Volcanoes/Mount St. Helens

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 (volcano number 321050) located at 46.2°N latitude, 122.18°W longitude, with a current summit height of 2549 masl, in Skamania County, Washington, the Pacific Northwest region of the United States. The volcano is part of the Cascade Range and the Cascade Volcanic Arc, a segment of the Pacific Ring of Fire that includes over 160 active volcanoes. It often exhibits ash explosions and pyroclastic flows.

Mount St. Helens is most famous for its catastrophic eruption on May 18, 1980, at 8:32 AM PDT (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.

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. The volcano is considered the most active in the Cascades within the Holocene epoch (the last 10,000 or so years).

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

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. The volcanic ash blanketed the region.

Radiation

Main resource: 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."

"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."

"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."

Lava domes

Main resources: Earth/Geology/Structures/Domes and Domes

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)."

"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."

"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."

"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."

"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."

"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."

"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."

"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."

Lahars

Main resources: 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

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."

"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."

"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."

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

"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."

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."

Volcanic activity

Main resources: Volcanoes/Volcanic activity and Volcanic activity

A "Whaleback" feature occurred in Mount Saint Helens new lava dome. Credit: Steve Schilling, USGS.{{free media}}

Four "stages of volcanic activity—Ape Canyon, Cougar, Swift Creek, and Spirit Lake—separated by dormant intervals [have been recognized]."

1. Ape Canyon (275 to 35 ka)
2. Dormant Interval 35 to 28 ka
3. Cougar (28 to 18 ka)
4. Dormant Interval 18 to 16 ka
5. Swift Creek (16 to 12.8 ka)
6. Dormant Interval 12.8 to 3.9 ka
7. Spirit Lake (3.9 ka to present)

"The sudden reawakening of Mount St. Helens in late September 2004 was surprising because the preceding four years had seen the fewest earthquakes since the 1980-86 eruption ended. In the early hours of 23 September 2004, a swarm of small-magnitude (< 1), shallow earthquakes (< 1 km or about 0.5 mi below the surface) began beneath the 1980-1986 lava dome. Over the next seven days, earthquake frequency and size increased and a growing welt formed on the SE margin of the 1980-86 lava dome and nearby portions of Crater Glacier. By September 26, 2004, the rise in activity led scientists to warn of an increased chance of explosions from the lava dome."

Tephrochronology

Main resource: Geochronology/Tephra layers

"Recognized tephra strata record more than 100 explosive eruptive events at Mount St. Helens; those tephra strata are classified as beds, layers, and sets. Tephra sets, each of which consists of a group of beds and layers, define in part the nine eruptive periods recognized at the volcano. Individual tephra sets are distinguished from stratigraphically adjacent sets by differences in composition or by evidence of clapsed time."

1980 tephra set

The plume photographed here rose nearly 3,000 ft (910 m) above the volcano's rim. Credit: Lyn Topinka.{{free media}}

This is the modern period from 1980 to 2008.

"There is data available for 44 Holocene eruptive periods."

1. 2004 Oct 1 to 2008 Jan 27 ± 10 days, VEI = 2, South of 1980-1986 lava dome.
2. 1990 Nov 5 to 1991 Feb 14, VEI = 3, North side of lava dome.
3. 1989 Dec 7 to 1990 Jan 6, VEI = 2, North side of lava dome.
4. 1980 Mar 27 to 1986 Oct 28 ± 3 days, VEI = 5, Summit and north flank.

"Plumes of steam, gas, and ash often occurred at Mount St. Helens in the early 1980s. On clear days they could be seen from Portland, Oregon, 50 mi (80 km) to the south. The plume photographed here rose nearly 3,000 ft (910 m) above the volcano's rim. The view is from Harrys Ridge, 5 mi (8 km) north of the mountain."

Uncertain tephra layers

5. [ 1921 Mar 18 ] to [ Unknown ].

6. [ 1903 Sep 15 ] to [ Unknown ].

7. [ 1898 Apr 5 ] to [ Unknown ].

Layer T

Goat Rocks age eruptive deposits are shown and Floating Island lava flow. Credit: Rick Hoblitt. {{free media}}

The Goat Rocks period lasted from 1800 to 1857.

There are 11 Holocene eruptive periods.

8. 1857 Apr to Unknown, VEI = 2.

9. 1854 Feb to 1854 Apr, VEI = 2, North flank.

10. 1853 Mar 15 ± 5 days to 1853 Aug (?), VEI = 2, North flank.

11. 1850 Mar to 1850 May (?), VEI = 2, North flank.

12. [ 1849 ] to [ Unknown ], VEI = 2, North flank.

13. 1848 Apr 1 (in or before) to Unknown, VEI = 2.

14. 1847 Mar 26 to 1847 Mar 30, VEI = 2, North flank (Goat Rocks).

15. 1842 Nov 22 to 1845 Sep 18 (?), VEI = 3, North flank (Goat Rocks).

16. 1835 Mar (?) to Unknown, VEI = 2, North flank (Goat Rocks area).

17. 1831 Aug to Unknown, VEI = 3, North Flank (Goat Rocks area).

18. 1800 Jan 15 ± 120 days to Unknown, VEI = 5, Dendrochronology, N flank--Goat Rocks area, Layer T.

"The Goat Rocks period was short and relatively small. An explosive eruption in 1800 produced a tephra deposit (set "T") that was followed in 1801 by an andesite lava flow, called the "Floating Island," on Mount St. Helens' north flank. Eruptions observed intermittently from 1831 to 1857 produced ash and the Goat Rocks Dome, whose growth also resulted in lahars and a small fan of volcanic debris."

Tephra layer Z

19. 1610 ± 40 years to Unknown, Radiocarbon (corrected), Pre-1980 summit dome, Tephra layer z.

Tephra set X

Worm Complex is on the southeast flank of Mount St. Helens—Middle Kalama lava flows in center of image. Credit: Rick Hoblitt.{{fairuse}}

Summit region is Kalama age–1479 to 1720 C.E. of Mount St. Helens looking northeast. Credit: Rick Hoblitt. {{free media}}

This tephra set occurred at the later end of the Kalama period ending in 1750.

20. 1525 ± 25 years to Unknown, Dendrochronology, tephra set X.

"The middle Kalama phase began about 1510 with eruption of andesite as pyroclastic flows (which generated hot lahars), a few lava flows, and tephra (set "X"). The middle phase peaked at about 1535 when many thick andesite lava flows erupted on all flanks of Mount St. Helens, including the Worm Complex flows [shown in the image on the right], near to the present day climbing route. This phase ended by 1570."

"The most significant event of the late Kalama phase was growth of a large dacite dome at the summit (Summit Dome). It probably took nearly 100 years for the Summit Dome to grow (between 1620 to 1720) and give Mount St. Helens its pre–1980 form. During growth, the dome shed material as pyroclastic flows and lahars on all flanks of the volcano, but the highest concentration was to the southwest, northwest, and southeast. Mount St. Helens acquired its pre–1980 cover of glaciers as a result of new elevation achieved by the growth of the Summit Dome."

Tephra set W

21. 1482 Jan 15 ± 120 days to Unknown, VEI = 5, Dendrochronology, Tephra layer We.

22. 1480 Jan 15 ± 120 days to Unknown VEI = 5, Dendrochronology, tephra Wn.

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.

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.

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

Layer D

East Dome erupted during Sugar Bowl time (C.E. 850-900) east flank view of Mount St. Helens. Credit: Rick Hoblitt / USGS. {{free media}}

The Sugar Bowl period lasted from 1,200 to 1,150 b2k.

23. 0780 ± 300 years to Unknown, Radiocarbon (uncorrected), NE flank (Sugar Bowl).

"During the Sugar Bowl Eruptive Period, three lava domes were built on the flanks of Mount St. Helens. Explosive eruptions associated with growth of the Sugar Bowl Dome produced a tephra layer (set "D") and two lateral blasts that affected an area about one-tenth as large as the 1980 eruption."

Tephra set B

File:Northeast crater wall annotated deposits.jpg

Northeast crater wall above Sugar Bowl, Mount St. Helens with annotated deposits are from some of the Spirit Lake Stage eruptive periods. Credit: Michael Clynne.{{fairuse}}

The Castle Rock Creek period, see the label in the image on the right, lasted from 2,025 to 1,700 b2k. This is layers Bi and Bu.

24. 0420 (?) to Unknown, Radiocarbon (corrected).

25. 0270 (?) to Unknown, Radiocarbon (corrected), Tephra layer Bu.

26. 0230 (?) to Unknown, VEI = 0, Tephrochronology.

27. 0190 (?) to Unknown, Radiocarbon (corrected), Lower E flank (East Dome), Layer Bi.

28. 0100 (?) to Unknown, VEI = 0, Radiocarbon (corrected), SW flank (Cave basalts).

29. 0100 BCE (?) to Unknown, Radiocarbon (corrected).

30. 0220 BCE (?) to Unknown, Radiocarbon (corrected), NNE flank (Dogs Head), Layer Bd.

31. 0250 BCE (?) to Unknown, Tephrochronology, Tephra layer Bo.

32. 0280 BCE (?) to Unknown, Radiocarbon (corrected).

"The Castle Creek Eruptive period produced tephra, pyroclastic flows, and many lava flows and domes. Several andesite to dacite lahars, thick dacite domes, and pyroclastic flows and tephra were emplaced about 2.0 ka. Shortly thereafter, Mount St. Helens erupted a widespread unite of dacite tephra ("Bi") followed by two andesite lava flows (North Rim and Red Rock) and pyroclastic flows associated with dacite dome eruptions."

Recent work has identified products from three separate mildly explosive basaltic eruptions ("Bu" 1, 2, and 3) that took place over a time span of about 150 to 200 years (about 1.9 to 1.7 ka). Several basalt to dacite lava flows erupted in the same time span, which transformed the Pine Creek-age cluster of domes into a classic cone shaped composite volcano. Three basaltic lava flows poured down the flanks of the volcano reaching up to 13 km (8 mi) from their source - the preCave (1.8 ka) and Cave basalt (1.895 ka) on the south flank, and the Castle Creek basalt (1.74 ka) on the north flank. The summit elevation at the end of Castle Creek time was about 2,450 m (8,000 ft)."

The Pine Creek period lasted from 3,000 to 2,500 b2k and produced set P, and the early B layers Bo and Bh.

33. 0530 BCE (?) to Unknown, VEI = 5, Radiocarbon (corrected), Pine Creek tephra layers Ps and Pu.

34. 0800 BCE (?) to Unknown, Radiocarbon (corrected).

35. 0830 BCE ± 75 years to Unknown, Radiocarbon (uncorrected).

36. 1010 BCE (?) to Unknown, Radiocarbon (corrected).

37. 1100 BCE (?) to Unknown, Radiocarbon (corrected).

38. 1180 BCE (?) to Unknown, Radiocarbon (corrected), Pm layer.

"During the Pine Creek Eruptive Period, Mount St. Helens ejected tephra, produced pyroclastic flows and dacite domes, and two small debris avalanches occurred on its north flank. Repeated collapse of hot, growing lava domes produced an extensive and broad fan of volcanic debris as much as 180 m (600 ft) thick on the south flank of the volcano. Similar deposits on the north flank can still be found as far downstream as the town of Toutle."

"Andesite and basaltic andesite lava flows erupted late in this period (Muddy River andesite) along with accompanying pyroclastic flow and lahar deposits of similar composition. Based upon analysis of exposed dacite domes found in the walls of the 1980 eruption crater, by the end of the Pine Creek Period the volcano was a cluster of lava domes that had grown to a maximum elevation of about 2,100 m (7,000 ft)."

Tephra set Y

The Smith Creek period produced set Y.

39. 1610 BCE (?) to Unknown, Radiocarbon (corrected).

40. 1680 BCE (?) to Unknown, Radiocarbon (corrected), Tephra layer ya.

41. 1770 BCE ± 100 years to Unknown, VEI = 5, Tephrochronology, Tephra layer Ye.

42. 1860 BCE (?) to Unknown, VEI = 6, Radiocarbon (corrected), Tephra layer Yn.

43. 2100 BCE ± 300 years to Unknown, Tephrochronology, Tephra layer Yd.

44. 2340 BCE (?) to Unknown, VEI = 5, Radiocarbon (corrected), Yb layer.

"During the Smith Creek period, two periods of highly explosive activity (3.90 to 3.85 ka and 3.5 to 3.3 ka) deposited large amounts of tephra (set "Y") and pyroclastic flows. The second period was initiated with a highly explosive eruption ("Yn") that was about four times larger than the one in 1980, making it the most voluminous eruption in Mount St. Helens' history. These tephras have been identified as far away as 950 km (590 mi) from source. During late Smith Creek time, a lava dome was extruded and huge lahars swept down the Toutle River and probably reached the Columbia River."

The set Y eruptions started shortly after 4,000 b2k and continued at least to about 3,300 b2k. 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. Both have been found several hundred km downwind.

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. 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. But, there does not appear to be a corresponding acidity peak in Greenland. The frost-ring date does coincide approximately with a large radiocarbon-dated eruption of Mount St. Helens.

Tephra set J

The Swift Creek period produced sets S and J.

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

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

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

Tephra set S

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.

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

Tephra set M

The Cougar stage produced sets M and K.

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

Tephra set C

Lava "domes erupted just west of the present volcano in two distinct periods—one from 275 to 250 thousand years ago (ka) and a second from 160 to 35 ka."

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.

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

Volcanism

Main resources: Volcanoes/Volcanism and Volcanism

"Volcanism during the Ape Canyon Stage produced a cluster of lava domes with maximum elevations of about 1,200 m (4,000 ft). Ash layers correlating to the two eruptive periods have been found as far east as central Washington, indicating that explosive eruptions also occurred. Much of the Ape Canyon Stage history is recorded in a Cougar-age debris avalanche, glacial deposits, and lahars in the Lewis River Valley. Many Ape Canyon-age rocks were altered hydrothermally (by volcanically heated ground water), indicating that an extensive hydrothermal system existed during the latter part of the stage. Eruptive History at Mount St. Helens including timing of stages and periods of volcanism."

"The Cougar Stage was probably the most active eruptive stage in Mount St. Helens' history before the Spirit Lake Stage. During this time the volcano produced explosive eruptions that ejected large volumes of ash, lava domes, lava flows, pyroclastic flows, a debris avalanche, and lahars."

"The Cougar debris avalanche was the most devastating event of the Cougar Stage, and was probably larger than the huge debris avalanche that triggered Mount St. Helens' 1980 eruption. The deposit is primarily made up of reworked Ape Canyon-Stage rocks. It originated near Butte Camp in the southwest part of the present-day edifice and left a 180–to 270–m (600– to 900–ft) thick, 17–km long (11–mi) deposit extending from the south flank of the volcano, into the Lewis River, which was temporarily dammed. Downcutting of the dam caused flooding downstream as far as the Columbia River and filled the lower Lewis River Valley with volcanic debris at least 60 m (200 ft) thick."

"The Cougar debris avalanche was immediately followed by a large explosive eruption producing pyroclastic flows that buried the avalanche deposits with up to 90 m (300 ft) of dacite pumice in ancestral Swift Creek. The Cougar–stage debris avalanche probably initiated onset of this explosive eruption. About midway through the stage, continued explosive activity deposited two sets of tephra ("M" and "K") and more pyroclastic flows."

"At about 18 ka, the Cougar Stage culminated with the eruption of the largest lava flow in the history of Mount St. Helens. Known as the Swift Creek flow, its thickness was up to 200 m and it reached nearly 6 km (3.7 mi) down the Swift Creek drainage, where it presently forms the divide between the West Fork and main stem of Swift Creek. The vent for this andesite lava flow, at an elevation of 1,830 m (6,000 ft) on the south flank of Mount St. Helens, marks the location of the volcano's summit at the end of the Cougar stage."

Dacites

Main resources: Rocks/Dacites and Dacites

File:Dacite-HotRock large.jpg

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."

"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."

See also

• Campi Flegrei
• Io
• Volcanoes, list
• Rainier
• Redoubt
• Somma
• Tambora
• Tephra layers
• Mount Vesuvius
• Volcanic activity
• Volcanic minerals
• Volcanic rocks
• Volcanism
• Volcanology