Entering the Blast Zone
The skies were clear blue as I headed out to meet with volcanologist Mariah Tilman for our hike at Shellburg Falls. In the distance, I caught a glimpse of snow capped Mount Jefferson, the second tallest Cascade volcano in Oregon. Though we weren’t going to get up close to this behemoth during our hike, I wondered if our walk in the foothills of the Cascades might offer a glimpse into Jefferson’s power. What forces are responsible for the formation of the Cascade peaks? Are these same forces at work in other parts of Oregon? Would we see any evidence of past volcanism during our hike through a humble state forest? Little did I know, I was about to enter the “blast zone” when it comes to volcano knowledge.
The Hike
Hike at a Glance
Trailhead: Shellburg Falls Trailhead
Distance: about 2.8 round trip. Out and back trail.
Elevation Gain: about 400 feet
Notes: There is no restroom at the trailhead. Parking is limited. Part of the hike is on private property so stay on trail. There are many variations to this hike with options for more mileage.
Volcanology Basics
The Shellburg Falls trail begins on a road through private pasture land before entering the forest. As we made our way through this area, Mariah and I talked a bit about what it is like to be a volcanologist, as well as why the profession is so important.
Of course, the job of a volcanologist is to study volcanoes. There are five USGS volcano observatories, all found in the western U.S., including the Cascades Volcano Observatory in Vancouver, WA. The main goal of these observatories is to monitor volcanic activity in order to predict and assess the risk associated with volcanic eruptions.
How do they do it? According to Mariah, there are a lot of tools a volcanologist will use to size up volcanic risk. Among the most important tools are seismometers. These can be placed throughout the landscape in order to detect movement of the earth, and give us an idea of what is happening below the surface. Another tool that is used is satellite imagery. Satellite imagery can be especially useful in monitoring the activity of volcanoes in remote areas, like Alaska, which has 52 active volcanoes, most of which are part of the hard to reach Aleutian islands.
Safety First
Public safety is the primary reason we study volcanoes. Besides the threat of lava and pyroclastic flows near the erupting volcanic vent, lahars- a hot mix of water and volcanic debris- can travel dozens of miles. If an eruption occurred during our hike, a lahar from Mt. Jefferson could easily travel far enough to reach us and neighboring towns. Yikes!
Then there is ash. Ash has the ability to travel large distances causing widespread disruption of natural and human systems. As Mariah explained, ash can be especially problematic for air traffic. In 1989, two jetliners nearly went down in an ash cloud generated by the eruption of Mount Redoubt in Alaska.
Fortunately, since the famous Mount Saint Helens eruption of 1980, scientists are better equipped to monitor and predict volcanic eruptions, sometimes even a year in advance. Given enough warning, communities can at least prepare for the onslaught.
With Mt. Jefferson looming “a little too close for comfort,” I asked Mariah if we should be concerned about it erupting. She reassured me that none of the Cascade peaks are currently predicted to erupt anytime soon. Phew!
It’s All Downhill
As we made our way into the forest, we encountered our first geological phenomenon- the remnants of an old landslide. Landslides occur when the shape of the land, climate, and geology work in concert to weaken the connection between overlying sediment and material beneath. When this occurs, gravity takes over, moving earth materials downhill where they accumulate. Though people often think of geology as the building up of land through plate tectonics and volcanism, the wearing down of the land by weathering and the movement of land by erosion, are also important geological processes.
But how do we recognize a wearing down process, like a landslide, in nature? What can we observe to understand the geological activity of a place? I asked Mariah what to look for.
Think like a Geologist
She explained, one of the best ways to begin thinking like a geologist is to look for patterns in the landscape. Differences in the landscape are important evidence to understand the geology of a place. Though the area where the landslide had occurred in the past was now overgrown with trees, moss, and other vegetation, Mariah pointed out that the shape of the land had changed.
There were other landslide clues as well. First, Mariah and I observed many large rocks strewn about the base of the hillside. Unlike in a river, where sediments are sorted by size as the river loses energy downstream, rocks in a landslide lose energy abruptly, falling into a jumbled piles. Second, the shape of the rocks was angular. Landslides happen quickly, so rocks in a landslide will be angular, instead of worn down and smooth.
Hotspot or Subduction?
As we continued our hike past the landslide, the shape of the land changed again . We started noticing outcroppings or rock of unknown origin to the left of us. Mariah and I began to speculate- How did these rocks form? Where did they come from?
Mariah narrowed down the source of these outcroppings to two likely possibilities. First, about 16.7 to 5.5 million years ago it is believed that the Yellowstone hotspot was under the Oregon-Idaho-Nevada border. This hotspot resulted in huge floods of basalt lava to cover large swaths of Oregon.
Secondly, about 35 million years ago and again 7 million years ago, tectonic activity along the Cascadia subduction zone built up the old and new Cascade volcanoes. Subduction occurs when an oceanic plate plunges beneath an overriding plate. As the descending plate heats up in the mantle it “sweats,” resulting in a build up of gases and pressure- the perfect conditions for explosive volcanic eruptions characteristic of the Cascades and other stratovolcanoes.
Igneous Rocks, Rock!
Though Mariah wasn’t 100% sure the origin of the rocks in the Shellburg Falls area, one thing was certain- these were igneous rocks. In general, rocks can be classified as igneous (molten rock that has cooled), metamorphic (rock that has been subjected to intense heat and pressure), or sedimentary (rock formed from compacted sediments). However, rocks can also be further described and classified depending on how they formed and their mineral content.
Rocks formed from hotspot volcanism, for example, are typically basalts, with high amounts of iron and magnesium and low amounts of silica minerals, giving them a dark color. In contrast, rocks like rhyolite, that are formed through subduction volcanism, have a higher amount of silica content, making them lighter in color. So rock color is a clue to the mineralogy, which in turn is a clue to rock formation.
However, Mariah warned, be careful of broad generalizations. Stratovolcanoes (those formed by subduction) actually form many types of rocks during their activity, including basalt. Also, the color of rocks can easily be distorted by weathering, making it difficult to identify the mineralogy based solely on color.
Count the Minerals
In order to effectively classify an igneous rock, you need to look at the mineral composition more closely. Basalt by definition should only be 49-50% silica, rhyolite should be 70-75%, with andesite falling in-between. Unfortunately, in order to get down to percent composition that requires magnification. Without a microscope in the field, using color and shape are often the best one can do.
With that in mind, and after cracking into a rock to get a better look at its color, we came to a conclusion that the outcroppings were probably basalt. Left in uncertain agreement, we hurried up the road.
Crystal Clear
Soon, we reached the bridge that leads over Shellburg Creek, just above lower Shelberg Falls. To the left, was a large outcropping of igneous rock. At Mariah’s suggestion, we stopped to examine the rocks here.
However, rather than trying to identify them, Mariah began searching the rocks for crystals. Mariah explained, in order to really understand the life of an igneous rock, knowing the type is not good enough- you have to look at the crystals!
A bit like tree rings provide the life history of a tree, crystals provide a record of where and for how long the magma the crystal formed in spent underground. Different crystals will form in magma depending on its temperature and depth. For example, olivine- a green colored mineral- forms at high temperatures and depth. While quartz forms at low temperatures and shallow depths.
Perhaps the most notable crystals that form in magma are those called plagioclase feldspars. The chemical composition of these crystals will change depending on temperature. Deep in the ground under high temperatures they are calcium rich, but as the crystal grows closer to the surface, calcium is gradually replaced by sodium. The results are concentric rings of crystals with different amounts of sodium and calcium that offer a record of the magma’s movement before an eruption.
Unfortunately, we didn’t find any distinct crystals in our wall of rocks, only some small grains. It seems the magma that formed this particular outcropping did not spend much time underground.
Right before the outcropping is a small dirt trail to the left that leads to upper Shellburg falls. We retraced our steps back a few yards to this junction and made our way onward toward our final stop- the falls.
Free Fallin’
As we walked along Shellburg creek, we could see large boulders of rock in the creek below. Where did they come from? These boulders were likely the remains of an old waterfall overhang- “old Shellburg falls.”
You see, waterfalls form when a hard rock overlays a soft rock. In the case of Shellburg Falls, basalt rock sits on top of sedimentary rock. The softer rock erodes over time creating a waterfall overhang. With enough weathering, the overhanging rocks stability can become compromised resulting in collapse. This process of weathering and collapse means a waterfall is always moving further upstream over time.
We would need to move further upstream to see”new Shellburg falls.”
Blanketed in Basalt
Before long, Mariah and I were in full view of the waterfall. The hard igneous rock cliffs that line the canyon, and form the waterfall overhang, stood out beautifully against the sedimentary rock below it.
But wait, look at the rocks to the left! The left wall of the canyon showed a familiar jointing pattern- columnar basalt! Columnar basalt looks sort of like a pipe organ, but with hexagonal pipes that aren’t pipes at all, but columns of lava rock. This pattern of basalt is the result of slow cooling, cracking, and contracting. Columnar basalt is not only useful for identifying rock as basalt, but it is a geological wonder in many regions around the world.
Ancient Waters
Things got even more interesting, as we made it into the large cavern behind Shellburg falls. From here, you could see how the soft sedimentary rock had been worn away below the basalt cliffs. However, rather than looking up at a ceiling of hexagonal columns of basalt like that observed outside the cavern, large bubbles of rocks protruded down towards us. We found pillow basalt!
Pillow basalt forms when lava flows into water and cools there. That means the location of present day Shellburg falls was once the location of another ancient body of water. Not only that, but this ancient body of water probably existed for some time. The sedimentary layer below the basalt was thick; it must have taken a good deal of time to collect so much sediment- possibly millions of years!
According to Mariah, the geological history of Oregon is not very long compared to other areas of the country. Oregon is young geologically speaking. Yet, so much has happened to take us up to the current day. Oregon of the past was a fiery furnace with lava flows and explosive eruptions; it faced deluges of water & ice; and experienced many changes in climate and weather. It has been built up and torn down countless times by the forces of nature. And it is just beginning! The ancient body of water that existed in the past may be long gone, but give it a few million years and Shellburg Falls will look completely different.
Rock on!
After continuing to the other side of the falls for a different perspective, Mariah and I decided to head back to the trailhead. Who knew that in just a few miles of trail, one could see so many signs of geological activity. From landslides to lava flows, from weathering to the formation of crystals, you don’t need to visit a volcano to see the drivers of geological activity in Oregon. Just pay attention to the landscape. And maybe pick up a rock or two.
Mariah Tilman is a volcanologist and igneous petrologist. She studied volcanoes at the University of Alaska, Fairbanks. In addition to volcanology, she also has a background in hydrology and water quality. She currently teaches Geology of the Pacific Northwest among other classes at Chemeketa Community College and Portland Community College.
