This week marks the 100th anniversary of the eruption of California's Lassen Peak. As the anniversary slides past, it leaves Mount St. Helens as the only Cascade Range volcano that has erupted over the last century. This means that although we have thirteen major composite volcanoes plus a multitude of smaller cinder cones and lava domes running from California into Canada, only one has experienced an eruption in the past 100 years. Does that mean we don't have to worry about the Cascades as a volcanic hazard anymore? That answer is decidedly "no" ... but why?

Why do the Cascade volcanoes exist, anyway?

The Cascade Subduction Zone, showing the location of the trench, the downgoing slab, the active volcanoes and some major earthquakes. NPS

It is all thanks to subduction, the process of recycling that sends old oceanic crust back into the mantle. For the Cascades, this is subduction of three very small tectonic plates: the Juan de Fuca, Explorer and Gorda Plates. These are all remnants of a much larger plate (the Farallon) that broke apart and has mostly been subducted (and helped form the Sierra Nevada).

Today, these microplates are sliding underneath North America at a rate of ~3.5 centimeters per year. As the oceanic plates slide under the continental plate of North America, they start to dip sharply (at ~55º from horizontal for the Cascades). This means that by the time the plate is ~70-100 kilometers from the trench (see above), it reaches a depth of ~100 kilometers ... and as the plate goes deeper in the Earth, it gets hotter.

This is the root cause of all the volcanism in the Cascades (and all volcanic arcs). However, it might not be what you think. The downgoing plate (as the oceanic plate is called) heats up but it doesn't melt to form the magma that eventually erupts in the Cascades. Instead, a pile of different mineral reactions occur to release water trapped in the structure of certain minerals.

These are dehydration reactions that they send water and other fluids upwards into the mantle that trapped between the downgoing plate (see above) and the overriding plate (in this case, North America). This water does to the mantle what salt does to ice: lowers the melting point. So, what was solid mantle begins to melt a little bit and it is this melting of the mantle that creates the magma that may eventually erupt.

So, without that tectonic action of these microplates sliding to their doom under North America, we would have no Cascade Range. The Cascades aren't the only place on Earth with volcanoes caused by subduction. The western coasts of South America, Japan, the Kamchatka Peninsula of Russia, New Zealand, Indonesia and many more places have active subduction and with it, lots of active volcanoes. The Cascades are merely one piece of the puzzle for how the Earth's crust keeps its balance, destroying crust as it creates new crust at the mid-ocean ridges.

Volcanic Activity in the Cascades

The USGS watches over the Cascade volcanoes, from Washington's Baker in the north to Lassen Peak in the south, through the Cascades Volcano Observatory and California Volcano Observatory. Most of the volcanoes show signs that they are still active volcanoes, whether it be small earthquake swarms from time to time, hot and cold springs, fumaroles or the occasional bouts of deformation. However, only St. Helens has erupted since the 1915 activity at Lassen Peak. Twice St. Helens roared back, first in the 1980's, including the cataclysm eruption for which it is most famous and then again in the 2000's, when domes of lava started to refill the scar left from the 1980 explosion and collapse.

For an entire volcanic arc, that seems quiet to many onlookers. Right now, the USGS status for the Cascades has it "green" across the board. This means that none of the volcanoes it monitors are showing any sign of activity. Compare that to some other similar subduction-related locations and the Cascades seem awfully still. In Kamchatka, there are sometimes four or more volcanoes erupting at the same time and if you glance at the USGS/Smithsonian Weekly Volcanic Activity Report, you'd see that most volcanic arcs have at least a volcano or two making trouble.

So why do the Cascades seem to be so different?

4000 years of Cascade volcanism. USGS

Before we can answer this question, we should probably look at the past activity in the Cascades. The USGS has a great graphic that shows the eruptions in the Cascades for the last few thousand years (see above). From this perspective, the Cascades don't look as quiet and for geologic processes, thinking in terms of hundreds to thousands of years is probably a better frame of reference than any human lifespan.

A few volcanoes really dominate the activity (St. Helens, Rainier, Medicine Lake and Shasta), so are they skewing our views of Cascade activity? I compiled the last confirmed eruption from each Cascade volcano (and some other volcanic areas in the range) to get a sense how odd this current quiet might be. If you take a look at the plot, you can notice a few things.

The most recent eruptions from each Cascade volcano, compiled from Global Volcanism Program data. Cinder Cone and Chaos Crags are part of the Lassen Volcanic Center. Collier Cone and Devil's Hills are part of the Three Sisters. Belknap Crater is a lava flow field between Jefferson and the Three Sisters. Erik Klemetti

First, it has been quiet over the last hundred years. However, if you back up to about 1700 AD, then the numbers climb rapidly. Over the last 3 centuries or so, Hood, Rainier, Shasta, Glacier Peak and Baker join St. Helens and Lassen Peak in the Eruption Club. Go back a little further, and we can add Cinder Cone (a small eruption at the edges of the Lassen Volcanic Center).

After that, we have to jump back to around 900-1100 AD for other volcanoes' last eruptions. This bunch includes Jefferson, Adams, Medicine Lake (Glass Mountain) and the eruption of the Chaos Crags domes at the Lassen Volcanic Center. The Big Obsidian Flow at Newberry Caldera erupted about 1330 years ago while some of the big lava flow fields in central Oregon formed about 1500 years ago.

Really, the only volcanoes that haven't joined in over the last 2,000 years are Crater Lake and the Three Sisters (not including Collier Cone, which might not be related directly to North Sister). However, both of those did produce some spectacular eruptions in the geologically-recent past, including the Devil's Hills on South Sister and the collapse of Mt. Mazama to form Crater Lake at ~5,700 BCE, the largest eruption recorded in the Cascades.

So, from a deep(er) time perspective, the Cascades aren't all that quiet ... but still, they definitely aren't as active as a lot of volcanic arcs.

Is there a tectonic reason?

If the Cascades are quieter than most arcs, you have to think it has something to do with the tectonic processes that form the magma that erupts. So, can we identify anything about the Cascade subduction zone that might make it different than other, more active, subduction zones?

A compilation of various subduction parameters for the Cascades and Kamchatka. Sources are linked to in the text. Erik Klemetti

[Speculation Alert!]

When we look at arcs, the age of the oceanic crust that is being pulled downward, the angle that it is heading and the rate that the plate is going down are thought to play an important role in how magma might be produced. If we want to compare our Cascades with a very active volcanic arc, let's say on the Kamchatka Peninsula in Russia, we might get a sense of the different between a very active and less active subduction zone.

The oceanic plates going down under North America are younger, dipping steeper and moving slower than their counterparts on the opposite side of the Pacific Ocean. Why might that make a difference? One reason might be the age of the crust that being shoved under North America.

Old, cold oceanic crust (some of the oldest on Earth), like what is going down under Kamchatka, has had a lot of time to have the mineral reactions that create water-bearing minerals, converting the oceanic crust into a "sponge" of water, ready to be heated and "squeezed" out as it plunges under the Eurasian plate. Off of North America, the oceanic plates are young and hot (almost 10 times younger than the Pacific Plate going down under Kamchatka), so they haven't experienced as much alteration so they might not have as much of those water-bearing minerals really needed to get a lot of magma to form.

The slab dip might be important too. For the Cascades, the plate plunges down at ~55º, both near the trench and under the volcanic arc. For Kamchatka, the slab is steep as it goes down and then gets shallower under the arc. This might allow for more dehydration of the slab under Kamchatka than the Cascades. Even the rate of subduction might play a role, where the faster subduction under Kamchatka allows for more "wet" oceanic slab to move through, releasing more fluids and forming more magma.

It appears that there are a few tectonic factors that might suppress the generation of magma under the Cascades, so the overall lower output might make sense. Although geologists are confident about what are the processes going on under a subduction zone are to form volcanoes, the balance of these factors—that is, which ones are most important for generating magma—is still being actively studied. So, right now all we can say is that differences exists between the Cascades and Kamchatka and they might be the root cause of the different levels of activity ... but exactly why is beyond our knowledge right now. However, this ought to operate over geologic timescales of thousands or more years ... what about the last hundred?

Washington's Rainier, with the industrial waterfront of Seattle in the foreground. Don't be lulled into complacency by its quiet century. Ted S. Warren

So, what is it?

There are a few things we can say about the Cascades after looking at all these data. First, the Cascades, although quiet now, aren't always so quiet. Second, compared to other arcs, the Cascades are less active. I tend to think that the quiet over the last 100 years is a product of statistics. The eruptions are mostly randomly distributed through time, so our 100 year time slice is just an anomaly in that distribution. We're just in a lucky/unlucky period where many of the big Cascade volcanoes aren't in eruptive cycles. However, looking at the distribution of eruptions for the last few thousand years, we can expect that this might not last.

The Cascades have the added bonus of likely being a volcanic arc that is near its end. The San Andreas fault system is slowly working its way up the North American coast as the small plates get completely consumed by subduction. This means that eventually, as the Mendocino Triple Junction moves north, the Cascade volcanoes will likely "shut off" when the source of their magma stops operating. This won't be for millions of years, but the clock is ticking.

The greater question of the overall lower volcanic activity in the long term for the Cascades is fascinating question for which we don't have the answer. In any case, we can't be lulled into a sense of complacency about the "quiet Cascades". Almost any of these volcanoes could erupt in our lifetimes (and I almost expect that one other than St. Helens will), so we need to be ready for that next Cascade eruption.