The High Cascades


The Pacific Northwest is full of surprises.

For one thing, the train from Eugene, Oregon, to Portland is sometimes a bus.

For another, from the top of the last big hill on I-5 before the bus reaches the Portland metro area you will see the white shoulders of Mount St. Helens, surprisingly close to this city (at least it elicits a gasp from those of us who aren’t from the area).

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Here is what Mount St. Helens looks like from the aerial tram in town. Luckily for Portland, 70 miles away, the lateral blast in May 1980 went north instead of south. (Jamidwyer, CC BY-SA 2.0)

But perhaps the biggest surprise for someone traveling up the Willamette Valley towards Portland is that most of what you see from the highway or railroad are rolling mountains, heavily forested, only a few thousand feet high.

Those are the Western Cascades.

This was unexpected. As a former Easterner, I thought the whole Cascade Range would look like Switzerland, or at least the Rockies.

Many people probably share this misconception, thanks to what makes the news–Mount St. Helens in 1980, for example, or Mount Hood in 2018 (the stranded climbers tragedy).

In reality, those are just part of the High Cascades–some individual stratovolcanoes that include (but aren’t limited to):

The much less impressive-looking Western Cascades, sitting at the feet of these majestic fire mountains, are volcanoes, too. They are remnants, going back many millions of years–long before the young giants arose.

The Pacific Northwest has very complex geology, and volcanic structures and rocks reflect that. It’s a geologist’s ideal outdoor lab, but the rest of us just wonder how the same volcanic zone can form both High and “low” Cascades.

The vast Willamette Valley, which separates the Cascades from the coastal mountains, is a big clue.

It didn’t form the way many vallies do. Instead of being underlain by soft rock that has eroded faster than the mountain rocks on either side, the Willamette Valley is a forearc basin.

The sea once flowed here (and still does in Puget Sound, a separate northern section of the same basin).

The clearest video I could find on forearc basins was made from a petroleum industry standpoint, but don’t worry: there’s no exploration going on in the Willamette, as far as I know. The farmers would protest.

Here in the Pacific Northwest, that downgoing slab is called the Juan de Fuca plate. It’s made of basalt (runny red lava, frozen into stone, basically). The granitic North American plate is overriding it.

This regional subduction zone is also why there is going to be another Really Big Quake here one of these days (the last one was in the 18th century).

The Cascades are volcanoes, not merely mountains like the Coast Range, because that is where, miles underground, the basaltic Juan de Fuca plate is hot enough to melt.

Hot matter always rises when it’s denser than its surroundings, so some of that molten basalt heads upward. Where it has reached the surface, there are now “low” Cascades volcanoes (and a few High ones). The basalt interacts with the country rock (the overlying granite rock of the North American plate) along the way, so the erupted lava is quite varied.

Like any seafloor, the Juan de Fuca plate also has a lot of water. This facilitates melting of basalt. It can also help melt the silica-rich rocks of the overlying North American plate as the magma and fluid rise. In that case, geochemical processes change the magma’s characteristics.

This type of rising magma becomes more sticky, for one thing, and it tends to squeeze up into a taller shape than a basalt formation.

Here is Mount St. Helens, rebuilding itself in a less harmful eruption that happened between 2004 and 2008. (Portland is about 70 miles away on the other side of that back crater.)

That’s definitely not Hawaiian-style lava!

Eruptions of this type of “gray” lava are more explosive, too, because the gases that you can see (and others you can’t see) in that video sometimes get locked into the sticky lava dome.

And since what goes up must come down, explosively erupted material piles up close to the vent, building over time those beautiful High Cascade volcanoes.

The person who made the video below concentrated on the volcano’s glaciers, but look carefully and you will also see that the ice has carved into the flank, exposing the various layers that were deposited during previous eruptions.

The “low” Cascades, in between the giants, on the other hand, have been dissected by water and wind, and sometimes winter temperatures; they are beautiful, too.

Together, the High and “low” Cascades offer some of the most scenic views in the world.


Featured image: Screen cap from Steve Smith’s video (above).


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