Lecture 11 - The Cascade Volcanoes

Outline

I. Formation of Cascade Volcanoes

II. Mt. Rainier National Park

III. Mt. St. Helens National Monument

IV. Crater Lake National Park

V. Lassen Volcanic National Park

I. Formation of Cascade Volcanoes

The linear trend of Cascade Volcanoes is located at a subduction zone where the Juan de Fuca oceanic plate is being subducted beneaththe North American continental plate. As the oceanic plate plunges deep into the earth and heats up, water released from sediments on the oceanic plate lower the melting temperature and the oceanic basaltic layer and the surrounding peridotite mantle begins to melt and produces primarily andesitic magma which rises to erupt at the different volcanoes. These are for the most part stratovolcanoes that consist of alternating layers of lava flows and pyroclastic (tephra) material.

The subduction process responsible for the Cascade volcanoes began in Cenozoic time (about 40 million years ago) and built volcanoes on top of what previously was a continental plain. We know this because we find sandstones, mudstones (continental plain deposits) and andesites (volcanics) of this age in this region. Younger rocks in this area indicate that volcanism continued with a large amount of magma solidifying underground to form the Tatoosh granodiorite (between granite and diorite in composition) that is exposed in Mt. Rainier National Park. The present volcanoes that we see today aregenerally less than 100 thousand years old.

II. Mt. Rainier National Park

The highest volcano in the Cascade Range (and the highest peak in the northwestern US) is about 25 thousand years old but was apparently over 1000 feet higher prior to about 6,000 years ago when violent eruptions (similar, but larger than the 1980 Mt. St. Helens eruption) destroyed the summit. The remains of the top part of Mt. Rainier can be found in the Osceola mudflow which traveled quite far from the volcano. In addition to actual volcanic eruptions (the last for Mt. Rainier was over 100 years ago), mudflows pose a serious threat from all Cascade volcanoes since the elevated temperatures and steep unstable slopes associated with the volcanoes coupled with the large volumes of snow and ice on their peaks are the ingredients needed to generate mudflows. Rockfalls and avalanches are also frequent on Mt. Rainier.

• Mudflows- ~5,800 years ago- Osceola mudflow runs down White River Valley over 80 km (50 miles) to Puget Sound
• ~600 years ago-Electronic mudflow runs down Puyallup River Valley similar distance to Puget Sound
• 1947- mudflow runs down Kautz Creek to Nisqually River destroying everything in its path
• Avalanches- 1963- Rockfalls and avalanches triggered by a steam explosion occurred on the north face of Little Tahoma Peak and traveled some 7 km (~4 miles) from its source and included building size boulders

Mass wasting processes are very important on Cascade volcanoes.

Other Interesting Features:

• Glaciers- There are more than 40 glaciers on Mt. Rainier and their influence on the mountain is profound but will be considered later when we discuss glaciers more completely.
• Ice Tunnels and Caves- steam still produced in the summit area of Mt. Rainier has melted tunnels and caves through the thick ice deposits that line the crater floor. These caves and their warm steam emissions (fumaroles) have saved the lives of many who have climbed this mountain.

III. Mt. St. Helens National Monument

Early History

Unlike Mt. Rainier, the early geologic history of Mt. St. Helens (MSH) is poorly known due to few if any exposures of the older volcanic rocks. MSH's is believed to be about 40,000, but most of the volcano was built in the last 4,000 years with volcanic eruptions about ever 100-200 years. Deposits are primarily pyroclastic accompanied by mudflows with relatively few lava flows. Extensive glacier systems formed on the mountain top.

Recent History

In 1978 US Geological Survey pointed out that MSH may be due to erupt soon and a warning system which included scientific monitoring and evacuation plans was established. March 20, 1980 a large earthquake was recorded on MSH and seismic activity continued. On March 27 a large ash explosion occurred and much of the area was evacuated.

May 18, 1980 Eruption

8:32 AM- An earthquake (M~5.1) triggered a large bulge that had been developing on the north side of the volcano to break loose and slide down the mountain, taking the mountain top with it. The unplugging of the volcanic vent allowed steam and ash to be released from the volcano and to hug the mountainside racing down the slope (pyroclastic flow that hugs the slopes is called nuee ardente, very dangerous!). This cloud of hot gas and ash moved with speeds as large as 700 miles per hour and devastated a 230 mile area north of the mountain. The volcano continued to spew ash for 9 hours.

Mudflows developed soon after the blast, destroying houses,bridges and logging operations along the Toutle and Cowlitz Rivers. The landslide material entered Spirit Lake, raising the water level by over 200 feet. 57 people died in this explosion. In just minutes MSH once the fifth largest peak in the northwestern US lost over 1300feet of elevation to become the 37th largest peak.

After May 18, 1980

Several smaller eruptions of ash and rhyolite, too thick to flow forming domes within the crater, occurred since the 1980 eruption. Presently, small ash eruptions continue but they appear to be due to pulverizing pieces of the rhyolite domes and no new magma. Who knows when another devastating eruption might occur. Of particular concern are: destruction of 3 reservoirs on the south side of the mountain that would cause huge floods; The Trojan nuclear power plant on the west side of the mountain; more mudflows like those that rushed down the Toutle River.

IV. Crater Lake National Park

Created from the catastrophic explosion of Mt. Mazama just lessthan 7,000 years ago. At this time Mt. Mazama was probably one of thehighest of the Cascade peaks. The volcanic deposits that make up therim of Crater Lake tell us about the former activity of Mt. Mazamaand the surrounding area. In addition to the main composite conemaking up Mt. Mazama, this region has many other composite cones.This suggests that at times the main vent got sealed by volcanicmaterial and the magma and gasses found different paths to thesurface producing other composite cones such as Mt. Scott (at 8926feet, it is the highest point in Crater Lake NP), Union and CraterPeak and cinder cones such as Bald and Desert Crater. Much of theinner workings of the volcano are revealed in andesitic dikes thatshow the paths that magma took to reach the surface. Examples ofthese include Devils Backbone and Phantom Ship. They are leftstanding because of their relative hardness compared with adjacentash deposits.

Formation of Crater Lake

1. Just under 7000 years ago Mt. Mazama erupted in a similar manner but with much more force than the 1980 Mt. St. Helens eruption. A huge volume of gas and pyroclastic material was ejected miles into the sky, avalanches of hot ash swept down the slopes (nuee ardente) at hurricane speeds carrying much volcanic debris, snow melted and produced large lahars, forests left standing burst into flames. Ash from this eruption reached from southern Nevada to Alberta Canada (see book Figure 34.9). The pinnacles region consists of welded ash material (tuff) from the hot avalanches.
2. The loss of large volumes of material from the interior of the volcano caused the cones upper section to collapse under its own weight creating a caldera 5 miles wide and 4000 feet deep.
3. Since this catastrophic eruption, few lava flows erupted within the caldera, the cinder cone of Wizard Island formed and water collected within the caldera to create the ~2000 feet deep clear blue lake.

V. Lassen Volcanic National Park

Lassen Peak is a dacite (intermediate in composition between andesite and rhyolite) dome that formed on the flanks of an older composite volcano called Mt. Tehama which was severely eroded away tens of thousands of years ago. Several peaks in the park southwest of Lassen are the remanents of Mt. Tehama (Brokeoff Mountain, Mt.Diller, Pilot Pinnacle, Diamond Peak and Mt. Conrad). Lassen is a bit unusual for a dome in that it does have a summit crater and although it was built by piling up thick, sticky lava, it has had some violent eruptions with the most recent being from 1914-1921. Other dacitedomes in the park are Reading Peak and Chaos Crags. Some shield volcanoes (Prospect Mountain) and Cinder Cones (Cinder Cone) are also present. The sequence of events at Lassen include:

1. Andesitic lava and pyroclastic deposits built up Mt. Tehama composite volcano over the last ~1 million years
2. About 11,000 years ago, dacite forced its way into andesitic composite cones on Raker and Reading Peaks, and built up Lassen Peak on Mt. Tehama's northwest flank
3. Renewed activity built Chaos Crags (dome) and Cinder Cone and caused crater formation at Lassen Peak. In 1914 activity at Lassen included steam eruptions, lava flows and explosions of gas and ash.

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