MT. ST. HELENS AND OTHER CASCADE VOLCANOES – Northwest U.S. Stratovolcanoes

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FIELD TRIP STOPS – MT. ST. HELENS NATIONAL VOLCANIC MONUMENT AND THE JOHNSTON RIDGE OBSERVATORY.

 

LOCATION: From Interstate 5 in Washington State, travel east along Route 504 for 53 miles to the Johnston Ridge Observatory. There are other roads surrounding the park that will lead you to other vantage points.

 

GEOLOGIC FEATURES: Stratovolcano; Basaltic (fluid) and Andesitic (more viscous - slica rich) Lava flows; Pumice; Ash; Pyroclastic Flows; Lateral Volcanic Blast; Landslide; Lahar; Cascades Mountains; Convergent Plate Margin; Subduction Zone; Lava Dome.

 

DESCRIPTION: The Cascade Mountain chain extends from northern California, through Oregon and Washington, into southwest Canada and besides Mt. St. Helens include Mt. Rainer, Mt. Adams, Mt. Hood and Mt. Jefferson among others.  They are the result of the subduction of the Juan de Fuca Plate underneath the North American Plate at a Convergent Plate Margin. Lavas that formed the volcano were commonly composed of molten rock which was thicker in composition (more viscous) than the more fluid lavas found in Hawaii.

     The explosion of Mt. St. Helens marked the first volcanic eruption in modern times to occur within the 48 states. On May 18, 1980 a magnitude 5.1 earthquake triggered by magma moving to the surface caused the north flank of the mountain to collapse in the largest landslide ever recorded by man. This earth movement resulted in a release of pressure and an accompanying lateral Pyroclastic Surge of superheated gas and ash directed northward (rather than the usual surge directed straight upwards). This sideways surge pushed the landslide material and totally decimated the landscape within 5 miles north of the volcano while blowing down trees and stripping their branches over about the next 10 miles. Subsequent denser Pyroclastic Flows filled the landscape north of the volcano with layers of pumice, ash and rock fragments.

     A column of ash rose nearly 15 miles into the air where the upper air flow carried some of the ash around the globe for a few years before settling to the ground.  Ash, mobilized by melting glacial water from the mountaintop,

dense Lahars up to 600 ft. in thickness that traveled about 17 miles down the neighboring Toutle River.

     In the end, Mt. St. Helens lost about 1300 ft of its summit which now appears as a horeshoe-shaped rim, its opening facing to the north and the observation deck of the Johnston Ridge Visitor Center.

 

STUDENT QUESTIONS:

(1) What event triggered the eruption of Mt. St. Helens?

(2) How does Lava differ from Magma?

(3) How was the direction of the initial blast different than for most volcanic eruptions?

(4) How did expanding gases contribute to the explosive nature of the eruption? What caused the gases to expand in the first place?

(5) How does the thickness (viscosity) of a magma relate to its ability to easily release pent-up gases?

(6) What are two primary causes of viscosity in a lava

(7) Place the following three lava types in order from least viscous (more fluid) to most viscous (thick consistency): Andesite, Rhyolite, Basalt

(8) What is Pumice? How are the holes (vesicles) in the rock created?

(9) What is a Pyroclastic Flow? How does it differ from a Pyroclastic Surge?

(10) What is a Stratovolcano and how does it differ from a Shield and Cinder Cone Volcano?

(11) Define a Lahar.How is a Lahar generated

(12) What is a Lava Dome?

(13) How did the Johnston Ridge Volcanic Observatory get its name?

(14) CHALLENGE: What might have caused Mt. St. Helens to erupt laterally rather than the usual vertical direction?

(15) CHALLENGE: How was the magma/lava at Mt. St. Helens originally generated?

(16) CHALLENGE: Draw a cross-section of the Convergent Boundary responsible for the Cascade volcanoes. Include the positions of the Subduction Zone, Ocean Trench, area of magma geneation and the position of Mt. St. Helens (or any volcano in the Cascade Mountain chain).

(17) CHALLENGE: How are tsunamis generated at Convergent Plate Boundaries?

(18) CHALLENGE: What will eventually happen to the lava dome presently found atop Mt. St. Helens?

SELECTED  REFERENCES:

-Taylor, Alan. The Eruption of Mount St. Helens in 1980. The Atlantic, May 18, 2015:

      https://www.theatlantic.com/photo/2015/05/the-eruption-of-mount-st-helens-in-1980/393557/ 

-USGS. Mount St. Helens. Accessed Jan. 1,2020: https://www.usgs.gov/volcanoes/mount-st-helens.

-USGS. Mount St. Helens. Accessed Jan. 1, 2020: https://www.usgs.gov/volcanoes/mount-st-helens/mount-st-helens-national-volcanic-monument

PHOTOS and VIDEOS:

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Figure 1 - One of the first views of Mt. St. Helens as seen from Route 504 towards Johnston Ridge Volcano Observatory.  The view is towards the south.  The lateral blast of May 18, 1980 was directed towards the north leaving a horseshoe shaped crater edge. (Photo Oct. 2009)

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Figure 2 - A close-up view of Figure 1 showing debris avalanches from the crater edge.  Note also the new lava dome building up at the base of the crater. (Photo Oct. 2009)

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Figure 3 - View from Johnston Ridge approximately five miles from the volcano.  Most of the sediment in the foreground is part of the Pumice Plain. Exposures of pumice are seen in the river banks. Pyroclastic Flow deposits are seen closer to the mouth of the north crater. The area in view was completely wiped clear of surface vegetation by the lateral blast. (Photo Oct. 2009)

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Figure 4 -  A close up view of the crater and Pyroclastic Flow debris near the north crater opening. (Photo Oct. 2009)

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Figure 5 - View of Mt. St. Helens from the Johnston Ridge Volcanic Observatory. This location was named in honor of geologist David Johnston of the the U.S. Geological Survey (USGS) who was at this spot as the volcano erupted. He felt the earthquake and observed the largest landslide ever witnessed by man followed by a lateral Pyroclastic Surge of hot gas, ash and rock that traveled towards him at speeds estimated to be up to 670 mph.  Today the mountain stands 1300 ft. lower in altitude than before the eruption.

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Figure 6 - Seismographs inside the observatory's Visitor Center keep track of the many minor earthquakes that occur in the area. Earthquakes just prior to the main eruption in 1980 were likely caused by the movement of magma towards the surface.

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Figure 7 - After a movie about the eruption in the Visitor Center, the screen rises to reveal this view of the mlountain.

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Figure 8 - Within the Visitor Center, a diorama installed with colored lights illustrates the extent of various phenomena associated with the eruption.  Here, lights show the area exhibiting complete instant deforestation  by the lateral surge. Note the sign  showing the location of the present Johnston Ridge Observatory. (Photo Oct. 2009)

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Figure 9 - Near the Johnston Ridge Observatory one can see the many trees that were blown to the right (north) and stripped of their branches by the blast. (Photo Oct. 2009)

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Figure 10 - Looking towards the east at the Observatory with downed trees pointing north. (Photo Oct. 2009)

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Figure 11 - Near the Observatory, if trees weren't uprooted, they were snapped at their base. (Photo Oct. 2009)

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Figure 12 - At Bear Meadow, about 11 miles from the mountain, hillsides are littered with downed trees, each pointing away from the blast. (Photo Oct. 2009)

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Figure 13 - A close-up of Figure 12 showing more detail of the fallen trees. (Photo Oct. 2009)

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Figure 14 -  Nearly 40 years after the eruption, new tree growth is seen scattered along the hillsides. (Photo Oct. 2009)

PHOTOS TAKEN ON A HIKE TO THE SUMMIT (JUNE 2015) by Eric Marintsch

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Figure 15 - Boulder Field of solidified lava along the flank of the volcano. (Photo June 2015 by Eric Marintsch)

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Figure 16 - From the summit of Mt. St. Helens looking towards the north. Spirit Lake is about 6-8 miles away. Mt. Rainer, seen in the distance, is about 85 miles away.  Here we can see the Lava Dome that will eventually either rise to form the shape of the mountain prior to its eruption in May of 1980, or it will blow up if enough pressure builds under the crater. (Photo June 2015 by Eric Marintsch)

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Figure 17 - Panoramic view of the summit. Volcanoes in the distance from center to right are Mt. Rainer, Mt. Adams, and Mt Hood. (Photo June 2015 by Eric Marintsch)

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Figure 18 - Selfie of explorer Eric Marintsch at the summit. Below are his videos taken inside the crater showing the smoke from heated fluids below the dome's top as well growth of reddish bacteria. The smell of sulfur dioxide and the presence of fine volcanic dust promote the use of an air filter mask. (Photo June 2015 by Eric Marintsch)

OTHER VOLCANOES OF THE CASCADE RANGE

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Figure 19 - Mt. Rainer, located 85 miles directly north of Mt. St. Helens. In the foreground is the classic minivan - a 2000 Toyota Sienna. (Photo June 2015 by Eric Marintsch)

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Figure 20 - Mt. Hood in Oregon found about 60 miles southeast of Mt. St. Helens (Photo Aug. 2017)

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Figure 21 - The deep blue waters of Crater Lake extend to a maximum depth of 1949 ft. making it the deepest lake in the U.S.  Wizard Island is a Cinder Cone volcano within the caldera. It stands 755 ft. above lake level. Some lava flows can be seen to project from the base of the volcano. (See the Crater Lake Field Trip for more information)