The Cascades Volcanoes

One of the first things you learn here in Oregon is that everybody’s very proud of the volcanoes – Mount St. Helens, Mount Rainier and Mount Hood, especially.  However, those are only a few of volcanoes in the Cascades, an arc that extends some 700 miles from British Columbia to northern California.

I hope to start visiting these natural wonders for the Sunday Morning Volcano as soon as possible, but let’s first take a look at the Cascades themselves. The sheer size of these things awes this Easterner!

Here is a YouTube video someone posted, taken from an airliner, of Mount Hood, Mount Adams and Mount Rainier; I believe that white lump to the left is Mount St. Helens, still impressive despite its massive eruption in May 1980.

Such majestic piles may seem ancient, but they’re kids in geologic terms – only 5-7 million years old. Their ancestral lineage, insofar as geologists have pieced it together goes back, goes back 37 million years, when the Juan de Fuca tectonic plate (named after a 16th-century explorer) gave rise to a chain of volcanoes that has been erupting in the region ever since. The first Cascades volcanoes weren’t as high as the ones we know today, but they left a good geologic record.

Time magazine’s Dick Thompson has written, in “Volcano Cowboys,” the neatest nontechnical description I’ve ever read of how a subduction volcanic arc like the Cascades works. It’s more like a furnace than anything else.

As the Juan de Fuca plate slowly dives beneath the North American plate, it carries with it water and minerals from the Pacific’s floor. At a depth of 60 to 120 miles, the water, calcium carbonate, carbon dioxide, sodium, potassium, and other deposits act as fluxes that lower the melting temperature of rock. The melted rock increases in volume and becomes less dense. Water and carbon dioxide infuse the liquid rock with gases, making it even more buoyant than the surrounding rock. So new magma begins moving to the surface. It snakes through fractures in the overlying plate. Often these passageways are blocked…at times, when more magma wells up from below, the magma puts even more pressure on…and shoulders the cold rock out of the way. When magma breaks through the rock like this, it sets off earthquakes or, if it happens at the surface vent, releases an eruption. The debris around the volcanic vent begins to pile up, eventually creating a mountain.

Some 5-7 million years ago, the geometry of the Juan de Fuca plate changed, and the modern Cascades began forming.


The United States Geological Survey monitors the Cascades through the Cascades Volcano Observatory in Vancouver, Washington. Thompson’s “Volcano Cowboys” also provides a history of the CVO’s early days.

To detect changes that could signal an upcoming eruption, these geologists monitor seismic activity, ground deformation and volcanic gases. They also map ground changes and collect rock samples, and have an acoustic network around Mount Rainier and Mount St. Helens to detect volcanic mudflows, also called lahars.

People here may love the volcanoes, but they also respect them. In the Cascades is one of the world’s most dangerous volcanoes, Mount Rainier. I’ll be taking a general look at that next Sunday.



Northwest Origins: An Introduction to the Geologic History of Washington State.” Catherine L. Townsend and John T. Figge.

“Volcano Cowboys.” Dick Thompson. Thomas Dunne Books, New York. 2000.

“Volcanoes of the Cascades: Their Rise and Their Risks.” Richard L. Hill. Globe Pequot Press, Guilford, Connecticut. 2004.

Image on front page is from “Volcanoes – Mount Rainier National Park.” National Park Service.

Categories: Sunday morning volcano, volcanoes

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