GO 326/ES 767
Volcanism of the Cascade Mountains

James S. Aber
Emporia State University

Table of Contents
Tectonic setting Cascade eruptions
Mount St. Helens Mount Shasta
Related websites References

Tectonic setting

The Cascade Mountains consist of several active volcanoes (triangles) as well as many volcanic centers that were active during the Neogene and Quaternary. These volcanoes are developed above a subduction zone that stretches from northern California to southern British Columbia. The subduction zone involves the Juan de Fuca and related oceanic plates that descend beneath the western edge of North America.

Unlike typical subduction zones, no trench is present along the continental margin. Instead, terranes and the accretionary wedge have been uplifted to form a series of coast ranges and exotic mountains--Klamath Mountains. Inland from these coastal mountains are a series of valleys--Shasta, Willamette, Puget. These valleys represent old forearc basins, which have been uplifted. Major cities of the region are located in these valleys today--Portland, Seattle and Vancouver.

Map modified from L. Freeman.

The Cascade Mountains include more than a dozen, large volcanoes. Although they share general characteristics, each has unique geological traits and history. The major volcanoes of the Cascades are listed below.

Major Cascade volcanoes. Listed from north to south; with dates of historical eruptions. Data from Harris (1988).
Volcano Location Eruptions
Mt. Garibaldi British Columbia none
Mt. Baker Washington 1792, 1843-65, 1870, 1880
Glacier Peak Washington 1750 (?)
Mt. Rainier Washington 1841, 1843, 1854
Mt. Adams Washington none
Mt. St. Helens Washington 1800-57, 1980-84

Volcano Location Eruptions
Mt. Hood Oregon 1854, 1859, 1865-66
Mt. Jefferson Oregon none
Three Sisters Oregon 1853 (?)
Newbury Caldera Oregon none
Mt. Thielsen Oregon none
Crater Lake Oregon none
Mt. McLoughlin Oregon none

Volcano Location Eruptions
Medicine Lake California 1910
Mt. Shasta California 1786, 1855
Cinder Cone California 1850-51
Lassen Peak California 1914-17
Chaos Crags California 1854-57

From this historical record emerge two geographic regions of current volcanic activity. Volcanoes are most active in Washington and northern Oregon, and a second region of eruptions is northern California. In contrast, central and southern Oregon are quiescent, as is southern British Columbia. The sectors lacking modern eruptions correspond to positions of fracture zones that offset the Gorda, Juan de Fuca, and Explorer oceanic ridges--Blanco, Nootka, and Sovanco fracture zones (see map above).

Cascade eruptions

Magma of the Cascades is generated by partial melting along the subduction zone. The most typical lava of Cascade eruptions is andesite, an intermediate igneous composition which is characteristic for subduction zones around the world. Other lava types span the spectrum of composition from basalt, to dacite and rhyolite. Each major Cascade volcano has a distinct signature in terms of its lava composition; some are quite consistent, whereas others show great variability.

Eruptions of andesite and basalt tend to be relatively calm events dominated by lava flows. Exceptions occur where such magmas encounter substantial groundwater (under snow cover, glaciers), which may create steam-driven explosions of the Strombolian type. Dacite and rhyolite magmas are more viscous and tend to develop higher gas pressures. Such magmas may explode violently in catastrophic eruptions that release immense volumes of tephra, ash, and pumice. Glacier Peak, Mazama (Crater Lake), and Mt. St. Helens have experienced this type of eruption.

Recent eruptions, particularly Mt. St. Helens in 1980, have provided important clues for better understanding of ancient volcanism in the Cascade system. One result of the 1980 eruption was recognition of the importance of landslides and mass movements in the development of volcanic landscapes. A huge section on the northern side of Mt. St. Helens slid away and created a jumbled landslide terrain many miles from the volcano itself. Accompanying pyroclastic flows and mudflows swept across the landscape. Similar events have occurred at Mt. Shasta and other Cascade volcanoes in prehistoric times.

Mount St. Helens

Mount St. Helens experienced a violent eruption on May 18th 1980. It was one of the most closely studied volcanic eruptions on record. The main eruptions displaced 2.7 km³ of rock and extruded ½ km³ of magma. In comparison to other major volcanic eruptions, this was not particularly large. For example, the Krakatoa eruption of 1883 was 6 km³, and some prehistoric eruptions have been 100s to 1000s of km³. Events leading up to the main eruption included several distinct phenomena (Decker and Decker 1981).

The main eruption was set off by an earthquake of 5.1 magnitude at 8:32 am on May 18. The quake struck under the northern side of the volcano and triggered a massive landslide. The whole northern side of the volcano began to slide down as a rippling mass. Seconds later a huge explosion of ash clouds blew out the side of the volcano. This lateral blast was directed over the terrain to the north. It consisted of superheated steam clouds filled with ash and rock debris. The lateral blast created three zones of destruction.

  1. Complete destruction within a few km of the volcano. All trees uprooted and blow/burned away; landslide and pyroclastic zone.
  2. Blow-down zone in which trees were broken off and knocked flat by the blast cloud.
  3. Outer zone where trees remained standing but were killed high temperature. Needles burned off.

Some 550 km² were devastated by the lateral blast, which was finished by 9:00 am. At that time, a vertical ash cloud began to build up; it reached an altitude of 20 km before waning in the afternoon. High-altitude ash was blown to the northeast all day across Washington and Idaho. The landslide, which initiated the eruption, also generated debris-avalanche deposits that were transported some distance down the North Fork of the Toutle River valley. This event was the largest such historical landslide, having a volume of 2½-3 km³ and extending 25 km from the source area (Lipman and Mullineaux 1981). In addition, the landslide generated mudflows, which surged along stream valleys, especially the Toutle River.

Map of Mt. St. Helens and surroundings showing eruption zones.
© Weyerhaeuser Co. (1988).

The eruption removed all or parts of most of the glaciers that had existed on the volcano (Brugman and Post 1981). Sixty-two people died in the eruption and $1 billion was lost in the lumber industry. The death toll would have been much higher on another day--the nearby timber industry was shut down on Sunday when the eruption occurred.

Appearance of Mt. St. Helens before the 1980 eruption. © Weyerhaeuser Co. (1988).
Eruption of vertical ash cloud at Mt. St. Helens. © Weyerhaeuser Co. (1988).
View from the northwest shortly after the eruption. Mt. St. Helens can be seen in right background with minor ash emission in progress. The foreground includes the area devastated by landslide and mudflow deposits. © Weyerhaeuser Co. (1988).
Closeup view of tree remains in the blow-down zone. Full-size trees are snapped off at the base and limbs are stripped away. © Weyerhaeuser Co. (1988).
North Fork of the Toutle River valley with Mt. St. Helens in the background. Avalanche debris partly fills the valley in the foreground. Photo date 5/99; © J.S. Aber.
Tree remains buried in avalanche debris, North Fork of the Toutle River valley. Photo date 5/99; © J.S. Aber.

Mount Shasta

Mount Shasta is the queen of the Cascade volcanic peaks. It is one of the world's largest stratovolcanoes, standing more than 10,000 feet (3000 m) above its surroundings. It is well known for myths, legends, and mysteries that have attracted many cult followers. Mt. Shasta is in fact a composite structure built of two major cones--Shastina on the northwestern flank and Hotlum on the summit, as well as several other eruptive centers and deposits. Many eruptions and intrusions of greatly varying character have taken place during the past 10,000 years to build the present edifice of Mt. Shasta (Harris 1988).

View of Mt. Shasta from the north. Elevation of the summit peak (Hotlum cone to left) is 14,161 feet. Shastina is the slightly lower peak to right. Photo date 5/99; © J.S. Aber.

Ancestral Mt. Shasta began to develop at least half a million years ago. The exact nature of this volcano is unknown, but it must have been quite large, as it generated a tremendous landslide between 300,000 and 360,000 years ago (Harris 1988). The enormous slide transported chaotic blocks of rocks into Shasta valley. The avalanche deposits extend 45 km northwest of the volcano and represent a volume of about 27 km³--about ten times the volume of the Mt. St. Helens 1980 avalanche deposits.

View of Mt. Shasta from a hot-air balloon looking toward the south. The hummocky landscape in the lower portion of view is part of the vast landslide deposit, which extends from the volcano into Shasta valley. Photo date 5/99; © J.S. Aber.
Late afternoon view of the northwestern side of Mt. Shasta. The hills in the near distance are large blocks within the landslide deposit. Photo date 5/99; © J.S. Aber.
Section through a landslide block, exposed at Lake Shastina. The block consists of volcanic strata transported, tilted, and deformed during the avalanche. Note house in upper left for scale. Photo date 5/99; © J.S. Aber.

Mount Shasta vicinity has experienced four main eruptive cycles during the past half million years--Sargeants Ridge, Misery Hill, Shastina, and Hotlum. Hornblende and pyroxene andesite lava flows dominate the eruptive cycles, which each terminate with intrusion of dacite domes (Harris 1988). These domes, being more resistant to erosion, form the summit peaks of Mount Shasta, Shastina, and Black Butte. The Hotlum eruptive center is considered to be active, as the summit dome is still hot. Boiling sulfur springs and steam emissions were seen in the 1850s, but these are reduced now to a single hot spring and small fumaroles.

Geology professor, Bill Hirt (right), leads a field excursion into a lava-tube cave near Mt. Shasta. Photo date 5/99; © J.S. Aber.
Section through a small cinder cone on the Deer Mountain shield volcano, northeast of Mt. Shasta. Compare with cinder cones in Eifel district. Photo date 5/99; © J.S. Aber.
Near-vertical dike within the cinder cone on Deer Mountain shield volcano. Photo date 5/99; © J.S. Aber.

Related websites

References

Return to tectonics syllabus.
GO 326/ES 767 © J.S. Aber (2008).