For Part 1 of this series, check out my previous post on Goat Rocks Wilderness Backpacking (https://bluemarbleearth.wordpress.com/2025/02/23/goat-rocks-wilderness-backpacking/).

I brought up a laundry list of geology questions at the end of my last blog post about the Goat Rocks Wilderness. Sorry for leaving you hanging for so long – let’s dive in.

When you think of the Cascade Range, you probably think of Mt. Rainer, Mt. Hood, Mt. Adams, Mt. St. Helens, and Mt. Baker. But in the history of Cascade volcanism, these are newcomers! Subduction and Cascade volcanism have been occurring for 40 million years – and our familiar volcanoes have only been around for approximately 500,000 years. Just 1.25% of the total Cascade story. We have evidence of the other 98.75% of Cascade volcanoes based on ancient lava flows and unearthed intrusive igneous rocks such as granite. Nick Zentner of Central Washington university did a fantastic video on these “Ghost Volcanoes” here; https://www.youtube.com/watch?v=ksnBYfRvxRE (https://www.youtube.com/watch?v=ksnBYfRvxRE).

Have you ever wondered what Mt. Rainier might look like a couple million years in the future? We can’t hop into a time machine, but Kyle and I did travel a few miles southwest to the Goat Rocks Wilderness to get a glimpse of what it may become. The Goat Rocks Volcanic Complex was active between 3.1 million years ago and 115,000 years ago. The figure below compares the ages of the volcanoes in its neighborhood:

The Goat Rocks Volcanic Complex is at an intriguing stage in its life – long dormant by our human standards, but not completely eroded. Geologists adore this kind of landscape. The volcanic system has been dissected by rain and glaciers so we can see the various components of the system like vents, dikes, feeder systems, and individual lava flows in cross section view. We can’t cut a cross section through Mt. Rainier, but the rugged landscape of the Goat Rocks is a close equivalent. Let’s start off with some basics – what kinds of rocks were Kyle and I hiking on? Time for a map.

Map (c) Courtney van Stolk

Like most volcanoes in the Cascades, Goat Rocks Volcanic Complex is built of andesite – a kind of intermediate extrusive volcanic rock. Intermediate, in that its mineral composition is halfway in between the oceanic and continental crust, and extrusive volcanic, in that it is formed from lava that erupted into the fresh air before cooling. It is similar to basalt, with the crucial differences that it has more silica and less of the minerals common in the oceanic crust such as amphibole and olivine. The increased silica content is especially important, as it increases the viscosity of andesite in its lava form. Andesite lava is thick like peanut butter and flows to a certain extent, but also breaks. Basalt lava, with its lower silica content, flows more like liquid honey. You can get a fantastic sense for molten andesite’s gooey viscosity in the flow-banded boulders in the photo below.

I reached out to Kellie Wall, a fellow Oregon State University Beaver who now works at the USGS (https://www.travelinggeologist.com/2017/03/into-the-goat-rocks-volcano-washington-with-kellie-wall/), to learn more about the rocks we saw. She did her PhD thesis on the Goat Rocks Volcanic Complex and was kind enough to return my emails during the federal government chaos of February 2025. During her fieldwork, she observed that the pale ribbons in the flow banding include more gas bubbles than the darker and denser surrounding rock. During their eruption there may have been been a small amount of explosive activity near the summit that broke up the lava a bit or emplaced scoria (a type of rock full of gas bubbles) on the lava as it was flowing, and then that material may have been re-incorporated, heated and sheared as it continued to flow.

Andesite can also break into flakes as the mineral structure cools and shrinks, seen in the photo below. These flakes then shatter to form the loose talus slopes that you see everywhere in the Goat Rocks Wilderness. Kellie mentioned that these fractures are most likely to form parallel to the flow direction and are especially common in the deposits around Old Snowy that have a particularly homogeneous texture of very small mineral crystals.

Fractured plates of Old Snowy Mountain’s andesite in the foreground.

Extrusive rocks cool quickly after eruption, creating very small crystals. Not all of the magma leaves the volcano – some remains behind in the vent, and more remains as a magma body far underground. The rock that cooled somewhere between the magma body and the fresh air takes an intermediate form – mostly very small crystals with a few larger crystals. On the map above, this is referred to as “Intrusive Andesite”. Kind of an oxymoron, but it’s what we have to work with. This intrusive andesite gives geologists insight on the location of the volcanoes’ plumbing.

We hiked very briefly on another type of rock on the way to Goat Lake along the Lilly Basin Trail – the Ohanapecosh Formation (the striped formation on the map above). We viewed much more of it looking north towards Mt. Rainier. “Volcaniclastic” describes rocks that were deposited as fragments of volcanic rock in lahars, landslides, and mud flows, and sandwiched between andesite lava flows. The Ohanapecosh formation dates from 36 to 28 million years ago, representing explosive traces of the earliest phases of Cascade volcanism. The timeline below describes some of the regional highlights.

The Ohanepecosh blankets almost 100,000 acres of southern Washington in a layer 1.9 miles thick, and forms the foundation beneath both the Goat Rocks Volcanic Complex and Mt. Rainier. It has a distinctive striped appearance due to its many layers of volcanic deposits. You can see this in the two annotated photos below.

Contact between the Goat Rocks Andesite and the older Ohanapecosh formation, with a phenomenal view of Mt. Rainier in the background. This is the kind of snack break backdrop that gets me out of bed happily at 5am.

This eroded landscape gives us the chance to walk right up to the surface where the first andesite flow of the Goat Rocks Volcanic Complex flowed out of the Lake Creek Volcano and over the ancient Ohanepecosh formation more than 1 million years ago!! Geologists find this thrilling (which geologist’s partners find bemusing, just ask Kyle). If you’re wondering, it’s most accessible on the rocks directly below Goat Lake where Kyle and I had the snack break with the marmots. To take a step over the boundary between the Ohanepecosh Formation and the Lake Creek lavas is to travel in geologic time from about 29 million years ago to 1 million years ago – 28 million years worth of rock was eroded between those two depositional events.

Not all the andesite in Goat Rocks Wilderness formed at the same time or even from the same volcano. Kellie has done extensive research on the age of volcanic rocks in the Goat Rocks (https://www.travelinggeologist.com/2017/03/into-the-goat-rocks-volcano-washington-with-kellie-wall/). Her research, along with the research of scientists who have studied this area in the past, tell the story of a series of ghost volcanoes that once loomed as tall as Mt. Hood on the horizon.

Geologists use precise percentage of minerals and trace elements to differentiate different magma sources. These chemical distinctions make it possible to distinguish multiple different volcanoes that overlapped, such as in the Goat Rocks. A combination of types of evidence – the eroded cross sections of volcanic sections, age dating using radioactive isotopes, as well as the chemical makeup, mineral composition, and mineral crystal sizes – allows scientists like Kellie and her mentors to do some deep time sleuthing into ancient Cascade Volcanism.

That’s how geologists hypothesize that a couple million years ago, the skyline in southern Washington may have looked like this, minus the modern interlopers (slide from Kellie Wall (https://www.youtube.com/watch?v=GqJTp3qKhV8))

Not all of these peaks existed at the same time.

• The Devil’s Washbasin volcano existed 3.6 million to 2.1 million years ago;

• The Bear Creek Mountain volcano was active 1.6-1.1 million years ago;

• The Lake Creek and Cispus Pass volcanoes were active 1.1 million to 456 thousand years ago.

• And the graphic doesn’t even show the volcanoes that currently make up the highest peaks of the Goat Rocks – the Old Snowy, Ives Peak, and Goat Ridge volcanoes were active 440 to 115 thousand years ago.

Most of the rocks we hiked on between Snowgrass Flats and the shortcut trail erupted from Old Snowy and Ives Peak volcanoes.

At Goat Lake, we hiked onto older andesite that had erupted from the Lake Creek Volcano. Based on the volume of associated extrusive rocks, the ancient Lake Creek volcano may have been as big as Mt. Hood! The map below shows geologists’ best guesses of locations of these ancient volcanoes and also describes rocks in a broader sense using their age and classification, instead of rock type.

Map (c) Courtney van Stolk

“Quaternary” describes the age of rocks younger than about 1.8 million years old – younger than the first ice age. “Tertiary” describes the age of rocks between 65 million years to 1.8 million years of age – younger than the last dinosaur. In the grand scheme of geologic time, these are “baby rocks” and “teenage rocks” respectively.

The map shows volcanic dikes as red lines. A dike is a linear feature where fresh lava shoved its way up through fractures in the overlying rock in order to erupt at the surface. They are often oriented in a radial pattern, pointing towards a volcano at the center of the pattern. This is a useful clue for locating ancient eroded volcanoes. The dikes around Goat Lake are key evidence for the location of the Lake Creek volcano.

For much more detailed information on each of these volcanoes, their lava flows, and their chemical fingerprints, check out Kellie’s presentation on Nick Zentner’s Central Washington University YouTube channel. It’s linked in the resource section at the end of this post. She shows off the breadth of her research both at OSU and the USGS, and goes way deeper into the details than I ever could in this post.

Time for the last question of this post – the one that led me down the Knife’s Edge trail. Why are the volcanic rocks such a fabulous range of colors? Kellie helped me out with this too. The green, red, purple, and yellow-tinted rocks belong to the Ohanapecosh formation and document the contact metamorphism caused by the renewed volcanic activity of the Goat Rocks Volcanic Complex. She says that the greenish and purplish colors are probably re-mineralized by hydrothermal fluids mobilized new minerals. The green tint could be caused by chlorite, and the purple tints caused by iron in various stages of oxidation. She saw pyrite crystals in the greenish volcanic rocks further south near Cispus Pass – I need to go there next! Black and grey colors indicate “fresh” lava rock, and yellow colors are caused as these rocks weather into clay minerals. The red tints are commonly caused by iron oxidation and tends to happen near volcanic vents where the rocks stay hotter for longer.

Green and purple tints in the Ohanapecosh formation northeast of Old Snowy.

Black, red and yellow tints in the Ohanapecosh formation north of Old Snowy, along the Knife’s Edge trail.

I hope you’ve enjoyed this post – if you want to learn more, check out the resources below:

• References

• Kellie Wall’s blog post on her thesis research: https://www.travelinggeologist.com/2017/03/into-the-goat-rocks-volcano-washington-with-kellie-wall/ (https://www.travelinggeologist.com/2017/03/into-the-goat-rocks-volcano-washington-with-kellie-wall/)

• Swanson and Clayton, Generalized Geologic Map of the Goat Rocks Wilderness, https://pubs.usgs.gov/of/1983/0357/report.pdf (https://pubs.usgs.gov/of/1983/0357/report.pdf)

• Nick Zentner video featuring Kellie Wall – “Construction of the Cascades “Andesite Factory” – the 3 million year evolution of the Goat Rocks Volcanic Complex” https://www.youtube.com/watch?v=GqJTp3qKhV8 (https://www.youtube.com/watch?v=GqJTp3qKhV8)

• Nick Zentner – Ghost Volcanos in the Cascades: https://www.youtube.com/watch?v=ksnBYfRvxRE (https://www.youtube.com/watch?v=ksnBYfRvxRE)

• Generalized geologic map of the Goat Rocks Wilderness and Roadless Areas (6036, Parts A,C, and D), Lewis and Yakima Counties, Washington https://pubs.usgs.gov/publication/ofr83357 (https://pubs.usgs.gov/publication/ofr83357)

• 7.5 minute geologic report of north-west-adjacent Packwood Quadrangle. https://pubs.usgs.gov/of/1996/0704/report.pdf (https://pubs.usgs.gov/of/1996/0704/report.pdf)

• Learn about one of the longest andesite flows in the world that was created by the Goat Rocks Volcanic Complex: Nick Zentner Tieton Andesite video – https://www.youtube.com/watch?v=X6pOs_kOqwU (https://www.youtube.com/watch?v=X6pOs_kOqwU)

• WA GEO 100 site on Goat Rocks – https://wa100.dnr.wa.gov/south-cascades/goat-rocks (https://wa100.dnr.wa.gov/south-cascades/goat-rocks)

• An inspiring off-trail traverse on the east side of the Goat Rocks Wilderness – https://climberkyle.com/2024/09/15/goat-rocks-high-route/ (https://climberkyle.com/2024/09/15/goat-rocks-high-route/)