Hiking up Tam McArthur Rim toward Broken Top is one of my absolute favorite hikes. With its mountain views, lakes, and windswept ridges frosted in wildflowers—it is the perfect hike for anyone that likes, well, perfect hikes.
It is also a hike chock full of geological curiosities! Lava rocks, volcanic cones, glacial lakes, and bisected mountains are all visible along the 5.6-mile trail. Each item offering a clue to the past, as well as the future, of the High Cascades of Central Oregon.
To help me understand the geological mystery of the region, I asked Derek Loeb—retired petroleum geologist, president of the Central Oregon Geoscience Society, and Sherlock Holmes of the Central Oregon Cascades—if he would meet with me to try and crack the case. Luckily, he agreed, and we headed up one late summer morning to the Tam McArthur Rim Trailhead for some good old-fashioned geological detective work.
Dinner Plate Andesite
It was a warm breezy morning when Derek and I started out on the trail— climbing up through pine trees and mountain hemlock with Three Creek Lake just below us. Derek and I hadn’t made it very far when we reached our first stop.
What do we have here? A platy outcropping of gray rock ran across the trail and along the hillside.
“You know the naming of volcanic rocks is based on chemical composition?” Derek inquired, as I puzzled over the fragmented rock.
You see, volcanic rocks are classified by their silica content, he explained. In general, the breakdown is as follows: basalts are 48-52% silica, andesites are 52-63%, Dacite 63-66%, and Rhyolite 68-77%.
Of course, silica is not something you can easily measure in the field. So, without a silica meter (does such a thing exist?), can one distinguish between the different types?
“Hard to tell just looking at [a rock],” Derek explained, but there are clues. “One of the clues you can use is how [the rocks] present themselves.”
Derek pointed to the fine fractures in the rock before us. “This is very typical of andesite,” he proclaimed. The “thinly bedded fracture pattern” is probably due to exposure to a local stress regime while cooling, Derek hypothesized—giving the rock shale-like appearance.
According to Derek, the shale-like fracture pattern in andesite is so prolific, that there are several lakes along the PCT called “Shale Lake”—despite the fact that shale is a sedimentary rock and has no business on the Cascade Crest.
“I assure you there is no shale on the crest of the Cascades,” Derek said. It is all andesite.
“I call it dinner plate andesite,” said Derek, picking up a piece.
“Stand back,” he called out and gently tossed the rock toward the outcropping where it pinged against the rocky face.
That ‘tink, tink’ seems to be a dead giveaway,” Derek mused.
Time Travel
Derek and I hiked past a few more outcroppings of dinner plate andesite, as we continued to climb up the dusty path through clusters of mountain hemlock trees. As we walked, Derek spoke about his interest in geology.
“You get to do time travel in the past and in the future,” he spoke adamantly. “A basic tenant of geology is the present is the key to the past, but the past is also the key to the future.”
For example, we might see dinner plate andesite and surmise that a lava flow came through the area sometime in the past—we can even date it and identify its source. At the same time, the andesite offers a window into what the area might look like in the future.
The job of a geologist is to look in both directions—understanding the past to predict the future.
Active, Dormant, Extinct
I considered this. Then, peering ahead of us up the trail, I asked Derek if the Central Cascade Volcanoes would erupt again.
His short answer was “yes,” but it is complicated.
Though most of the central Oregon Cascade volcanoes are considered extinct—meaning that they haven’t erupted in the last 10,000 years—recent eruptions have occurred in the vicinity.
For example, North Sister was constructed from 120,000 to 45,000 years ago—definelty extinct.
However, just north of it’s edifice is McKenzie Pass—“which has been very active as recently as 1600 years ago.” Not to mention the “recent” eruptions of South Sister 2,000 years ago.
So though North and Middle Sister, as well as Broken Top, are considered “extinct” by way of the 10,000-year-eruption rule, the Three Sisters as a region is volcanically active.
In addition, Derek pointed out, the distinction between active, dormant, and extinct isn’t all that useful taken alone. Assessing volcanic threats requires a closer look at the volcanic hazards, as well as the risk of exposure to the hazard.
“Not all volcanic eruptions are a problem hazard-wise,” said Derek. He used the examples of a small cinder cone eruption in Newberry National Volcanic Monument.
“There might be some local impact,” he remarked, but being a moderately hazardous eruption type and in a remote location means the threat of this sort of eruption would be quite low.
“However, a large cinder cone eruption closer to Bend could be a big problem,” said Derek. “Cinder cones will frequently produce a late-stage lava flow as the gas is depleted. Most of the east side of Bend was ‘paved’ by lava flows produced by Newberry cinder cone eruptions about 70,000 years ago,” he added.
Similarly, a small rhyolite flow from South Sister might block the Cascade Lake Highway and disrupt recreation. But a more gas-rich, violent rhyolitic eruption that produces pyroclastic flows that travel toward the basin—like the series of eruptions dating back to somewhere between 200,000 and 600,000 years—that would be catastrophic!
Either way, Derek and I agreed, the Instagram threat assessment for any eruption would be off the charts.
What Lies Beneath
Still winding our way up the trail over eruptive material from the past, I questioned Derek about how we know volcanic activity is occurring. Can we see what lies beneath the earth’s surface?
As it turns out, we can, and geoscientists do so in a variety of different ways.
The first way Derek mentioned was using what is called seismic tomography—essentially using the patterns of seismic wave return patterns during earthquakes to interpret Earth’s internal structures, including potentially active magma chambers.
“Think of it like a CT scan” Derek suggested. “Hotter, more plastic rock is slower than solid, cold rock,” he explained, “producing an anomaly that you can map.”
The use of GPS stations and tiltmeters is another method used to monitor surface topography changes.
“GPS is now accurate enough you can measure small changes,” said Derek.
Of course, satellites can be used to detect change using a technique called interferometry. Derek explained how repeat satellite passes can use a type of radar wave to measure topography and detect changes. Repeated passes for the same location constructively stack up when the Earth is static. A little movement will change that and cause the waves to interfere.
“If things are changing, the travel time will change and the waves won’t stack,” explained Derek. “That is how they first detected the bulge on South Sister,” he went on.
“The fourth way is to go to local lakes and streams and sample gases,” said Derek. “Picking up increasing gasses associated with magma can start to raise the alarm.”
Whatever the methods used, volcanologists are good at using the data to warn of pending eruptions. Unfortunately, the timeline of the eruption is, as Derek put it, “nebulous.”
From the time of the warning, it could take an unpredictable amount of time before the eruption will occur. “It may not happen in a week, month, or year…” Derek speculated. “It is hard to get people to pay attention.”
Luckily for us, there was no eruption warning in place, as we were probably only about three miles from Broken Top and six miles from South Sister.
Where did you come from?
At this point, the steep grade of the trail leveled off a bit and the rocks we were passing by no longer resembled dinnerware. Instead, clusters of large rocks of varying shapes and colors lay scattered next to the trail.
Many rocks were in shades of red, black, or gray; some smaller rocks were nearly white. Many of the rocks were massive, but others had large or small vesicles in them. One interesting specimen was a large, maroon-colored rock swimming with dark blogs of grey. As I would later find out, this separation of color was probably due to slight differences in the chemistry.
I needed to know what was going on! Were these new rock forms indicative of anything?
“The question you need to ask,” Derek pulled me back, “is this [rock] in place, or was it transported?”
In other words, do the rocks actually describe the subsurface geology? Or did they get washed in by water, blown in by the wind, or fell from somewhere higher up by gravity?
The surest sign that a rock is from the place where you found is if you can find it’s nearby source.
Otherwise, you must rely on clues. Does it look like it’s been moved? Does its orientation make sense? Is its original structure intact or has it been reshaped through transport?
Quaternary Rhyodacite
Of course, another surefire way to know if the rocks match the subsurface is to bring Derek along.
Derek whipped out his phone and pulled up a georeferenced map from USGS for the Bend Quadrangle—a rectangular area of land that is equivalent to roughly about 41 to 71 square miles, or 7.5-min longitude by 7.5-min latitude.
“We are now in the QRD unit,” said Derek looking down at his phone. “That is a quaternary rhyodacite.”
The underlying geology had changed from the less silica-rich andesite to more silica-rich rhyolite and dacite rocks.
The Many Faces of Rhyolite
Now as you may recall, rhyolite and dacite rocks have a higher silica content than other volcanic rocks, like andesite or basalt—a measurement that can only be determined through chemical analysis. However, just like with andesite, there are clues that can help to tell them apart!
“Rhyolites are some of the most interesting of the volcanic rocks because they are the shapeshifters,” Derek explained.
Derek picked up a small piece of white pumice from the ground. We had seen several of these small, pieces of volcanic rock, earlier as we hiked through a few of what Derek called pumice flats.
Pumice, he explained, can have the exact same chemical composition as obsidian—a black, shiny rock that has no gas bubbles in it, and, in fact, no crystalline structure. Pumice and obsidian couldn’t be more different, yet they are both rhyolites.
The Physics of Color
These are not the only forms of rhyolite either. “Rhyolite can be black, gray, purple, maroon…,” Derek went on. “It really covers the bases.”
As for dacite, it too is variable in color, though not as much as rhyolite, and is often a paler, bluish grey. Basalt and andesite are also usually grey—though often on the darker side.
“Color,” Derek explained, “comes from the physics of light.”
Mineral Mayhem
We continued up the trail, observing the rocks along the way. At one point, Derek noticed a clean face on a piece of grey rock—perhaps an andesite based on the color.
For example, pumice is rhyolite from an eruption high in gases that expanded the rock creating millions of gas bubbles that can scatter light in all directions, sort of like a cloud might—giving it whitish colors. In contrast, obsidian contains a lot of micro inclusions of iron oxide minerals, like magnetite, that absorbs rather than scatters light—hence the deep blackish colors.
Derek took a closer look at the broken face of the rock.
“Probably plagioclase feldspar,” he declared.
Plagioclase is a term used to describe a group of feldspar minerals that are chemically very similar, only varying in their percentage of sodium and calcium. Feldspar minerals in general follow the chemical formula AT4O8 (where A is potassium, sodium, or calcium, and T is Si or Al).
“Feldspar is the most common mineral in the Earth’s crust,” Derek told me, but it also comes in many forms. It is often the trace elements that fill in the crystal lattice that give it its characteristics.
For example, rubies and sapphires are both the same mineral (corundum), but ruby has chromium as an impurity and sapphire has titanium and iron.
In the case of feldspars, they can range in color from white or pink to very dark grey. One of the most important plagioclase feldspars to Oregon is the Oregon sunstone—a labradorite that, like other sunstones, contains small inclusions of copper or iron oxide (either hematite or gothite) giving the gemstone an orange color.
You won’t find any rubies or sapphires in Oregon, or sunstones, for that matter, in the Central Cascades.
“The Three Sisters Wilderness is mineral poor in terms of classic rock hounding,” said Derek.
But that doesn’t mean it isn’t fun to look closer at the less “classic rock hounding” minerals in the rocks. And a fresh face is a great place to do so.
“Which is why our [geologists’] favorite investigation tool and anger management tool is a rock hammer,” laughed Derek.
Particulars on Vesicular
We continued past the open-faced rock, toward the rim. We were getting closer to the final push to the top. In the meaning time, it seemed like there was an endless supply of rocks to examine as we wandered along.
At one point, Derek picked up a massive rock from the trail and handed it to me.
“Feel how dense it is,” he said encouraging me to feel the weightiness of the rock. “This would indicate that it is flow and not a gas-heavy eruption.”
He then picked up a smaller rock, riddled with small vesicles (holes), and handed it to me. It felt much lighter.
This second rock would have been from a heavy gas eruption, he explained. “Scoria,” Derek called it, “usually associated with cinder cones.”
You see, vesicles are a good indicator of the presence of gas, but the particulars for each type of vesicular rock depends on conditions.
For instance, scoria is usually formed from low silica lava, high in gases that expand as they rise during an eruption and the lava cools usually in flight. Pumice, on the other hand, forms from high silica lava that is thicker and stickier resulting in frothy lava that erupts violently and cools quickly in the air.
“Pumice can rise thousands, even ten thousand feet high,” said Derek. “It is a cold ash flow. It isn’t molten when it hits the ground.”
Flow Boundaries
However, vesicles are not reserved for high gas eruptions. Many flows also contain vesicles.
At one point, Derek and I stopped at a collection of rhyolite-dacite rocks with large vesicles to discuss what was going on.
“Rhyolite and dacite are very viscous, so gas cannot escape in a controlled way,” he went on. So, “while it cools, it [the gas] will migrate upward, and might accumulate into bigger vesicles.”
In short, vesicles in flows of lava are generally found near the top. This can be useful for a couple of reasons.
For one, they tell you where the flow boundaries are. “The cooling interfaces are the ground and atmosphere,” explained Derek. And vesicular rocks, as well as rugosity, or roughness, occur at these boundaries.
Second, they help geologists determine which way is up. “I would look at the vesicles and orientation of the vesicles,” said Derek, “this should be related to the free surface.”
Geology meets Botany in the Pumice Flat
As we walked over another small rise, the trees faded behind us and we entered a large, flat open space, hemmed in by a large hill just ahead. Again, we had entered a pumice plain.
Though devoid of any large trees, like the mountain hemlocks we had been walking through for most of the hike, pumice plains are often inhabited by a few different wildflowers. We saw a couple of species of buckwheat, along with purple lupine, and a low-growing Newberry’s knotweed.
Earlier I had asked Derek about the connection between botany and geology—and here on the pumice plain, seemed like the perfect opportunity to discuss.
“Different plants will seek out different geological environments,” Derek said.
The pumice plain is not an ideal environment for most plants. Pumice creates soil that drains quickly and doesn’t hold onto nutrients well. Cold temperatures and low moisture are also challenging. Few plants can tolerate this environment.
“[In the pumice plain] it comes down to austerity and competition—there are not a lot of resources in the pumice plain, little water, and nutrients… but that also discourages competition.”
It takes a special sort of plant to survive the harsh conditions and set the stage for other plants to come in. One example of a species that does this is lupine.
“Lupine is a member of the pea family and can convert nitrogen from the air to a useable from at its roots and therefore make its own fertilizer,” Derek explained. “This gives it an advantage, so it is frequently the pioneer species that then enables other hardy plants to grow in the vicinity.”
Layers of Lava
After climbing over a couple of steep hills we reached a viewpoint. Looking down you could see Three Cree Lake again and the steep cliffside that we had walked up.
The underlying architecture of the lava flows that made up the cliff was exposed in all its many layers. There was the andesite layer, with its platy structures, and many rhyodacite layers with looser pyroclastic layers in-between layers of ash and pumice. It was a magnificent edifice built from a variety of rock types, built from a variety of lavas.
Looking at the layers, I tried to imagine just how each lava flow would have moved across the land so many years ago.
Flow with It
If you recall, volcanic rocks can be classified by their chemistry—specifically their silica content—with basalt and andesites being lower in silica than dacite and rhyolite. This not only affects their form as rocks but more importantly it affects their flow.
“If it has more silica products, it isn’t going to flow far,” Derek explained, “It is thicker and will build up vertically.”
According to Derek, a high silica flow might travel a few miles, maybe 10 miles at most, and at an almost imperceptibly slow pace.
“The big obsidian flow in Newberry National Volcanic Monument is a classic example of a rhyolite flow,” Derek suggested. “Or if you go to Wickiup Plain, you can see Rock Mesa,” another great example.
In comparison, basalt or andesite will erupt as a fluid stream of lava that flows over top of each other—“like many coats of varnish,” Derek described. Also, “Basalt flows can flow much, much further,” Derek went on, “especially if they form lava tubes.”
McKenzie Pass is a good place to see basalt flows.
Even better, Derek suggested watching videos of a Kilauea eruption to truly appreciate the movement of lava in general.
Of course, it should be kept in mind that not all high silica volcanic products are released in lava flows. Pyroclastic flows—best described as a rushing flow of hot volcanic rocks, ash and gas can also travel far. “Perhaps even 100s of miles,” estimated Derek.
Banded
As we walked the last hundered or so feet to the top, several colorful red and black colored rocks caught my attention.
“This looks cool,” I said, pointing to one of them.
It was another rhyolite or dacite specimen, like we had seen before—only it had thick bands of black running through it.
“Rhyolite and dacites are very viscous,” Derek explained, “As they cool, any variation in silica will change the melting point and it will tend to start segregating by silica content forming these bands.” This process, Derek explained, is called flow banding.
“Reminds me of petrified wood,” said Derek.
It was gorgeous.
The Sculptor’s Hand
Finally, we reached the high point on the rim—and the end of the “maintained path.” We stood on the cliff’s edge looking down at a rocky face that dropped down steeply into a basin.
At this point, Derek asked me what I thought about what we were seeing.
“What would you call this type of topography?” He queried.
I must have looked apprehensive to answer because offered a subtle hint—pointing out the “near semi-amphitheater” shape.
“A glacial cirque?” I responded questioningly.
“Yes, a glacial cirque!” replied Derek in a congratulatory tone. “A classic glacial cirque. There is another one over there too,” he remarked referring to the Three Creek Lake Basin area. “It is not an isolated phenomenon…
“And…” he went on pointing to the east toward the lakes, “dollars to donuts, there is some glacial till, moraine material, creating the lumpy topography.”
Glacial cirques are bowl-shaped valleys formed by glacial erosion—the removal of rock and sediment as the glacier flows downslope. When this material is deposited a moraine forms—an accumulation of this debris known as glacial till. Finally, when a glacier retreats and the depression left behind fills with water, a lake can form—a “moraine lake”
Mystery solved. Tam-McArthur Rim is a glacial cirque. And Three Creek Lake is a moraine lake. It isn’t all about the lava, but the ice!
“Glacial processes in the Cascades tend to be underappreciated by the general public,” Derek sighed. Yet, glaciation is just as responsible as volcanism for creating what we see today in the High Cascades
“The volcanic processes provided the raw material,” explained Derek, “the glacial processes provided the sculptor’s hand.”
Geometry of Volcanoes
Of course, standing at the top of the rim, it was hard to ignore the many voluminous peaks filling up the skyline. Broken Top and the Three Sisters were most prominent, but you could also see out toward Mount Washington, Three-Fingered Jack, and Jefferson, as well as Black Butte and Mt. Bachelor.
Volcanoes are often grouped into three major types distinguished by the geometry of the cone—stratovolcano, shield, and cinder cone.
Stratovolcanoes are often the tallest with steep sides; some during a catastrophic eruption may lose their top, like Mt. St. Helen’s, for example. Shield volcanoes are large with shallowly sloping sides, often formed from low silica lava that flows. Cinder cones small and conical, built up by pyroclastic fragments of a single eruptive event.
Changing Geometry
The problem is geometry changes.
“Broken-Top is a generally misunderstood peak,” said Derek pointing to its ragged open maw.
According to Derek, many people assume, based on its shape, that Broken Top catastrophically erupted. “But it is mainly because it has been through a couple of glacial cycles…” explained Derek, that it looks like its top was blown off.
Glaciation has sculpted all the peaks to some degree in the Cascades. Only, volcanoes like Broken Top (active 300,000-150,000 years ago) and Three-Finger Jack (active 500,000-250,000 years ago) were built much earlier than Three Sisters (North Sister, the oldest, active 120,000-45,000 years ago) so they have experienced a lot more glacial erosion.
Derek pulled out a diagram that showed some of the Cascades Volcanoes’ building phases alongside a graph of time vs. temperature data taken from ice cores from Greenland. From the diagram, you could see how long each volcano was in a building phase compared to the number of glacial cycles it experienced.
“Broken Top has been through two glacial cycles,” Derek said pointing to the graph. While “South Sister’s ice cream scoop shape is because it was active during the last glacial period while it was still forming.”
The pointed top of Mt. Washington was visible on the horizon. “Mount Washington is another one that people make assumptions about,” said Derek—“One-fingered George, I call it.”
With its spire-shaped top a lot of people might mistake Washington for a stratovolcano, but, in fact, it is a shield volcano.
“Imagine what the original shield geometry is,” Derek suggested. We traced the line of the slopes that slanted down gently away towards what remained of its top. “What you see left is the central magma conduit,” glaciation took the rest.
I asked about Back Butte, as I remembered it was older than Mt. Washington, but still retained more of its shape.
“Black Butte is an oddball,” Derek replied, “It is the oldest of the Cascade volcanoes—1.4 million years old…It wasn’t heavily glaciated because it is lower elevation and far enough east,” Derek explained.
I tried to imagine what Black Butte (a stratovolcano by the way) would look like if it had been heavily glaciated. Would it even be here now if glaciers had carved it all those 1.4 million years?
Who knew? Ice. Impressive.
Non-Maintained
After exploring the rim, Derek and I decided to continue along the well-used, but non-maintained trail toward Broken Top. The terrain was mostly flat, at first, clumps of whitebark pine bowed over next to the path. Broken Top was striking in the near distance.
As we hiked, we walked through an area littered with what looked like andesite rocks, suggesting a flow in the vicinity, though we never identified the source.
Little Broken Top
However, soon the geology shifted toward a less definable area that contained a mixture of different rock types. Hidden among the varied rocks was a ragged piece with red and black bands that caught Derek’s eye.
“It looks like a miniature Broken Top,” he claimed.
When you look at Broken Top you can see thick bands of color following its slopes on a diagonal—each band of color is a different lava flow, according to Derek.
“Color almost always has to do with the oxidation state of the iron,” explained Derek.
Though not completely understood, fluids that form the magmas and ascend as lavas are oxidized—meaning (in this case) iron is being stripped of electrons as it chemically bonds with oxygen. The different combinations of iron with oxygen are what is responsible for the different colors.
“You can get hematite [Fe2O3] which is red, limonite [FeO (OH) *nH2O] is yellow, and magnetite [FeO] is black,” said Derek.
I snapped a picture of the miniature Broken Top with its bands of color before continuing up the trail toward the true Broken Top—its oxidized lava rock bands almost glimmering in the distance.
Volcanic Bombs
The trail steepened, as we reached a nice viewpoint, and stopped for lunch. After lunch, we decided to take the trail just a little bit further—Derek had one more artifact to show me.
“These are volcanic bombs,” said Derek, pointing to a large, elongated reddish volcanic rock, “some of the best examples, as they are relatively intact.”
The rock was slightly pointed on one end and looked pulled or stretched—lines ran through the rock parallel to its lengthier side—like pulled taffy.
“If you have cohesive blobs of lava ejected, in flight they will adapt an aerodynamic teardrop shape,” Derek explained.
We continued up the trail in pursuit of a few more bombs that Derek had seen on a previous visit.
The trail wound up a red pile of cinders—the rocks oxidized to red hematite—before reaching a narrow ridge with several massive volcanic bombs.
These bombs were huge—the size of a large dog. One lay open—its guts exposed for us to see—probably broken from the impact.
“Probably from basalt or low silica lava,” Derek decided as we examined its innards—this is typical of lava bombs.
All of It
I was struck by the beauty of the place—the arc of volcanoes, the sparse vegetation, the open expanse, and these amazing rocks that made an impact some 10s-100s of thousand years ago. All of it.
“It is what Central Oregon has to offer,” said Derek.
Breathtaking geology.
And with one last look, we headed back.
