A Brief Explanation: Why the Pacific Northwest of America Has Volcanoes

Mt. St. Helens Eruption, 1980. [Image Source]

If you've had the opportunity in your lifetime to set out to the Pacific Northwest of America, then in all likelihood you've come across a volcano or two; potentially without even realizing it. In the most geologically active region of North America, the Pacific Northwest is home to its fair share of active volcanoes (roughly 20 major volcanoes), which have the potential to effect millions (10 million) of people. These volcanoes reside in a volcanic arc of the Cascade Range, which spans a distance of over 700 miles (1,100 km) through British Columbia, Washington, Oregon and Northern California. The two most recent volcanic eruptions to have occurred in this arc were Lassen Peak (1914-1921) and the infamously well known, Mt. St. Helens (1980).

While many people are well aware of these volcanoes, few understand why they exist in the first place. Rather than giving you an in depth explanation of the Cascade Volcanic Arc, whose history spreads over the course of 37 million years (that's a lot of years), I'd like to focus on the process which allows these volcanoes to occur. The main culprit that attributes to their manifestation, is due to a process called subduction, which derives from the theory of plate tectonics.

Diagram of the subduction process in the Pacific Northwest [Image Source]

The above diagram gives you a visual of what is taking place in the Pacific Northwest. Slowly, but surely the Juan de Fuca Plate is pushing itself under the continental crust of the North American Plate. This occurs due to the ocean floor– predominantly basalt (igneous rock)– being spread apart by divergent plate boundaries (Juan de Fuca Ridge). As the denser basaltic ocean crust moves towards the North American Plate, it begins to "sink" underneath the less dense continental crust. As subduction occurs, there are numerous processes taking place e.g. accretion, earthquakes, mountains, volcanoes, etc. If you look closely at the above diagram, you will see that the oceanic crust is eventually given the name "subducting slab." This area is of the upmost importance when trying to understand why the majority of volcanoes of the Pacific Northwest exist.

As the oceanic crust begins to move under the continental crust, it begins to come under extreme pressure and heats up. When this occurs, rocks and minerals melting points become far more volatile– but it's not pressure alone that causes this melting point to be reached.

In igneous petrology, there is a process known as "flux melting." Unfortunately, this has nothing to do with flux capacitors or time travel. Flux melting occurs when down-going oceanic plates carry water and other volatile components, which then are released (a process known as metamorphic dewatering) into the hot ultramafic rock (rocks composed of silicate minerals) of the upper mantle wedge. Essentially, the water in the down-going plate lowers the melting point of the mantle wedge residing above the subducting slab, due to extreme heat and pressure.

Diagram displaying the process of flux melting. [Image Source]

As the ultramafic rock begins to melt, it carries the same attributes of what anything else hot does– it begins to rise. It will consistently rise (provided the process continues) until it reaches the surface of the continental crust. This is when a volcano is formed! While volcanoes can form a multitude of different ways , the process of flux melting especially applies to the highly combustible composite stratovolcanoes of the Pacific Northwest. Overall, these volcanoes have remained relatively quiet, but have no doubt, they are very active and should be of the upmost concern for the populations living near them.

Well, I hope this helps anyone that may have not understood why the Pacific Northwest is riddled with volcanoes. While I could have gone more in-depth with terms, history and other information, I figured keeping this post short would be much better to spark curiosity for those curious types who might appreciate geology or science in general. If you have any questions or anything else you'd like me explain, please ask me in the comments below.

Bibliography:

https://en.wikipedia.org/wiki/Flux_melting
https://en.wikipedia.org/wiki/Pacific_Northwest#Population
https://en.wikipedia.org/wiki/Plate_tectonics
https://en.wikipedia.org/wiki/Cascade_Range
https://en.Cac.org/wiki/Cascade_Volcanoes#List_of_volcanoes
https://en.wikipedia.org/wiki/Juan_de_Fuca_Plate

Marshak, Stephen Essentials of Geology 4th Ed. W.W. Norton & Company, 2012