Cascade Volcanoes

The Cascade Range is a majestic volcanic arc paralleling the Cascadia Subduction Zone, from northern California to southern British Columbia. For all intents and purposes, this section discusses the Cascades south of I-90, since they are geologically distinct from the North Cascades, which can be found north of I-90 in Washington. The Cascade Volcanic Arc is chiefly composed of Cenozoic volcanic rocks less than 37 million years old, while the North Cascades are chiefly composed of Mesozoic intrusive and metamorphic rocks that were accreted onto the western margin of North America from various localities.

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Capped by the massive Mount Rainier (14,410’), the Cascades are a textbook example of a volcanic arc. They are composed of over 4,000 volcanoes, including stratovolcanoes, cinder cones, shield volcanoes, and everything in between, and began forming 37 million years ago when the Juan De Fuca Plate began subducting underneath the North American Plate. The Cascades are quite a young mountain chain as well, as most of the peaks within the range are younger than 2 million years old. The Cascades host 20 active volcanoes, with the most recent eruption being the 1980 eruption of the infamous Mount Saint Helens in Washington. Cascade volcanoes are quite diverse geochemically speaking, with all main magmatic compositions represented, from mafic basaltic shield volcanoes such as Medicine Lake Volcano or Newberry Volcano to felsic rhyolitic domes of Mt. St. Helens and Mt. Rainier. Moreover, the lion’s share of the stratovolcanoes within the Cascades are mainly andesitic (intermediate) in silica content. For more information on the nature of volcanism, visit the “Volcanoes” section of the Geology Page here.

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Though the Cascades are quite a young volcanic arc at only 37 million years old, there are several eras of illustrious geologic history within this 37 million year time span (which I guess is technically a pretty long time in terms of human history). The first such era occurred from the inception of the arc 37 million years ago (Ma) to 17 Ma, and is dubbed the “West Cascades Period”. During this 20 million year period, the Cascades were further west than they are today and were exceptionally active and explosive. One such volcano, Mt. Aix, located near Wenatchee, WA, was especially explosive. The remains of this period of Cascade history are well preserved in the North Cascades, in the form of the 26-29 million year old Chilliwack Batholith, a large plutonic unit representing the cores of ancient Cascade volcanoes and the early “plumbing system” of the volcanic arc. The Snoqualmie Batholith is another example of a granite unit preserving the cores of ancient Cascade volcanoes that outcrops in the Cascade Range. It is 22-28 million years old, and can be found in the vicinity of I-90 just east of Seattle near Snoqualmie Pass.

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From 17 to 9 Ma, the Cascade Volcanic Arc was uncharacteristically quiet, with the lion’s share of volcanic activity occurring east of the arc in the Columbia River Flood Basalts. Why this is exactly is still a mystery in the geologic community. For whatever reason, the volcanic activity in the Columbia Plateau caused a significant dip in the volcanic activity in the Cascades. My personal hypothesis is that the magma reservoirs generated by the subduction of the Juan De Fuca Plate in the Cascade Volcanic Arc at this time were being “robbed” by the Columbia River Flood Basalts, but that is a totally unscientific postulation. I will say, basalt is much hotter than andesite or rhyolite due to a litany of petrological factors, and perhaps the basalts from the Columbia River Flood Basalts melted into the magma chambers of the Cascades and assimilated the magma chambers into the flood basalts. Moreover, it is widely accepted that the flood basalts were a result of hotspot magmatism, and I postulate that this further aided the “robbery” of Cascade magma. Faults, fissures, and underground conduits of magma could have branched out of the magma chambers in the Cascades into the Columbia River Flood Basalts, and as aforementioned, the flooding of hot basalt could have assimilated the rhyolite and andesite produced by subduction, making it hotter and less viscous, and allowing it to travel via underground conduits east to the Columbia Plateau. But who knows, maybe I should just put my tinfoil hat back on.

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From 9 Ma to the present day, the Cascades have been rocking and rolling (or I guess more accurately, building and belching) fairly regularly. There was an 800,000 year window of heightened activity from 6.25-5.45 Ma dubbed the “High Cascades Period” where volcanic activity in central Oregon was especially intense. Even today, central Oregon and Oregon in general is the epicenter of much of the volcanic activity in the Cascades. Within the last million years, older generations of volcanoes died out, and the main volcanoes that dominate the Cascades today were built. Starting 600,000 years ago, Mt. Shasta and Glacier Peak were built. Mt. Rainier was built 500,000 years ago, Mt. Adams 450,000 years ago, and Mt. Mazama (Crater Lake) 420,000 years ago. Since the construction of these volcanoes, they have erupted fairly regularly, though the 12,276’ Mt. Adams (technically the largest volcano in Washington by volume) has been slumbering peacefully for the last thousand years. The Cascades have had their share of explosive and destructive eruptions, and several come to mind. The eruption of Mt. Mazama that created Crater Lake occurred 7,700 years ago and was very violent. Roughly 5,000 years ago, Mt. Rainier blew off 1,600’ of its top in an explosive eruption that created the infamous Osceola Mudflows, a series of deadly lahars that made it all the way to Renton (roughly 60 miles from the volcano). This eruption lowered Mt. Rainier to a height of 14,100’. It’s absolutely crazy to think that Mt. Rainier was 15,700’ in elevation before this eruption, as the mountain is already extremely massive and foreboding. Lastly, Mt. St. Helens blew 1,300’ off its top in 1980 with the infamous landslide and eruption of the volcano, and it erupted again from 2004-2008. Gas and steam still exit from the crater today, and Mt. St. Helens is by and large the most active volcano in the Cascades today. Volcanoes such as Mt. Rainier and Mt. Hood pose major threat to Seattle and Portland, respectively. More is discussed about those hazards here.

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The Cascades are very rich ecologically speaking. Not only do they create a rain shadow effect, allowing for several different species to live on their slopes, they also have very rich volcanic soil. On the windward side of the range, you can find deep forests of Douglas Fir, Western Hemlock, Oregon White Oak, Sitka Spruce, Western Red Cedar, and White Alder. In the subalpine regions of the range, species such as Subalpine Fir, Engelmann Spruce, Mountain Hemlock, Noble Fir, Pacific Silver Fir, Whitebark Pine, and Western White Pine can be found, along with a litany of wildflower species. The leeward eastern slopes of the range are drier, and are dominated by Ponderosa Pine, Lodgepole Pine, and Douglas Fir. The lowest slopes on the east side of the Cascade Range are dominated by sagebrush and Western Juniper.

 

In terms of fauna, the Cascades are home to several large mammals, including mule deer, mountain goat, elk, mountain lion, coyote, and black bear. Marmots, various squirrels, fishers, and various weasels are small mammals that reside in the range. Mt. Hood (Oregon), Mt. Adams (Washington), and Mt. Shasta (California) host grey wolf packs, and the North Cascades are abundant with wolves. Bald eagles, golden eagles, ospreys, peregrine falcons, and several species of hawks dominate the skies, while a plethora of songbirds inhabit the trees. Salmon, Trout, and several other species of fish dominate the rivers, lakes, and streams of the Cascades.

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The Cascade Volcanoes are some of the most beautiful areas in the United States and have no shortage of pretty areas to explore. The North Cascades, while still a part of the Cascade Range geographically, is a completely different section which can be seen here. From north to south from I-90 in Washington, Mt. Rainier is definitely the first marquee locality to explore. It is the highest mountain in the Cascades (14,410’), and the most prominent mountain in the contiguous US, as it rises basically from sea level. Mt. Rainier is absolutely massive, and it hosts several glaciers. The entire Mount Rainier National Park is beautiful, and it spans several biotic zones, from deep forest to glacial tundra. Further south in Washington is Mount Saint Helens, one of my personal favorite locations in the Cascades. Spirit Lake Highway goes to the Johnson Observatory, where you can look into the caldera of Mt. St. Helens and still see steam and smoke rising from the mountain from time to time. Spirit Lake Highway also traverses the area that was destroyed in the 1980 eruption, showcasing all of the scars on the land from the eruption. There are still thousands of logs in Spirit Lake from the trees getting blown out of their roots due to the eruption. South of Mt. St. Helens is Ape Caves, a gigantic lava tube on the south side of the volcano. Mount Adams majestically rises to the east of Mt. St. Helens. The Columbia River Gorge would undoubtedly be the next marquee Cascade location to check out, straddling the Washington-Oregon border. Multnomah Falls is of course the famous waterfall in the gorge, but there are several of them. Mount Hood, standing at 11,239’, stands tall in the backdrop of Portland and hiking and skiing on and near the mountain is a must. Further south, exploring Newberry National Volcanic Monument near Bend is a must. Otherworldly volcanic landscapes dominate, with hundreds of cinder cones and lava tubes. Moreover, views of the High Cascades are astounding from Pilot Butte in Bend. Further south, hitting Crater Lake is a must. It is the deepest lake in the US (1,949’ deep), and sits in a caldera from an explosive eruption of Mt. Mazama 7,700 years ago. In California, exploring the volcanic landscapes of Medicine Lake Volcano and the 14,162’ Mt Shasta is critical, as is exploring Lassen Volcanic National Park. As I’m sure you’ve realized, there is no shortage of exploration in the majestic Cascade Range, and a road trip up the range from California to Washington is an experience of a lifetime.