The Cascades

The Cascades The Cascade Mountain Range is about 700 miles long, extending from southwestern British Columbia, Canada south through Washington and Oregon, into north-central California (see map). This mountain chain consists of a series of volcanoes and innumerable smaller vents and cones located about 80 to 150 miles inland from the Pacific Ocean. Four (4) national parks are situated within the Cascades, including North Cascades National Park in north-central Washington, Mount Rainier in west- central Washington, Crater Lake in south-central Oregon, and Lassen National Park, centered on Mount Lassen, the southern-most Cascade peak, situated in north- central California. Figure 1 – Cascade Range Many peaks exceed 10,000 feet (3,000 m), including Mount Hood (11,235 feet [3,424 m], highest point in Oregon) and Mount Rainier (14,410 feet [4,392 m], highest in Washington and in the Cascade Range). Most of the summits are extinct volcanoes, but Lassen Peak (10,457 feet [3,187 m]) and several others have erupted in the recent past. Mount Baker (10,778 feet [3,285 m]) steamed heavily in 1975, and Mount St. Helens (8,365 feet [2,550 m]) erupted in 1980 and again in 1981. Page 1 of 11 The modern-day definition of the extent of the Cascades is generally accepted to mean a volcano between the end members (Mount Lassen to the south and Mount Meager to the north). There is also some discussion as to whether “Three Sisters”, near the center of the range is really 1 vent or 3 volcanoes very close to one another. Every orogenic belt, or mountain range has their own unique method of formation, and the Cascades are no different. Off of the western coast of this portion of the North American plate, there is a “spreading center” along the Juan de Fuca Ridge, where new ocean crust is forming. As this geologic process continues, a relatively small plate of the earth’s crust, the Juan De Fuca plate is pushed eastward, into the western edge of the North American plate. As the Juan de Fuca plate is pushed beneath the North American plate in a process referred to as “subduction”, it is bent downward. Both the plate and the sediments that are riding on the plate begin to melt as it descends into the mantle, releasing steam and other gasses. Occasionally the steam and melted rock are “vented” through an eruption of a volcano (see Figure 2, right). Figure 2 – Cascade Tectonic Overview Page 2 of 11 Tectonic Setting The tectonic setting along the margin between the eastern edge of the North Pacific Ocean and the western cost of British Columbia, Washington, and Oregon is relatively complex, and is bounded by another world-famous geologic feature, the San Andreas Fault. The subduction zone that is the source of the material and energy for the Cascades is essentially a 700-mile interruption of the San Andreas Fault, which extends down to the south, to the Gulf of California. It appears that the Cascade Range, or some early configuration of it began erupting about 37 million years ago. Not all portions of the Range are equally as “active”. It appears that the central portion of the range is much more active, with more eruptions and venting that the northern end. However, the older lavas appear to be found in the northern portion of the Range. The Cascade Range is also home to numerous alpine glaciers, with 25 glaciers on Mount Rainier alone. Glaciers form when local annual snowfall exceeds that which melts during the summer. At altitude, these icefields form against the mountains, and will eventually begin to slide downhill under the effects of gravity. Page 3 of 11 Unique Geologic Setting At the southwestern-most corner of the three (3) tectonic plates that are being subducted beneath the North American Plate there is what geologists refer to as a “triple junction”, where three (3) tectonic boundaries converge (see below). Location and Geometry of Mendocino Triple Junction As shown, there is “right-lateral” movement across the San Andreas Fault, where the Pacific Plate is grinding northward along the western margin of the North American Plate. West of the Triple Junction there is also “right-lateral” movement of the Gorda Plate as it passes to the east along the Mendocino Fault. North of the Triple Junction lies the subduction zone where the Gorda Plate is descending beneath the North American Plate. In this diagram are also representations of the three (3) types of tectonic boundaries which include mid-ocean spreading centers, subduction zones, and transform faults. The location of the southern-most Cascade peak, Mount Lassen is shown by the yellow arrow. There are no Cascade peaks south of the Mendocino Fault. Page 4 of 11 Why are the Cascade Volcanoes Dangerous? Eleven of the Cascades have erupted at least once in the past 4,000 years, and several have done so in just the past 200 years (see Figure 3, below). Some volcanoes appear to have erupted 100 times or more, while others are dormant, and have not erupted in 100,000 years. This variability in both timing and location only add to the dangers this mountain range presents. Lahar pathways from three (3) separate events on Mount Rainier (yellow arrow) Page 5 of 11 In addition to the lahars, there is also a considerable risk of damage caused by the ash and fallout that is typically ejected from a volcano during an eruption. Such pyroclastic releases often shoot ash 30 miles (50 km) into the atmosphere, and cover broad areas. Billowing eruption with pyroclastic and ash ejecta – Mount St. Helens, May 1980 Left: Extent of pyroclastic outfall from Mount Mazama Right: Pyroclastic deposit at Mount Mazama Page 6 of 11 Figure 3 – Recent Eruption Summary of the Cascade Range in the US The most recent significant eruption in the Cascades happened in late May 1980 when Mount St. Helens erupted. This event provided a great deal of data on the mechanics of an eruption. When Cascade volcanoes do erupt, pyroclastic flows, lava flows, and landslides can devastate areas more than 10 miles (>16 km) away; and huge mudflows of volcanic ash and debris, called lahars, can inundate valleys more than 50 miles (>80 km) downstream. Falling ash from explosive eruptions can disrupt human activities hundreds of miles downwind, and drifting clouds of fine ash can cause severe damage to jet aircraft even thousands of miles away. Mount St. Helens has been among the most active of the Cascade peaks for a very long time. Despite weeks of prolonged geologic activity, many people wanted to see an eruption. On May 18 at 8:32 a.m., the northern slope of Mount St. Helens gave way in a tremendous landslide (below, right), allowing the chamber beneath the mountain to depressurize. The eruption killed 57 people and destroyed 250 homes, 47 bridges, and 185 miles of roadway. In the time since, a cone has begun to re-grow (photos, below). Page 7 of 11 May 17, 1980 May 18, 1980 Mount St. Helens “The glacier-covered summits of Mount Baker and other Cascade volcanoes greatly increase the hazards of any eruption because of the possibility of a “lahar”, or volcanic mudflow, being triggered when snow and ice, suddenly transformed to water and steam, mix with hot ash and other debris making a slurry. A lahar roars down a mountain valley destroying everything in its path. The lahar set off by an explosive eruption at Mount St. Helens was not as descriptive as it might have been because reservoir (water) levels had been lowered when earthquake swarms and steam eruptions indicated that the volcano might erupt”. Page 8 of 11 An excerpt from the USGS Volcano Hazards Program: “Because of its elevation (14,410 feet or 4,392 meters), relief, hydrothermal alteration, icecap, glacier-fed radial valleys, and proximity to encroaching suburbs of the Seattle- Tacoma metropolis, Mount Rainier (photo, left) is the most threatening volcano in the Cascades. Its next eruption could produce volcanic ash, lava flows, and avalanches of intensely hot rock and volcanic gases, called pyroclastic flows. Some of these events swiftly melt snow and ice and could produce torrents of meltwater that pick up loose rock and become rapidly flowing slurries of mud and boulders known as lahars. The greatest risk at the volcano comes from its potential for generating huge lahars triggered by sector collapse or magma- water-ice interaction rather than from an eruption itself.” Mount Rainier 55 miles southeast of Seattle Modern-day measurements and computer modeling conducted by the USGS show that there is a possibility that a lahar that is generated on the northwestern flank of Mount Rainier would flow all the way to Puget Sound, a distance of about 50 miles. In short, what makes Cascade Range volcanoes so dangerous is the combination of hot rock and ejecta (tephra) that mix with meltwater from glaciers and snowfields, making a debris flow that has the consistency of wet cement (lahar). As this heavy, wet slurry runs downhill, it will collect trees, buildings, structures, and so forth. What makes them even all the more dangerous is the proximity to population centers. As discussed in The Geology of National Parks, “In the Cascades, volcanic eruptions have been relatively infrequent in terms of human life spans. Before Mount St. Helens erupted, many residents did not believe the geologists and other scientists who warned of potential hazards. However, the 1980 explosion and subsequent less-intense Page 9 of 11 eruptions made believers out of skeptics. What is not recognized is that major evacuations are really the only adjustment that human beings can make when a volcano becomes threatening”.

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