Volcanoes/Mount St. Helens

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This image shows Mount St. Helens, one day before the devastating eruption. Credit: Harry Glicken, USGS/CVO.{{free media}}

Mount St. Helens' is an active stratovolcano in Skamania County, Washington, in the Pacific Northwest region of the United States. The volcano is located in the Cascade Range and is part of the Cascade Volcanic Arc, a segment of the Pacific Ring of Fire that includes over 160 active volcanoes. This volcano is well known for its ash explosions and pyroclastic flows.

Mount St. Helens is most famous for its catastrophic eruption on May 18, 1980, at 8:32 am PDT[1] (20 b2k) which is the deadliest and most economically destructive volcanic event in the history of the United States. It is an example of a plinian eruption. A massive debris avalanche triggered by an earthquake measuring 5.0 on the Richter scale, caused the eruption, reducing the elevation of the mountain's summit from 9,677 ft (2,950 m) to 8,365 ft (2,550 m) and replacing it with a 1 mile (1.6 km) wide horseshoe-shaped crater. A sudden surge of magma from the Earth's mantle caused the earthquake.[2]

Mount St. Helens is geologically young compared with the other major Cascade volcanoes. It formed only within the past 40,000 years, and the pre-1980 summit cone began rising about 2,200 years ago.[3] The volcano is considered the most active in the Cascades within the Holocene epoch (the last 10,000 or so years).[4]

The plinian deposit from the May 18, 20 b2k, eruption shows a break-in-slope (thickness vs. distance) at about 27 km from source.[5] This break is too far from source to be explained by the transition from column margin to umbrella cloud sedimentation.[5] The most distal segment is composed of low Reynolds number particles.[5]

Ice cores from the Upper Fremont Glacier (UFG) in Wyoming, USA, taken in 1981 and 1980, 600 km from the volcano and directly upwind of the UFG, have a mercury containing tephra layer from the 20 b2k Mount St. Helens eruption.[6] The volcanic ash blanketed the region.[6]


Main source: Radiation
The diagram depicts the volcanic explosivity index. Credit: chris.

"The Volcanic Explosivity Index, or VEI, was proposed in 1982 as a way to describe the relative size or magnitude of explosive volcanic eruptions. It is a 0-to-8 index of increasing explosivity. Each increase in number represents an increase around a factor of ten. The VEI uses several factors to assign a number, including volume of erupted pyroclastic material (for example, ashfall, pyroclastic flows, and other ejecta), height of eruption column, duration in hours, and qualitative descriptive terms."[7]

"In the figure [on the right], the volumes of several past explosive eruptions and the corresponding VEI are shown. Numbers in parentheses represent total volume of erupted pyroclastic material (tephra, volcanic ash, and pyroclastic flows) for selected eruptions; the volumes are for uncompacted deposits. Each step increase represents a ten fold increase in the volume of erupted pyroclastic material."[7]

"A series of small to moderate explosive eruptions from Mono-Inyo Craters Volcanic Chain, California, during the past 5,000 years ranged from VEI of 1 to 3. The 18 May 1980 eruption of Mount St. Helens was a VEI 5 with an erupted volume of about 1 km3."[7]

Lava domes[edit]

Image of the rhyolitic lava dome of Chaitén Volcano during its 2008–2010 eruption. Credit: Sam Beebe.
One of the Mono Craters is an example of a rhyolite dome. Credit: Daniel Mayer.
Lava domes in the crater of Mount St. Helens. Credit: Willie Scott, USGS.
Photo showing the bulging cryptodome of Mt. St. Helens on April 27, 1980. Credit: Peter Lipman.
Chao dacite coulée flow-domes (left center), northern Chile, is viewed from Landsat 8. Credit: Robert Simmon, NASA Earth Observatory, USGS Earth Explorer.

Def. a roughly circular mound-shaped bulge that builds up from the slow eruption of viscous felsic lava from a volcano is called a lava dome.

Lava domes are rarely seen in shield volcanos, but are common in stratovolcanos because the latter have more silicic magmas.

Mount St. Helens has been building a new lava dome since the May, 1980 eruption.

"Streams of molten rock that ooze from gaps or vents in the Earth’s surface are called lava flows. Though generally slow-moving, these rivers of rock pose a hazard to everything in their paths. They can bury or burn homes and roads, ruin farmland for generations, and transform glaciers into muddy landslides (lahars)."[8]

"Lava flows can take many shapes and move at very different rates depending on the viscosity of the magma, the slope of the land, and the rate of an eruption. Some of the speediest flows travel 60 kilometers (40 miles) per hour; the slowest creep along at less than 1 kilometer (0.6 miles) per hour. They can sometimes even flow for more than a year after an eruption has ended."[8]

"Viscous (or sticky), non-explosive flows produce distinctive landforms known as lava domes. These circular mounds form as lava slowly oozes from a vent and piles up on itself over time. Lava domes tend to have steep, cliff-like fronts at their leading edge and wrinkle-like pressure ridges on their surfaces."[8]

"The Chao dacite is a type of lava dome known as a coulée. These elongated flow structures form when highly-viscous lavas flow onto steep surfaces. On May 14, 2013, the Operational Land Imager (OLI) on NASA’s Landsat 8 satellite acquired the image above, which highlights some of the distinctive features of a coulée."[8]

"The Chao dacite sits between two volcanoes in northern Chile: the older and partially-eroded Cerro del Leon and the younger Paniri. The dome itself is a giant tongue of rock that extends southwest from the vent. Curved pressure ridges known as ogives dominate the surface of the 14 kilometer (9 mile) dome."[8]

"Volcanologists think the Chao dacite dome formed over a period of about 100 to 150 years. A pyroclastic flow during the Chao I phase left light-brown deposits of tephra and pumice at the leading edge of the flow. Pyroclastic flows are avalanche-like events that bring mixtures of hot gas and semi-sold rocks surging down the flanks of volcanoes at speeds as fast as 100 kilometers (60 miles) per hour."[8]

"This period was followed by the Chao II phase, when 22.5 cubic kilometers (5.4 cubic miles) of lava erupted. This flow has 400-meter tall (1,312 feet) fronts that stand out with their dark shadows on the southwest end. The final, Chao III phase added another 3.5 cubic kilometers (0.8 cubic miles) of denser lava with a lower viscosity. This type of lava is less likely to form pressure ridges, so surfaces in this part of the flow are comparatively smooth."[8]

"It’s not clear why the Chao dacite erupted as a flow and formed a dome rather than erupting explosively. However, some researchers have noted that there are a number of other domes in the area (such as Chillahuita), suggesting that the domes may be the leading edge of a broader magmatic system that erupted along pre-existing faults. Though much larger, a series of lava domes along the eastern side of California’s Sierra Nevada range—the Mono-Inyo chain—offers a possible analog for what might be happening in this part of Chile."[8]


Main sources: Sediments/Lahars and Lahars
An explosive eruption of Mount St. Helens on March 19, 1982, sent pumice and ash 9 miles (14 kilometers) into the air, and resulted in a lahar (the dark deposit on the snow) flowing from the crater into the North Fork Toutle River valley. Credit: Tom Casadevall.

Def. a volcanic mudflow is called a lahar.

Part of the Mount St. Helens lahar entered Spirit Lake (lower left corner of the image on the right) but most of the flow went west down the Toutle River, eventually reaching the Cowlitz River, 50 miles (80 kilometers) downstream.

Volcanic ashes[edit]

Volcanic ash consists of rock, mineral, and volcanic glass fragments smaller than 2 mm (0.1 inch) in diameter, which is slightly larger than the size of a pinhead. Credit: D.E. Wieprecht, USGS.
Close view is of a single ash particle from the eruption of Mount St. Helens. Credit: A.M. Sarna-Wojcicki, USGS.

"Volcanic ash collected in Randle, Washington, [in the image on the right, is] located about 40 km NNE of Mount St. Helens."[9]

"The north edge of the eruption cloud of May 18, 1980, passed over Randle and deposited between 1 and 2 cm of ash on the community. At the same distance along the axis of the eruption cloud, however, about 7 cm of ash and larger-sized tepra fell to the ground."[9]

"Volcanic ash consists of rock, mineral, and volcanic glass fragments smaller than 2 mm (0.1 inch) in diameter, which is slightly larger than the size of a pinhead. Volcanic ash is not the same as the soft fluffy ash that results from burning wood, leaves, or paper. It is hard, does not dissolve in water, and can be extremely small--ash particles less than 0.025 mm (1/1,000th of an inch) in diameter are common."[9]

"Ash is extremely abrasive, similar to finely crushed window glass, mildly corrosive, and electrically conductive, especially when wet."[9]

"Volcanic ash is created during explosive eruptions by the shattering of solid rocks and violent separation of magma (molten rock) into tiny pieces. Explosive eruptions are generated when ground water is heated by magma and abruptly converted to steam and also when magma reaches the surface so that volcanic gases dissolved in the molten rock expand and escape (explode) into the air extremely rapidly. After being blasted into the air by expanding steam and other volcanic gases, the hot ash and gas rise quickly to form a towering eruption column directly above the volcano."[9]

The second image down on the right is a close "view of a single ash particle from the eruption of Mount St. Helens; image is from a scanning electron microscope (SEM). The tiny voids or "holes" are called vesicles and were created by expanding gas bubbles during the eruption of magma."[9]

Tephra set W[edit]

Eruption of the dacitic set W began the Kalama period late in the 15th century, probably in 520 b2k. The initial event produced the large-volume, pumiceous layer Wn, the second largest Holocene tephra from Mount St. Helens. Layer Wn is overlain by several smaller pumiceous tephras, including the moderate-volume layer We. Both layers Wn and We have been traced for hundreds of kilometers downwind.[10]

Using contiguous sampling, magnetic susceptibility measurements, wet sieving, light microscopy, and electron microprobe analysis of glass in pumice fragments, the 518-519 b2k Mount St. Helens We tephra layer is identified in sediments from Dog Lake in southeastern British Columbia (some 650 km away), suggesting that the plume drifted further north than previously thought.[11]

The GISP2 based calendrical age of Mount St. Helens Wn tephra dates the eruption that produced the Wn tephra layer at 520-521 b2k.[12]

Tephra set Y[edit]

The set Y eruptions started shortly after 4,000 b2k and continued at least to about 3,300 b2k.[10] The tephra set consists chiefly of two voluminous coarse pumice layers: Yn, the largest volume of Holocene tephra known from Mount St. Helens, and Ye.[10] Both have been found several hundred km downwind.[10]

A notable frost, tree ring event occurred in 4035 b2k among subalpine bristlecone pine observed at localities from California to Colorado, over a distance of some 1,300 km, that is attributed to Mount St. Helens.[13] This is the most severe frost event in the entire tree-ring record, as it occurs in all trees sampled and caused severe anatomical damage.[14] But, there does not appear to be a corresponding acidity peak in Greenland.[13] The frost-ring date does coincide approximately with a large radiocarbon-dated eruption of Mount St. Helens.[15]

Tephra set J[edit]

Eruptions between about 10,500 and 12,000 b2k deposited dacitic pumice layers near the volcano, recognized out to hundreds of km east.[10]

Following tephra set J is a dormant period lasting between about 4,000 b2k to 10,500 b2k.[10]

The mid-Holocene is a period of inactivity at Mount St. Helens.[11]

Tephra set S[edit]

Eruptions from about 13,000 b2k during the early Swift Creek stage deposited a few large-volume dacitic pumice layers near the volcano recognized up to hundreds of km east.[10]

Another mostly dormant interval occurred between 14,000 and 19,000 b2k.[10]

Tephra set M[edit]

About 20,500 b2k there are a set of tephra layers none of which is more than a few mm thick near the volcano.[10] Nevertheless, one ash bed has been recognized in Nevada.

Tephra set C[edit]

During the Ape Canyon stage (36,000 - 50,000 b2k) there is a set of tephra layers that contains at least two large-volume dacitic pumice layers. One erupted near the end of the Ape Canyon stage, records one of the largest volume tephra eruptions known to Mount St. Helens, and has been recognized as far away as Nevada.[10]

There is a dormant interval between the Ape Canyon and Cougar stage eruptions of about 15,000 yrs (21,000 - 36,000 b2k).[10]


Main sources: Rocks/Dacites and Dacites
Close view is of dacite lava from the May 1915 eruption of Lassen Peak, California. Credit: USGS.

Def. a rock with a high iron content is called a dacite.

"Dacite lava is most often light gray, but can be dark gray to black. Dacite lava consists of about 63 to 68 percent silica (SiO2). Common minerals include plagioclase feldspar, pyroxene, and amphibole. Dacite generally erupts at temperatures between 800 and 1000°C. It is one of the most common rock types associated with enormous Plinian-style eruptions. When relatively gas-poor dacite erupts onto a volcano's surface, it typically forms thick rounded lava flow in the shape of a dome."[16]

"Even though it contains less silica than rhyolite, dacite can be even more viscous (resistant to flow) and just as dangerous as rhyolites. These characteristics are a result of the high crystal content of many dacites, within a relatively high-silica melt matrix. Dacite was erupted from Mount St. Helens 1980-86, Mount Pinatubo in 1991, and Mount Unzen 1991-1996."[16]

See also[edit]


  1. Mount St. Helens National Volcanic Monument. USDA Forest Service. http://www.fs.fed.us/gpnf/mshnvm/. Retrieved 2006-11-26. 
  2. May 18, 1980 Eruption of Mount St. Helens. USDA Forest Service. http://www.fs.fed.us/gpnf/mshnvm/education/teachers-corner/library/volcanic-eruption-summary.shtml. Retrieved 2007-08-11. 
  3. Mullineaux, The Eruptive History of Mount St. Helens, USGS Professional Paper 1250, page 3
  4. USGS Description of Mount St. Helens, USGS.gov . Retrieved 15 November 2006.
  5. 5.0 5.1 5.2 Bonadonna C, Ernst GGJ, Sparks RSJ (May 1998). "Thickness variations and volume estimates of tephra fall deposits: the importance of particle Reynolds number". J Volcanology Geothermal Res 81 (3–4): 173–187. doi:10.1016/S0377-0273(98)00007-9. http://www.geo.mtu.edu/~raman/papers2/Bonadonnaetal1998JVGR.pdf. 
  6. 6.0 6.1 Schuster PF, Krabbenhoft DP, Naftz DL, Cecil LD, Olson ML, DeWild JF, Susong DD, Green JR, Abbott ML (June 2002). "Atmospheric mercury deposition during the last 270 years: a glacial ice core record of natural and anthropogenic sources". Environ Sci Technol. 36 (11): 2303–2310. doi:10.1021/es0157503. PMID 12075781. http://webhost.ua.ac.be/mitac4/instr/Hg_270years_EnvSciTech_36_2002_2303_2310.pdf. 
  7. 7.0 7.1 7.2 C. G. Newhall and S. Self (29 December 2009). VHP Photo Glossary: VEI. Menlo Park, California USA: United States Geological Survey. http://volcanoes.usgs.gov/images/pglossary/vei.php. Retrieved 2015-02-28. 
  8. 8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7 Erik Klemetti and Adam Voiland (21 November 2013). The Shapes that Lavas Take, Part 1. Washington, DC USA: NASA. http://earthobservatory.nasa.gov/IOTD/view.php?id=82424. Retrieved 2015-02-18. 
  9. 9.0 9.1 9.2 9.3 9.4 9.5 D.E. Wieprecht (18 May 1980). VHP Photo Glossary: volcanic ash. Menlo Park, California USA: USGS. http://volcanoes.usgs.gov/images/pglossary/ash.php. Retrieved 2015-03-09. 
  10. 10.00 10.01 10.02 10.03 10.04 10.05 10.06 10.07 10.08 10.09 10.10 Mullineaux DR (1996). "Pre-1980 Tephra-Fall Deposits Erupted From Mount St. Helens, Washington". USGS Professional Paper (1563). http://vulcan.wr.usgs.gov/Volcanoes/MSH/EruptiveHistory/summary_msh_eruptive_stages.html. 
  11. 11.0 11.1 Hallett DJ, Mathewes RW, Foit FF Jr (May 2001). "Mid-Holocene Glacier Peak and Mount St. Helens We Tephra Layers Detected in Lake Sediments from Southern British Columbia Using High-Resolution Techniques". Quart Res. 55 (3): 284–292. doi:10.1006/qres.2001.2229. http://www.nau.edu/~envsci/DJHallett/downloads/QR01Hallett.pdf. 
  12. Fiacco RJ Jr, Palais JM, Germani MS, Zielinski GA, Mayewski PA (1993). "Characteristics and possible source of a 1479 A.D. volcanic ash layer in a Greenland ice core". Quart Res 39 (3): 267–273. doi:10.1006/qres.1993.1033. 
  13. 13.0 13.1 LaMarche VC Jr, Hirschboeck KK (January 1984). "Frost rings in trees as records of major volcanic eruptions". Nature 307 (5946): 121–126. doi:10.1038/307121a0. http://fp.arizona.edu/kkh/nats101gc/PDFs-09/LaMarche.Hirschboeck.1984.all.edt.opti.pdf. 
  14. LaMarche VC Jr, Harlan TP (1973). "Accuracy of Tree Ring Dating of Bristlecone Pine for Calibration of the Radiocarbon Time Scale". J Geophys Res. 78 (36): 8849–8858. doi:10.1029/JC078i036p08849. 
  15. Simkin T, Siebert L, McClelland L, Bridge D, Newhall C, Latter JH (1981). Volcanoes of the World. Stroschberg: Hutchinson Ross. 
  16. 16.0 16.1 DaciteUSGS (17 July 2008). VHP Photo Glossary: Dacite. Menlo Park, California USA: USGS. http://volcanoes.usgs.gov/images/pglossary/dacite.php. Retrieved 2015-03-11. 

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