Case History: Mt. St. Helens

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Terence Webster
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Case History: Mt. St. Helens EAS 458 Volcanology Introduction 1980 eruption of Mt. St. Helens was particularly interesting and violent eruption with an unusual lateral blast.

1 Case History: Mt. St. Helens EAS 458 Volcanology Introduction 1980 eruption of Mt. St. Helens was particularly interesting and violent eruption with an unusual lateral blast. In the 1970 s, the USGS (Crandell( and Mullineaux) identified Mt. St. Helens as the most dangerous volcano in the Cascades. USGS continues to consider it highly dangerous. It illustrates hazard management done well (mostly). 1

2 Eruptive History Eruption This period includes the cataclysmic May 18, 1980 eruption, the destruction of the summit and the (initial*) growth of a new dome in the summit crater. *(Future geologists will probably consider the present activity as a continuation of the 1980 eruptive period - but we will treat that separately). 2

1980 Eruption Precursor Activity Increase in seismic activity in March, 1980 Magnitude 4.2 event on March 20. Built to continuous shaking within a week. Most quakes < 3 km depth.

3 1980 Eruption Precursor Activity Increase in seismic activity in March, 1980 Magnitude 4.2 event on March 20. Built to continuous shaking within a week. Most quakes < 3 km depth. Portable seismometers installed on March 21. First Eruption Since earthquakes did not follow typical shock- aftershock pattern, scientists decided by March 24 that these were of volcanic nature. By March 25, more than 174 earthquakes > M 2.6. Minor phreatic eruption on March 27 confirmed that St. Helens was reawakening. By March 28, more than a dozen explosions (all lacking juvenile material). March explosions recorded 3

4 Harmonic Tremor and Craters Two craters formed in the summit, which eventually merged into 1 large one. In April, first harmonic tremor observed. In the meantime, the Forest Service evacuated posts and set up road blocks. (Pressure from loggers later resulted in temporarily easing restrictions). Explosions continue, with plumes rising to 20,000 ft (but still no juvenile material). 4

realize that the north flank had moved outward by 300 ft!

5 The Bulge On April 23, scientists compared aerial photos taken on April 7 with ones from previous year and realize that the north flank had moved outward by 300 ft! Crater developing into a graben Bulge sliding down and away 5

Changing View from Timberline Viewpoint 1964 April 8, 1980 April 26, 1980

Bulge and surrounding area slide away in gigantic debris avalanche.

km Overtops 380 m Johnson Ridge 62 km 2 of debris filled the valleys to the

6 Changing View from Timberline Viewpoint 1964 April 8, 1980 April 26, 1980 May 2, 1980 May 18, 1980 Magnitude 5.1 strikes at 8:32 AM. Bulge and surrounding area slide away in gigantic debris avalanche. (Earthquake probably caused the slide, but opposite cannot be ruled out). Top 400 m of the mountain is lost km 3 Debris avalanche reaches speeds of km/hr Travels as far as 24 km Overtops 380 m Johnson Ridge 62 km 2 of debris filled the valleys to the north 150 x 10 6 m 3 of mud deposited by lahars. Release of pressure on the cryptodome initiates a plinian eruption. 57 people killed or missing. 6

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10 Comparing Expectation and Actuality 10

Debris avalanche swept through Spirit Lake, producing 250 m high tsunami. Lateral Blast This was entirely unexpected by geologists - only one previous known example - Bezymianny,, Kamchatka, 1956.

11 Debris avalanche swept through Spirit Lake, producing 250 m high tsunami. Lateral Blast This was entirely unexpected by geologists - only one previous known example - Bezymianny,, Kamchatka, Based on observations at Mt. St. Helens, other lateral blast deposits have since been identified. Release 13 megatons of thermal energy Temperatures as high as 650 C Knocked down and sand blasted trees over 600 km 2 area. Max velocity may have been supersonic. Deposit thickness varies from 1 m near volcano to few cm at distal end. 11

Lateral Blast Effects Lahars Much of the property damage and economic loss (beyond the loss of forests to lumber companies) resulted from lahars.

12 Lateral Blast Effects Lahars Much of the property damage and economic loss (beyond the loss of forests to lumber companies) resulted from lahars. Source of the water not entirely clear - snowmelt (150 ft deep glaciers on Mt. St. Helens), ground water released from collapse, Spirit Lake contributed South Fork Toutle River lahar (and several smaller lahars north & east of volcano) began shortly after blast Largest lahar, North Fork Toutle River, did not begin until afternoon - fed by dewatering of debris avalanche. It was erosive, and peaked in the Cowlitz River that night. Enough mud and debris reached the Columbia River that channel depth decreased from 12 to 4 m, stranding 31 ships upstream and necessitating dredging. 12

13 Lahar Deposits, Muddy River Lahar Deposits Cowlitz River 13

Plinian Eruption Plinian Eruption Juvenile dacitic ash & pumice began to erupt shortly after blast, tapping shallow root of cryptodome. Eruption column quickly reached 14-16 km.

14 Plinian Eruption Plinian Eruption Juvenile dacitic ash & pumice began to erupt shortly after blast, tapping shallow root of cryptodome. Eruption column quickly reached km. Eruption rate increased at about noon, perhaps heralding arrival of pumice from magma body at 7 km depth (column reaches 19 km). Experimental work indicates this reservoir was a a pressure of 220±30 MPa, P water P total, and T = 930 ± 10 C. Partial column collapse produced pyroclastic flows that covered the debris avalanche deposit, producing the pumice plain. Ash deposited up to 1500 km away. Eruption ended that night. 67 killed. Numerous subsequent plinian and subplinian eruptions that year. 14

Five dome extrusions lasting 1981-1986 several days preceded by small explosions in 1981 March 19, 1982 plinian eruption, lahar into Toutle valley, followed by dome extrusion lasting a month Two more

15 Five dome extrusions lasting several days preceded by small explosions in 1981 March 19, 1982 plinian eruption, lahar into Toutle valley, followed by dome extrusion lasting a month Two more dome extrusions in 1982 Feb 7, 1983 subplinian eruption, lahar followed by a year of continuous dome extrusion more dome extrusion events lasting up to 2 weeks. By October 1986, dome volume had reached 74 x 10 6 m 3 (0.07 km 3 ). Seismic Constraints on the plumbing system Precursor earthquakes to May 18 were and confined to small, shallow volume; Increase just before May 18, then seismicity dies out. Subsequent eruptions also preceded by seismic swarms that die out after eruption. Post May 18, seismicity is as deep as 20 km. Within 2 weeks earthquakes were occurring in two distinct zones, one north and one south of the volcano Post 1986 seismicity mostly intermediate depth 15

16 Seismicity and the Magma Chamber Earthquakes at depths of 7 to 12 km surround an earthquake free zone thought to be the magma chamber. Volume: 5-7km 3. Focal mechanisms were consistent with contraction after magma withdrawal. Using subsidence of volcano after May 18 (~0.5m), assuming purely elastic response and that evacuation of volume = eruptive products, depth calculated (for point source) is about 8 km. Petrology May 18 pumice: SiO 2 initially 64-65%, but late in the day more variable % Magma chamber may have been zoned, with newer, more mafic magma near the bottom. opx, plag, amph., Fe-Ti oxide phenocrysts appearing in that order. Fe-Ti oxide geothermometry indicates T = C; log ƒ O2 of to Glass inclusions in phenocrysts have oxide totals 4.6% less than matrix glass, indicating volatile content of 4.6% In subsequent eruptions, SiO 2 declined to 61-62% in 1981 and then rose to ~64% 16

May 18 Magma Ascent Rates Change in color of ash and increase in eruption rate around noon on May 18 suggest arrival of magma from magma chamber. Equation for buoyancy driven flow: u = g(! c "!

17 May 18 Magma Ascent Rates Change in color of ash and increase in eruption rate around noon on May 18 suggest arrival of magma from magma chamber. Equation for buoyancy driven flow: u = g(! c "! L )r 2 8# u is velocity, ρ c is crustal density, ρ l is magma density, r is conduit radius, and η is viscosity (=2.3 x 10 6 Pa-s) Magma supply or eruption rate be given by: MSR =! l "r 2 u Carey & Sigurdsson estimated magma supply rate from column height, and combining the two equations determined u = 3.6 km/hr, r = 47 m. Mt. St. Helens Seismicity

18 Seismicity

19 Mt. St. Helens Earthquakes Oct 1, 2004 USGS Cascade Volcano Observatory: Seismic activity at Mount St. Helens has accelerated significantly, which increases our level of concern that current unrest could culminate in an eruption. We are increasing the alert level to the second of three levels. Earthquakes are occurring at about four per minute. All are still at shallow levels in and below the lava dome that grew in the crater between 1980 and This suggests that the ongoing intense earthquake activity has weakened the dome, increasing the likelihood of explosions or perhaps the extrusion of lava from the dome. October 1, 2004 Later that day (Oct 1), an explosion lofted steam and ash several thousand feet above a vent blasted through the fractured glacier, and hurled rock fragments at least ½ mile across the western half of the glacier and lava dome. Four more steam and ash explosions occurred through October 5. Glacier surrounding 1986 dome visibly deformed. 19

In late October, a larger whaleback-shaped extrusion of solid lava emerged immediately southeast of

20 October 27, 2004 On October 11, a spine of solid, but still hot, lava punctured the surface of the welt. As this initial spine grew upward, several smaller spines appeared nearby. In late October, a larger whaleback-shaped extrusion of solid lava emerged immediately southeast of the initial spine. The early spines plus the whaleback extrusion are referred to as the new lava dome. February

Dome Growth 2005-2006 As of August 18, 2006, was 85.0 million cubic meters (111 million cubic yards) and was growing at an average rate of less than 1 cubic meter per second (1.

21 Dome Growth As of August 18, 2006, was 85.0 million cubic meters (111 million cubic yards) and was growing at an average rate of less than 1 cubic meter per second (1.3 cubic yards per second). This volume is approximately equal to 150 Rose Garden Arenas (Portland, Oregon). The dome's high point was 2,369 meters (7,772 feet), 1300 above the crater floor, with long-axis length being 1,010 meters (3,314 feet) and width being 510 meters (1,678 feet). Scientists working on the old part of the new lava dome found evidence to suggest that the lava dome was essentially solidified within several hundred yards beneath the crater floor. The outer 5 to 10 feet of the lava dome is composed of ground rock that transitions to solid rock with numerous fractures. These findings support the stick-slip model of lava dome extrusion. If the model is correct, it may help explain the origin of many of the million plus small, shallow earthquakes that have occurred during the eruption. Occasional partial dome collapses have led to minor explosive events and ash plumes, such as this one on March 8, Lava composition similar to Explosions 21

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Why was this eruption important?

Why was this eruption important? Mount St. Helens Mount St. Helens has a long geological history (>3,000 yrs) of explosive eruptions. The 1980 Events: Initial relatively mild steam and ash (Vulcanian) eruptions. Sustained plinian eruption