ISNS4359
EARTHQUAKES AND VOLCANOES
Spring 2005
Steve Bergman, Instructor
Lecture 19. Historic Eruptions III. 1980 Mt. St. Helens, Popocateptl, & Ol Doinyo Lengai
Mt. St. Helens, WA 1980 The most active Cascade Range Volcano [In 1792, George Vancouver named it after
Alleyne Fitzherbert, Baron St. Helens (1753-1839) British Ambassador to Spain; Native American names Lawelatla meaning
"One from whom smoke comes," Loo-Wit, “Keeper of the Fire” or Tah-one-lat-clak, “Fire Mountain”].
1. Location: 46.2N, 122.2W, elevation: 2950 m before, 2549 m after; one of 13 active volcanoes of the Cascade
Volcanic Arc, but displaced ~50 km to the west of the main N-S trend.
2. Size & Type of volcanic feature: 8 km diameter stratovolcano.
3. Relationship to Plate Boundaries/ Names of relevant plates: eastward subduction of Juan de Fuca Plate
oceanic lithosphere beneath North American Plate continental lithosphere.
4. Date and duration of eruption: 27Mar1980 – 8:32AM 18May1980 - 18Oct1980 – 21Oct1986
5. Type of Eruption/Deposits/Magma Composition: Dacite debris avalanche, plinian, peleean eruptions
producing pyroclastic surge, flow and fall deposits; eruption terminated by emplacement of a dacite lava dome.
6. Volume of erupted products: ~4 km3, VEI= 5.
7. Area impacted by the eruption: The 0.2 km3 landslide covered 60 km2 and moved at 100-200 km/hr. The lateral
blast covered an area of >600 km2, moving 10-30 km within minutes at <500 km/hr at temperatures from 100-300C.
Pyroclastic flows covered 16 km2, up to 8 km from the vent. Over 80,000 km2 were covered by ash falls.
8. Summary of volcanic events: The eruptive history of MSH began about 40,000 years ago with dacitic volcanism,
which continued intermittently until about 2,500 yr ago. This activity included many explosive eruptions over periods of
hundreds to thousands of years, which were separated by apparent dormant intervals ranging in length from a few hundred to
about 15,000 yr. A Mw=4 EQ occurred below the volcano 20 March 1980, followed by several others. After 350 years of
quiescence, on 27 March 1980, an 80 m wide crater was blasted in the summit dome. During the next month, the northern
flank began to bulge (@2m/day), reaching 0.1 km x 1x 1.6 km. On 7 May, an ash and steam eruption formed a 4 km high
cloud. Over 8000 tremors had been detected in 40 days. On 12 May, a M w=5 EQ caused a small debris avalanche on its
north slope. At 8:32AM, 18May1980, a magnitude 5 EQ occurred at 1.6 km depth and an enormous debris avalanche swept
3 km3 of rock downslope at >400 km/hr for ~10 km. The release of pressure caused a massive blast and plinian eruption that
removed the uppermost 0.5 km of the mountain within minutes, producing a 24 km high ash cloud (volume 1.1 km 3). Most
pyroclastic flows (0.12 km3) moved at 50-80 mph (T>800°C) terminated within 8 km of the volcano, but lahars extended
down some valleys at least as far as 75 km. Fans of lahars and pyroclastic flows on the north side of the volcano dammed the
North Fork Toutle River to form the basin of an ancestral Spirit Lake between 3,300 and 4,000 yr ago.
9. Casualties/Type of hazards: The 9 hour eruption killed 57 people, >7,000 big game animals, destroyed 250 homes,
27 bridges, 300 km of roads, forests, caused >$2 billion in damage (damage, cleanup, timber losses=4 billion board feet), and
injected 0.1 million tons of SO2 aerosols and 1 km3 of ash into the upper atmosphere. The debris avalanche, lateral blast, and
pyroclastic flows caused most casualties. Lahars moving at 10-25 mph filled river valleys and caused floods, eventually
disrupting the Columbia River delta and Port of Portland to ship traffic.
10. Subsequent activity: Following 18May1980, MSH remained active and several ash and pyroclastic flow eruptions
occurred on 12Jun and 7 Aug, with the last major display on 16Oct1980. The summit dacite dome (1 km diam, 267 m tall,
0.13 km3) slowly grew during 17 episodes from Oct1980-Oct1986. Since then, only several periods of increased seismicity
and minor dome explosions have occurred, with renewed seismicity in August 2004, followed by steam and ash eruptions
beginning on 1Oct2004, resulting in the evacuation of the park within 10 miles of the crater. MSH awakened in late 2004
with several months of increased shallow seismicity (Mw <3) until small pyroclastic and steam eruptions began 1Oct2004
followed by lava dome inflation and eruption 1-2 weeks later at rates of 8 m3/sec. Dome growth and occasional explosive
eruptions continue to the present.
11. Notable Items: Pres. Jimmy Carter toured the aftermath and remarked “the moon looks like a golf course
compared to what we have here.” In contrast, in perhaps the largest eruption in the last 10,000 years, Mt. Mazama,
OR erupted ~6850 yr ago, releasing ~50 km3 of magma to the surface and collapsed to produce ~10 km wide Crater
Lake, one of the world's best known calderas. The 1980 Thermal Eruption Energy=1018 Joules (~M8.8 EQ, ~2 days
US energy consumption, ~400 million tons TNT); Peak Plume (9 hr) eruption rate = 20 million kg/sec; Mean lava
dome eruption rate (1980-1986) = 700 kg/sec
Selected References
Foxworthy, B., and M. Hill, 1982. The volcanic eruptions of 1980 at Mt. St. Helens: the first 100 days. USGS PP 1249.
Keller, S.A. (ed.), 1986. Mt. St. Helens: 5 years later. Eastern Washington Univ. Press, 441 pp.
Lipman, P.W., and Mullineaux, D.R., eds., 1981. The 1980 eruptions of Mount St. Helens, WA, USGS PP 1250.
Popocatepetl, Mexico [Mexico’s most dangerous volcano; Aztec for “Smoking Mountain”]
1. Location: 19.0N, 98.6W, elev.: 5452 m; ~71 km SE of Mexico City, and 40 km SW of the city of
Puebla, near the E end of the 1000 km long Mexican Volcanic belt situated above anomalously thick
continental crust (40-50 km).
2. Size & Type of volcanic feature: ~25 km diameter conical stratovolcano (500 km2 area) consisting of
interlayered andesite to dacite lava flows and pyroclastic deposits; the Popo crater is oval shaped,
measures ~820x650 m, 300 m deep, with near vertical walls, and has a small spatter cone that rises 35 m
above the crater floor.
3. Relationship to Plate Boundaries/ Names of relevant plates: NE-directed subduction of Cocos
plate oceanic lithosphere beneath continental lithosphere of the N American Plate.
4. Date and duration of eruption: 12-18Dec2000; 22Jan2001
5. Type of Eruption/Deposits/Magma Composition: Andesite Plinian and Peleean eruptions, and lava
domes.
6. Volume of erupted products: ~0.02 km3
7. Area impacted by the eruption: <10 km2
8. Summary of volcanic events preceding the particular event: Popo was born ~23,000 years ago.
The modern cone was constructed in two stages following a cataclysmic eruption with edifice collapse
and debris avalanche. El Fraile volcano was formed > 10,000 years ago and was partly destroyed by
three episodes of explosive activity, the last from about 5,000 to 3,800 years ago. The current summit
was formed by repeated lava eruptions until about 1,200 years ago. During the last 10,000 years, major
Plinian eruptions occurred every 1000-3000 years. Most historical eruptions were mild-to-moderate
steam and ash emissions, with larger explosive eruptions between 3195 and 2830 BC, 800 and 215 BC,
675 and 1095 AD, 1519 and 1663. These major eruptions produced a cycle of minor ash falls and flows
followed by debris-avalanches and associated Plinian ash clouds and flows, and finally extensive
mudflows and lahars. Popo has erupted >36 times in historic times. Activity in 1920-22 produced
intermittent explosive eruptions and a small lava plug in the summit crater. Minor ash clouds were also
reported in 1923-24, 1933, 1942-43, and 1947. Popo awoke from a six decade slumber on 21Dec1994,
and 6 years later on 12Dec2000 for several weeks, forming pyroclastic flows reaching up to 5 km from
the vent, igniting forest fires, and spewing clouds of ash ~8 km into the air over the Valley of Mexico.
75,000 Residents were evacuated within a week in 1994 & 2000 because of concern more massive
eruptions or mudslides would occur. Following several minor eruptions on 22-24Dec2000, the danger
zone was reduced to within 15 km of the summit, and citizens returned home.
9. Casualties/Type of hazards: No deaths/Pyroclastic flows, debris avalanches/landslides, lahars.
10. Subsequent activity: Subsequent pyroclastic eruptions occurred 22Jan2001 and 29Jan2001 resulting
in 8 km high ash clouds and pyroclastic flows which reached 8-15 km from the vent.
11. Notable Items: North America's second highest volcano (4.2 km above the surrounding
plains). The cone is covered with yellow sulphur spots and volcanic gases escape from many
fumaroles in its flank as well as from the main crater walls. These inner walls are made of
horizontal layers of lava flows alternating with pyroclastic deposits. Goff et al. [1998] found SO2
emission rates of up to 50,000 tons/day in 1997, the largest single source in the world at the time.
Over 1 million people live within 35 km of the summit, and 10 million within 70 km. Despite
being 1300 km from DFW, Popo is our closest major volcanic hazard. Southerly winds would
carry ash to E TX in the event of a large plinian eruption.
Selected References
Goff, F., et al., Geothermal Surveillance of Magmatic Volatiles at Popocatepetl Volcano, GSAB., 110, 695– 710, 1998.
Macias & Siebe, 2005. Popocate´petl’s crater filled to the brim:significance for hazard evaluation. JVGR, 141, 327-30.
Robin & Boudal, 1987. JVGR, 31, 115-130.
Siebe, et al., 1996. Repeated volcanic disasters in Prehispanic time at Popocateptl, central Mexico: Past key to the
future? Geology, v. 24, p. 399-402.
Ol Doinyo Lengai, Tanzania [Earth’s only active carbonatite volcano; Lowest temperature and
Lowest viscosity lavas; translation: 'Mountain of God' ‘one of a kind’]
1. Location: 2.75S, 35.9E, elevation: 2886 m, rises ~2 km above the Rift Valley floor south of Lake
Natron.
2. Size & Type of volcanic feature: 10 km diam. conical stratovolcano with a 500 m diam. summit
crater and dozens of small conical vents.
3. Relationship to Plate Boundaries/ Names of relevant plates: Near the W fault scarp of the East
African Gregory Rift Valley caused by continental rifting within the African Plate.
4. Date and duration of eruption: nearly continuous for the last several decades.
5. Type of Eruption/Deposits/Magma Composition: The volcano has erupted sulfur lavas at 150C,
and sodium-carbonatite (liquid baking soda) at 540-600C with very low viscosity (0.3-120 Pa sec)
forming 5-25 cm thick lavas that appear like oil or brown foam, and travel <10 m before freezing. The
lavas turn white when exposed to moisture. These are the lowest temperature and lowest viscosity lavas
known on Earth. Aa lavas formed when effusion rates are > 0.2 m3/sec whereas pahoehoe textures
formed at lower rates <0.04 m3/sec.
6. Volume of erupted products: Individual eruptions <100 m3
7. Area impacted by eruptions: <1 km2
8. Summary of volcanic events preceding the particular event: The bulk of the volcano was
constructed from 370,000-15,000 years ago by pyroclastic rocks and lavas. The earliest recorded
eruption occurred in 1880. Recent eruptions ranging from quiet lava effusion to violent ash explosions
occurred in 1940, 1950s, 1960s, 1988, 1993-2001. A 4 km high ash cloud was formed in 1966.
9. Casualties/Type of hazards: No known deaths. A German geologist’s foot was severely burned in
Oct2000 when he stepped into a 25 cm thick flow that he thought had solidified. Hazards include
explosive eruptions, suddenly appearing lava fountains, cone collapses, lava flash floods.
10. Subsequent activity: activity continues to the present.
11. Notable Items: Ol Doinyo Lengai is unique because it is the world's only active volcano that erupts
sodium-carbonatite lava. It is also the largest and only active volcano in this part of the East African Rift
Valley.
Selected References
Bell, K., and Keller, J., eds., 1995, Carbonatite volcanism of Oldoinyo Lengai – petrogenesis of natrocarbonatite:
IAVCEI Proceedings in Volcanology, Springer-Verlag, Berlin.
Dawson et al., 1968, Activity of the carbonatite volcano Oldoinyo Lengai, 1966: Geol. Rundsch, 57, 865-79.
Dawson et al., 1990. Physio-chemical properties of alkali carbonatite lavas: Data from the 1988 eruption of
Oldoinyo Lengai, Tanzania. Geology, 18, 260-3.
http://it.stlawu.edu/~cnya/; http://mtsu.edu/~fbelton/pearlygates.html;
http://www.skyboom.com/magmaimages/lengai.html
http://www.volcano.si.edu/gvp/volcano/region02/africa_e/lengai/var.htm