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Volcanoes in the PNW

Mount Hood (11,239'), an active volcano, is a part of the Cascade Volcanic Arc. Seen from the southeast in Wasco County, OR.

Introduction

Volcanoes form much of the iconic landscapes here in Washington and the PNW, from the likes of the colossal Mt. Rainier to the sobering Mt. St. Helens. Volcanoes are places of great mystique and energy, and have been the subject of many mythological tales from all over the world. Due to the Pacific Northwest’s active geology, most notably the processes associated with the Cascadia Subduction Zone, hundreds of volcanoes line the coast from northern California to British Columbia. Before we dive into the different types of volcanoes that can be found here and their eruptive threats to major population centers, let’s discuss what exactly volcanoes are and how they’re formed.

What is a Volcano?

Generally speaking, a volcano is defined as a rupture in the crust that allows hot lava, ash, and gasses to escape from a subterranean magma chamber- a reservoir of magma. Volcanoes exist on any planet or moon with a solid crust, but obviously we’re focusing on volcanoes that exist on Earth for these purposes. Before expounding, it’s important to know the distinction between lava and magma, since these will be terms that are discussed ad nauseam when referring to volcanoes. Lava is molten rock that has erupted out of a volcano onto the surface of the Earth, while magma is molten rock that has not erupted and is subterranean. Basically, all lava starts out as magma and magma turns into lava when it erupts out of the Earth. Volcanoes are present all over the world, but by and large, the vast majority of them form on or near tectonic plate boundaries, specifically where they (the tectonic plates) are converging or diverging. Moreover, volcanoes can form in rift zones where the earth’s crust is thinning and being stretched apart, as well as in the middle of a tectonic plate where there is a mantle plume (an anomalous plume of magma rising from the mantle) present, forming what’s known as a hotspot (think Hawaii). The volcanoes that are present in the PNW are formed due to the convergence of the Juan De Fuca Plate with the North American Plate at the Cascadia Subduction Zone.

Oregon's Mount Jefferson (10,502') is a textbook example of a stratovolcano.

How are Volcanoes formed in Subduction Zones?

Several processes are undergone in the formation of volcanoes at subduction zones, but the arguable defining condition of the formation of volcanoes in this geological environment is, perhaps surprisingly, the presence of water. Water is paramount to lowering the melting point of the crust in a subduction zone to allow for volcanoes to be formed. The process of how volcanoes are formed in subduction zones will be outlined in the following passages.

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Subduction Zones are characterized by denser oceanic crust subducting underneath lighter continental crust as both tectonic plates collide. The dense oceanic crust is composed of mafic igneous rock- rock that is derived from iron-rich, silicon-poor magma, such as basalt/gabbro. Conversely, continental crust is mainly composed of felsic igneous rock- rock that is derived from silicon-rich, iron-poor magma, such as rhyolite/granite. Due to the chemical makeup of the magma/rock, oceanic crust is denser than continental crust (iron is heavier and denser than silicon), and as such, will sink under the lighter continental crust in zones of subduction. It is important to note that all magma on earth begins as mafic, iron rich basalt, and in order to become silica-rich rhyolite, the magma has to evolve- undergoing chemical reactions involving water. As tectonic subduction occurs, great amounts of water from the ocean get dragged into the zone of subduction, altering the mineralogy and chemistry of the crust, evolving the iron-rich basalt to silicon-rich rhyolite, effectively forming new continental crust.

As the Juan De Fuca Plate subducts underneath the North American Plate, the magma evolves from basaltic magma to rhyolitic magma. The water dragged into the subduction zone from the ocean (both physically and chemically in the structure of minerals such as serpentine) lowers the melting point of the subterranean rock near the plate boundary, melting the solid rock into magma and allowing for metasomatism (chemical alteration of rock) to occur as the iron-rich basalt incorporates water into its chemical structure, changing the chemical composition of the magma. It is important to note that temperature is a large variable here too- mafic magma is far hotter than felsic magma, and the water lowers the temperature of the crust immensely.

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As this occurs, iron-rich minerals such as Olivine and Pyroxene crystallize and exit the melt in a process known as fractional crystallization. The more mafic (iron-rich) minerals (Olivine, Pyroxene) crystallize and exit the melt first because they have far higher melting points (roughly 1200 degrees Celsius) than the more felsic (iron-poor) minerals such as Quartz (600 degrees Celsius). This process is outlined by Bowen's Reaction Series. As these iron rich minerals exit the melt, the melt loses much of its mass as it is now enriched with lighter, more silicon-rich minerals such as Quartz and Orthoclase Feldspar. The evolved magma then rises through the crust to the surface, since it is now buoyant and lighter than the crust, which has incorporated the heavier, iron-rich minerals that were fractionally crystallized from the melt. Additionally, assimilation (partial melting of the existing felsic continental crust) is another key player in evolving the composition of the magma at subduction zones. As this magma erupts at the earth's surface above the subduction zone, volcanoes are created. It is important to note that all types of magma- from basalt to rhyolite erupt in subduction zone volcanoes, and fractional crystallization and magmatic evolution do not always occur everywhere in subduction zones, though those processes are present.

Map of major active volcanoes and population centers in the PNW, by Solomon Feinstein.

Rudimentary diagram outlining the process of volcanism in the Cascadia Subduction Zone, drawn by Solomon Feinstein.

Types of Volcanoes in the Pacific Northwest

Now that we know what volcanoes are and how they are formed at subduction zones, let’s discuss the different types of volcanoes present at the Cascadia Subduction Zone and some examples of them. In the PNW, Stratovolcanoes, Cinder Cones, Calderas, and Shield Volcanoes are present.

Stratovolcanoes

Stratovolcanoes, also known as composite volcanoes, are tall, conical, steep volcanoes built by many layers of viscous hardened lava, tephra, and pyroclastic flows. Viscosity is defined as the resistance to flow due to internal friction. As a rule of thumb, felsic, rhyolitic lava is generally very viscous, flowing almost like maple syrup. Conversely, mafic, basaltic lava is not viscous, flowing almost like water. Stratovolcanoes are formed when magma continuously erupts over a long period of time (500,000 years in the case of Mt. Rainier). Effectively, they are composed of thousands of lava flows stacked on top of each other, and this is reflected in the structural geology of the mountain. Geologists are able to analyze every layer of the volcano and attribute different eruptive patters of the volcano layer by layer, as stratovolcanoes are composed of interspersed layers of volcanic rock, some suggesting quiet eruptions that were more constructive (helped build the mountain up) than destructive (eruptions that destroyed part of the mountain), others suggesting destructive and violent eruptions. Geologists interpret the nature of eruptions by analyzing the rock. Intact rock such as andesite, basalt, rhyolite, and dacite suggest quieter, constructive eruptions, where as tephra, ejecta, ash, pumice, and breccia suggest more violent eruptions. All of the highest volcanoes in the Cascades are stratovolcanoes, including Mt. Baker, Glacier Peak, Mt. Rainier, Mt. Adams, Mt. St. Helens, Mt. Hood, Mt. Jefferson, The Sisters, Mt. Mazama (Crater Lake), Mt. Shasta, and Lassen Peak.

Cinder Cones

Cinder Cones are much smaller than stratovolcanoes and are often created by minor volcanic vents rather than major volcanoes. They are defined as steep conical hills, composed of loose volcanic material such as ash, pumice, and/or scoria (highly gaseous volcanic rock that was ejected as a molten blob during an eruption and cooled in the air) that has been built around a volcanic vent. Cinder Cones are often found on the flanks of stratovolcanoes, shield volcanoes, and calderas. Hundreds of them can be found throughout the Cascades, most notably in central Oregon near Newberry Volcano (Over 400) and northern California near Lassen Peak.

Mt. Baker (10,781'), an active stratovolcano near Bellingham, WA.

Mt. Hood (11,239'), an active stratovolcano near Portland, OR.

An unnamed cinder cone near Bend, OR.

Calderas

Though not technically a distinct type of volcano, Calderas are unique in a litany of manners. Calderas are defined as large depressions that occur after a volcano has erupted inwards and collapsed itself. Oftentimes, calderas fill with water, especially in wetter climates. The most famous example of a caldera in the PNW is Crater Lake, which exists in a caldera created 7700 years ago when Mt. Mazama, a stratovolcano, imploded. It is now filled with the deepest lake in the United States, at a depth of 1,946’.

Crater Lake, located in southern Oregon, is a classic example of a caldera- it's the collapsed remains of Mt. Mazama.

Shield Volcanoes

Shield Volcanoes are a type of volcano exemplified by their low profile, often resembling a shield lying on the ground. They are formed by eruptions of highly fluid lava, most notably mafic basalts with low viscosity. These lava flows are thinner and travel further than more viscous, felsic lava flows, thus creating an edifice that is not as steep as a composite volcano (stratovolcano). The most notable examples of shield volcanoes are the Hawaiian Islands, specifically Mauna Kea and Mauna Loa, but there are several shield volcanoes present in the Cascades, including Indian Heaven in Washington, several shield volcanoes in central Oregon, and Medicine Lake Volcano in California.

Three-Fingered Jack (7,844'), the eroded remains of a large shield volcano in central Oregon.

Volcanic Threats in the Pacific Northwest

As you are now aware, there are hundreds of volcanoes in the Cascadia Subduction Zone. Several of them are active stratovolcanoes, and many of them are within close proximity to major metropolitan areas, including, but not limited to, Seattle and Portland. Several cities in the Pacific Northwest would be directly affected by volcanic eruptions of volcanoes that exist near each city. From north to south, Mt. Baker poses a direct threat to Bellingham and Mount Vernon, WA. Glacier Peak is relatively remote, but lahars from Glacier Peak could reach Everett and Marysville, WA. Mt. Rainier is an absolute behemoth that poses great threat to and looms over the Seattle-Tacoma Metropolitan Area, from downtown Seattle to Olympia. Mt. St. Helens, though remote, is the most active stratovolcano in the Cascades and explosively erupted in 1980, decimating thousands of square miles of wilderness and blanketing a large area with ash for days. Mt. Adams, though relatively quiet, is still considered active and poses direct threat to Yakima, WA. Mt. Hood, a sleeping beauty, is a direct threat to the entirety of the Portland Metropolitan Area in Oregon and Washington. Mt. Jefferson looms over Salem, and the Sisters pose a direct threat to both Eugene and Bend, OR. Newberry Volcano also poses a threat to Bend. Mt. Mazama, though relatively remote, created the huge and picturesque caldera known as Crater Lake, and will most likely erupt again in the future. Mt. Shasta, another 14,000+’ behemoth, poses a direct threat to Yreka and surrounding areas in California. Lastly, Lassen Peak, an explosive stratovolcano that last erupted in 1921, poses a direct threat to Redding and Red Bluff, CA. All this is to say that there are over a dozen active volcanoes that pose direct threats to cities in the Pacific Northwest. But what exactly are the threats posed by the volcanoes?

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Pacific Northwest Volcanoes are extremely hazardous for a multitude of reasons. Three main hazards associated with the various volcanic edifices in Washington and the rest of the PNW include ash fall, pyroclastic flows, and lahars.

Ash Fall

Ash Fall is exactly what it sounds like- falling ash. Though that sounds relatively innocuous, ash fall can be extremely hazardous for a litany of rationales. Firstly, it’s important to know what exactly ash is- specifically volcanic ash. Volcanic Ash is different from run-of-the-mill ash from a campfire. It is composed of fine particles of glass that’s been ejected from a volcano, and as such, inhaling volcanic ash can be very dangerous. Not only does it pose the risk of serious lung injury upon inhalation (such as silicosis), it is also very destructive to internal combustion engines and air filters, clogging and virtually destroying them. If any Cascade volcano were to seriously erupt, flights to and from Seattle, Portland, and elsewhere in the PNW would be disrupted for quite some time. If an eruption was large enough, it could also disrupt global flight patterns, similar to Iceland’s major volcanic eruption in 2010. Additionally, due to the density and mass of volcanic ash, merely 20 cm of it could destroy a building. Moreover, volcanic ash can blanket crops, produce lightning storms, and if the volcanic eruption was large enough, even disrupt global climate for a period of up to a few years. Due to prevailing winds, communities east of the Cascade Range face the greatest threat of volcanic ash fall if an eruption were to occur, including Yakima, the Tri-Cities, Spokane, Bend, and Klamath Falls. A significant amount of volcanic ash from a large cascade eruption could likely travel as far as Boise, ID and Reno, NV as well. When Mt. St. Helens erupted in 1980, roughly 540 million tons of ash was ejected from the volcano and blanketed an area of over 20,000 square miles. St Helens’ ash cloud traveled as far as the central United States, and some of the ash traveled around the entire world. Yakima, a major city east of Mt. St. Helens, was covered in an inch of volcanic ash from Mt. St. Helens, the ash completely blocking out the sun, turning day into night.

Ash Fall from Taal Volcano, Philippines. Photo from Preview.ph.

Map of extent of ash fall from 1980 Mt. St. Helens Eruption. Courtesy of USGS.

Pyroclastic Flows

Pyroclastic Flows are another major hazard from stratovolcanoes, especially those in the Pacific Northwest. A pyroclastic flow is defined as a “hot, fast moving ‘cloud’ of gas, ash, and rock debris known as tephra” (Pacific Northwest Seismic Network). Pyroclastic flows can reach speeds as fast as 700 km/hr and temperatures as high as 1000 degrees celsius. They basically destroy everything in their path, and due to their density, they tend to “hug the ground” rather than creating a plume in the air. Though they are harbingers of death, they luckily tend to fizzle out before going too far from the volcano in which they erupted from. The best way to avoid them is to avoid a volcano when an eruption is imminent or when the volcano is in a period of unrest. The 1980 eruption of Mt. St. Helens created an absolutely colossal pyroclastic flow, due in part to the fact that the volcano erupted laterally rather than vertically. Taking a drive up to Mt. St. Helens showcases this perfectly- much of the forest that was leveled by the pyroclastic flow is still trying to recover, 43 years later.

Destruction of forest on Mt. St. Helens from pyroclastic flow; 43 years later the slopes are still bare.

Lahars

Lahars are perhaps the most dangerous volcanic hazard to major metropolitan areas in the PNW, particularly to Seattle, Tacoma, and Portland. Lahars are volcanic mudflows that contain at least 60% volcanic material (ash, tephra, lava, rock, etc), have the density of wet concrete, and travel down valleys from the volcano they erupted. Unlike pyroclastic flows, they travel much further from the volcano that they originated from due to their liquidity, but like pyroclastic flows, they destroy everything in their paths. Lahar threats are exceedingly high for Mt. Rainier in particular, due to Mt. Rainier’s sheer size and glaciation. Looming over the southern Puget Sound region at a whopping 14,410’ in elevation, Mt. Rainier is the most glaciated mountain in the contiguous United States. Glaciers are a major contributor to the formation of lahars, particularly when they rapidly melt from hot volcanic material during an eruption and mix in with the hot lava, ash, tephra, and rock. This creates a perfect storm of destruction when a major eruption occurs on Mt. Rainier. The southern suburbs of Seattle (Renton, Kent, Auburn) and downtown Tacoma stand right in the path of the deadly lahars Mt. Rainier would produce as they race down the Puyallup, White, and Nisqually River Valleys. Lahars alone from a Mt. Rainier eruption could kill over 80,000 in the southern Puget Sound region. Lahars from past Mt. Rainier eruptions traveled as fast as 70-80 km/hr (45-50 mph) and were as deep as 150 m (490’), absolutely plowing everything in their paths. Due to the lahar threat at Mt. Rainier, the peak is listed as a “Decade Volcano”- one of the world’s deadliest.

Lahar hazard map for Mt. Rainier, USGS.

Mt. Rainier (14,410'), seen from an airplane leaving Seattle en route to Boise.

Surviving a Volcanic Eruption in the Pacific Northwest

Unlike “The Big One”, we will luckily get some warning before the next Cascade volcano blows. Cascade volcanoes are some of the most monitored volcanoes in the world, and several signs precede a major volcanic eruption. Seismographs are present on most major volcanoes in the PNW, and volcanic gas output is highly monitored by Geologists. Before eruptions, volcanoes tend to “wake up”- outputting more volcanic gas, hosting swarms of earthquakes as magma moves underground, and even visibly bulging and deforming as magma rises from the subterranean magma chamber towards the surface of the volcano.

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As of the time of me writing this (June 2023), no volcanoes in the PNW have shown signs of impending eruption, though several certainly have shown signs of activity. Just two weeks ago, Mt. Hood experienced a swarm of 28 earthquakes, the strongest a magnitude 3.0. These earthquakes were all very shallow, suggesting magma movement within the volcano, though Geologists postulate that they aren’t indicative of an impending eruption. If earthquake swarms continue at Mt. Hood and increase in quantity and frequency, then I’d start to worry about an impending eruption, but for now, signs don’t point to one. Additionally, The Sisters in Oregon have been bulging for over a year now (detected in January 2022), but nothing suggesting an impending eruption has occurred. Mt. Baker, located near Bellingham, WA, constantly emits volcanic gas from fumaroles (openings on or near a volcano where hot volcanic gas emerges), though there have not been any recent changes in this fumarole activity. All this is to say that volcanoes in the Pacific Northwest are alive and well and will continue to show signs of activity, though nothing out of the ordinary suggesting an impending eruption has been observed. Moreover, the reason why we are able to observe and document all of these subtleties is due to how extensively monitored each volcano is in the PNW, which bodes well for alerting the public of an impending eruption when that time inevitably comes around.

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Knowing that the volcanoes in the Cascades are extensively monitored and that we will have some semblance of warning before they erupt, what can we do to survive a major Cascade eruption? The short answer is, it’s complicated and depends on where you live and what the weather is like when the volcano does erupt. With that being said, here are a few rules of thumb. Firstly, if you live in a lahar zone, get out. Immediately. Have enough food and water to live for a few weeks, take your most prized belongings, and leave. Do not visit the volcano that is about to erupt. Another pointer I would personally give is to have a supply of N-95 masks, because when ash fall occurs, breathing without a mask could be fatal. Ash particles as small as 10 microns can be present in the air, causing serious damage to your lungs if inhaled. Furthermore, having goggles to protect your eyes is also advisable. I would also have an abundance of extra water, as water supplies could be disrupted by an eruption. Wear protective clothing as well, and have supplies to block out ash. When ash fall occurs, refrain from driving as well. As long as you aren’t on the mountain or in the lahar zone, as long as you have some key supplies such as N-95 masks, goggles, extra food, and extra water, you should actually be fine. I’m personally far more worried about a Magnitude 9.0+ earthquake on the Cascadia Subduction Zone or a 7.0+ rupture on the Seattle Fault than I am a volcanic eruption. Hopefully I don’t eat crow for that position…

Mt. St. Helens (8,366') from Spirit Lake Highway, Cowlitz County, WA

Mt. Baker (10,781') from Artist Point, Whatcom County, WA

Mt. Adams (12,276') from Camp Muir, Pierce County, WA

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