaged by the blast and debris avalanche that took place in 1980. After the initial blast a debris avalanche roared toward the shores of the lake. When the flow slammed into the shoreline a tidal wave surged up to 800 feet above opposite shore, damming up the source of the lake. As the wave receded it pulled downed trees into the lake forming a lag mat that is still clearly visible today. Water in Spirit Lake has slowly risen since becoming dammed, and is now controlled by a 1.7 mile man-made tunnel that drains the lake into South Coldwater Creek (NMIA trail guide). Along the trail we will see outcrops of Tertiary welded tuff with flattened pumice fragments, a dacite intrusion, columnar jointing, scars left by the wall of water, and a view of the crater and dome. At Harmony Falls welded tuff is exposed which has been cut by a light greenish gray dike. Minor stop: road cuts along FR99 expose tephra layers.
Stop 2: Smith Creek Viewpoint –( 46°15’8.82″N, 122° 7’7.01″W) This stop is along FR 99 about 10.7 mi from FR25. View four distinct flows from the blast in 1980. Hummocky deposits left from lahars that swept through Smith Creek valley. Primary and secondary pyroclastic flows and accretionary lapilli are also found within the complex deposits exposed within the Smith Creek Valley. Depending on the weather Mount Adam and Mount Hood can be viewed from this viewpoint (Pringle, 2002).
Stop 3: Windy Ridge Viewpoint – (46°15’0.24″N, 122° 8’13.62″W)
This is one of the best places to get an overview of the devastated area and views within the crater. The viewpoint is located toward the end of FR99. It exposes views of the pumice plain, crater and southeastern flank of the volcano. Debris avalanche deposits can be seen to the west and indicate the travel path. The road becomes very treacherous past this point, but a short hike (1 to 1.5 miles) around Windy Ridge will expose spectacular views of the volcano and the destruction caused by the lateral blast.
Stop 4: Lahar Viewpoint and Lava Canyon Trail – (46°10’0.43″N, 122° 5’4.54″W)
This stop is located about 10 miles along FR83. Many various features are located within walking distance of parking area. The pyroclastic flows that were produced by the 1980 eruption spilled over and scoured through the glaciers that were present prior to the eruption, massive lahars. These interbedded tephra deposits and lahars are well exposed in outcrops at this stop. Scarred and dead trees, mudlines, and evidence for stream channel adjustments can also be found within the area. Lahar Viewpoint shows northerly views of Shoestring Glacier, Pumice Butte, East Dome, Ape Canyon, and muddy fans from the expansion of the lahars. From Lava Canyon Trail platy jointing can be seen in andesitic lava flows, as well as a spectacular waterfall. Alternate Stop: Trail of Two Forests – (46.09940° N, 122.21320° W) This stop was not planned, but since the snow had not melted at our other stops the roads were not accessible. 1,900 years ago, Mount St. Helens erupted a pahoehoe lava flow that engulfed a forest on the south side of the volcano. The trail took us to many tree casts that formed when the lava flowed around the trees and cooled, leaving impressions of the forest. Along the trail there is a 60 foot long cast tunnel that you can crawl through. The pahoehoe flows in this area were erupted during the Cave Basalt stage in the Castle Creek period, within the Spirit Lake stage eruption (Greninger, 2004).

Picture taken in the 60 ft tree mold at the trail of two forests.

Stop 5: Ape Cave – (46° 6’29.37″N, 122°12’41.63″W)
Last scheduled stop. Back down FR83 and it is just a few miles down the road from Stop 4. Ape Cave lava tube is one of the largest lava tubes in the world which formed when the surface of a lava flow cooled and hardened, but the molten lava beneath the surface continued to flow. It is two miles long and was constructed of a pahoehoe flows from the Castle Creek period which produced the Cave Basalt eruptive stage 1,900 years ago (Pringle, 2002). The cave also has cool lava stalagmites and stalactites. The Gifford Pinchot National Forest trail guide recommends having at least two sources of light per person, sturdy shoes, and warm clothing because the temperatures in the cave average 40 degrees F on a summer day. The lower cave is the more popular and is a relatively easy 0.75 mile hike that takes about one hour round trip. The upper cave is 1.5 miles long and takes about 2.5 to 3 hours to complete. It is more challenging and requires climbing over large piles of rocks (Topinka, 2007). While we were in the lava tube we noticed that there were lahar deposits on the cave floor with pumice and rhyolite fragments. The flows were also noticeable along the walls of the lava tube toward the end of the lower trail. The layers represent subsequent flow pulses down the tube. The matix of the flow was made of ash and mud and was semi-consolidated, like wet concrete. We could make out two separate flows. The lower flow was finer grained and fined upwards. The upper layer was much coarser with pumice and rhyolite angular fragments, but remained poorly sorted and coarse grained throughout. The floor of the cave had stretched pumice fragments throughout the entire length of the cave. The lengthwise direction of the stretched pumice can indicate the direction of flow. We did not see any prominent lava “stalagtites” in the lower cave but there were many spatter marks along the roof and sides of the tube.

Stretched pumice fragments from a lahar flow on the floor of the lava tube.
Some of our group in the lower leg of Ape Cave

Alt, David D. and Donald W. Hyndman, 1987, Roadside Geology of Washington: Mountain Press Publishing Company: Missoula, Montana, p 105-123.
Greninger, Suzanne ed., 2004, Mount St. Helens National Volcanic Monument: Gifford Pinchot National Forest: Northwest Interpretive Association trail guide, Hemlock Printers Ltd. P. 72.
Pringle, Patrick T., 2002, Roadside Geology of Mount St. Helens National Volcanic Monument and Vicinity: Washington Department of Natural Resources, Division of Geology and Earth Resources Information Circular 88.
Topinka, Lyn, 2007, Mount St. Helens, Washington: Points of Interest: Cascades Volcano Observatory, Vancouver, Washington: USGS.