Case Studies
Lectures 8-10 are accompanied by several case studies. The case studies only give examples and the list is by no means complete. These case studies DO NOT imply that a particular feature (e.g. mud flows/lahars) is observed only at a particular volcano or a particular eruption. In fact, most eruptions exhibit a combination of these.
The Increasing Fatality Rate from Volcanic Eruptions
It is important to understand how volcanoes erupt because volcanoes have become an increasing
cause of deaths over time. It is important to recognize the hazards associated with certain types
of volcanoes and try to mitigate hazards and prevent losses from eruptions.
Number of Fatal Eruptions |
Century | # Fatal Eruptions |
14th | 14 |
15th | 6 |
16th | 20 |
17th | 32 |
18th | 45 |
19th | 105 |
20th | 215 |
(* source: Simkin et al., 2001, Science, 291, pp. 255)
This does not mean that there are more volcanic eruptions now than there were a
few hundred years ago. But there are two main reasons why the fatality rate is increasing:
- the world's population has grown exponentially over the centuries
- people tend to move to volcanoes because volcanic soil is very fertile
What is a Volcano?
A typical volcano is a structure at the surface that is formed during volcanism, the process that transforms magma (molten rock at depth) into lava (molten rock at the surface).
A typical volcano has a crater (typically < 500m across), a vent and a magma
chamber.
Gases originally dissolved in the magma drive an eruption as the magma ascends from the magma chamber.
For a typical eruption, rock has to melt (through decompression or addition of volatiles) and escape through an opening (crater or crack/fissure). The eruption is driven by the release of dissolved gases during further decompression on the way up to the surface. A volcanic eruption is much like opening a coke can.
Steps involved, as shown in the lecture:
- magma rises (magma is hot and buoyant) against pressure gradient (i.e. pressure increases with depth so rising magma experiences a decrease in pressure)
- gases previously dissolved in magma are released and form bubbles
- gas bubbles now take up space and increase the ambient pressure on the rest of the magma
- as magma rises further, more gases are released, more bubbles form and ambient pressure is increased
- this happens until the pressure exerted from the bubbles overcomes the pressure from the overlying crust and
volcano
- this pressure imbalance causes a volcanic eruption
Examples of volcanoes: Kilauea (Hawaii), Grimsvötn (Iceland), Mt. Etna (Sicily), Mt. Vesuvius (Italy), Mt. St. Helens (Cascades, Washington), Mt. Pinatubo (Philippines), Nevado del Ruiz (Colombia), Mt. Fujiyama (Japan), Mt. Klyuchevskoi (Kamchatka), Merapi (Indonesia), Krakatoa (Indonesia), Kilimanjaro (extinct; East Africa).
Active, Dormant and Extinct Volcanoes
There is no exact definition of an active vs. dormant volcano but the Smithsonian Global Volcano Program defines a volcano that has erupted in the last 10,000 years as an active volcano. Others (such as the U.S. Geologic Survey) classify as active a volcano that currently shows any sign of activity, including seismic (tremors and earthquakes) and gas emissions. A dormant volcano has erupted in historic times (last 10,000 years) but currently shows no sign of activity. E.g., the USGS classifies Mt. Shasta, CA, that had 11 eruptions in the last 3400 years, as "currently not active" (dormant) because is currently shows no signs of activity. Mt. St. Helens, WA can be dormant for 1000s of years between eruptions. It was dormant since 1857 before it erupted on 18 May, 1980. An extinct volcano is one that is currently not active nor dormant and is unlikely to erupt again (e.g. Mt. Kilimajaro in Africa, old volcanoes on the Hawaiian island of Oahu).
How many are there and how often do they erupt?
There are about 1500 volcanoes that are known to have erupted within the last 10,000 years. Volcanic eruptions can be quite regular (e.g. Stromboli/Italy every 15 min or so, for the last 1000 years; Arenal/Costa Rica is currently continuous), but eruptions at volcanoes are usually irregular.
Some volcanoes may be dormant for hundreds of years lulling residents in a false sense of safety. The number of eruptions and the number of active distinct volcanoes at any given time is sometimes difficult to pin down because some blasts may belong to the same eruption or come out of the same volcanic field but may "look" distinct (e.g. two different cinder cones). Here is a summary from the Smithsonian volcano web site.
Time span | # eruptions |
erupting now | ~ 20 |
each year | ~50-70 |
each decade | ~160 |
historical eruptions | ~550 |
known in last 10,000 years | ~1300 |
known and possible in last 10,000 years | ~1500 |
The 16 Decade Volcanoes
The International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) compared Earth's volcanoes with respect to their
- history of large, destructive eruptions
- proximity to large populated areas
and designated 16 volcanoes as having the greatest potential for being destructive again. The designation of a "decade volcanoes" aims at
- better understanding of the volcano
- raise public awareness about volcanic hazards
A volcano may be designated a Decade Volcano if it exhibits more than one volcanic hazard), shows recent geological activity, is located in a populated area; is politically and physically accessible for study, and there is local support for the work. As of 2017,
the decade volcanoes are: Rainier, Colima, Mauna Loa, Santa Maria, Galeras, Teide, Etna, Vesuvius, Santorini, Nyiragongo, Avachinsky-Koryaksky, Sakurajima, Unzen, Taal, Merapi, Ulawun.
Follow link to Wikipedia.
Some Special Features
- Cinder Cones (or Scoria cones): form when the volume of ejected lava is relatively small (and volatiles relatively high)
- Caldera: forms after large amounts of magma escaped from the magma chamber; the roof of the magma chamber and the volcano above collapses due to the loss of support below
- Phreatic: (or phreatomagmatic, also hydrovolcanic) from Greek "phrear" for well. Water interacts with lava to form vigorous eruptions.
- underwater volcanic eruptions (e.g. Surtsey/Iceland)
- volcano on dry land intersects aquifer
- lava or pyroclastic flow moves over water saturated sediments
- water runs over hot rock to form steam explosions
- subglacial eruptions
Eruption Types
- Icelandic: large amounts of very low-viscosity lava; non-explosive; forming plateaus
- Hawaiian: low-viscosity lava; non-explosive; forming shield volcanoes
- Strombolian: relatively small amounts of moderately high-viscosity lava; usually peaceful; forming scoria or cinder cones
- Vulcanian: high-viscosity lava; moderately violent eruptions; moderately high-volatile content; moderately large eruption cloud
- Plinian: very high-viscosity lava; violent eruptions; very high-volatile content shoots tephra high into atmosphere; pyroclastic flows; tephra alternating with lava flows form strato volcanoes
The Volcanic Explosivity Index (VEI)
A scale from 0-8 that describes the violence of a volcanic eruption. Effusive eruptions typically have VEI values of 0-1. Very
explosive eruptions with lots of tephra/pyroclastic material being injected into the stratosphere typically have VEI values of 4 and greater.
Cataclysmic (very catastrophic) eruptions are 6-8. Icelandic and Hawaiian eruptions typically fall into the 0-1 category (though there are exceptions!), while Plinian eruptions are VEI 5 or greater.
Examples for VEIs
VEI index | Volcano |
0 | Lake Nyos (Cameroon) 1986; Kilauea (Hawaii) 1974 |
1 | Mt. Unzen (Japan), 1991; Kilauea (Hawaii) 1983-2003; Nyiragongo (Dem.Rep.Congo) 2002 |
2 | Stromboli (Italy) 2003 |
3 | Surtsey (Iceland), 1963; Heimaey (Iceland), 1973; Nevado del Ruiz (Colombia) 1985; Soufriere Hills (Montserrat) 1995 |
4 | Laki (Iceland) 1783; Pelee (Martinique) 1902; Soufriere (St. Vincent) 1902; Paricutin (Mexico) 1943; Eyjafjallajökull (Iceland) 2010 |
5 | Mount St. Helens (Washington) 1980 |
6 | Santorini (Greece) B.C. 1650; Vesuvius (Italy) 79; Krakatau (Indonesia) 1883; Pinatubo (Philippines) 1991 |
7 | Crater Lake/Mt. Mazama (Oregon) B.C. 5000; Tambora (Indonesia) 1815 |
8 | Yellowstone (Wyoming) B.C. 650,000 |
Volcanic Hazards in General
Depending on the eruption type, volcanoes pose threats to property and lives through a variety of volcanic characteristics.
- lava flows
- ash/tephra fall (ground and air)
- pyroclastic flows (mixture of tephra and volcanic gases)
- lahars (mud flows, mixture of tephra and water)
- emission of volcanic gases (e.g. Lake Nyos in Cameroon)
- erosion
- tsunami (typically when island volcanoes collapse and form calderas; e.g. Krakatoa, Santorini)
- submarine landslides, only recently discovered as hazard (e.g. Hawaii)
- earthquakes
- climate change (e.g. Mt. Pinatubo, 1991; Tambora, 1815 "the year without a summer)"
Volcanic Material
the three major groups of volcanic products are: Lava flows, pyroclastic debris and volcanic gases.
- lava flows
- basaltic: pahoehoe (low-viscosity; flows easily; ropy structure);
colder a'a' (low-to-medium viscosity; somewhat stagnant; blocky structure)
- andesitic: high viscosity; usually short; sometimes gets stuck in the vent and forms a lava dome clogging the vent; gas pressure can build up underground and lead to an explosion
- pillow lava: when lava gets in contact with water; the outer
surface solidifies instanteneously; cracks force the lava to ooze out into another blob
- pyroclastic debris
- ash: powder size; < 2mm; sharp glassy particles
- lapilli or cinder: marble-to plum-size
- bombs: basketball-to house-size
- volcanic gases
most magma contains dissolved gases, incl. H2O, CO2, SO2, H2S (up to 9%). Generally lava on continents (rhyolitic; see lecture 9) contains more gas than lava involving oceanic crust (mafic; see lecture 9). Volcanic gases can still escape long after an eruption and may be the only sign of volcanic activity
(e.g. dormant volcanoes). Volcanic gases escape in fumaroles.
Hazards from Volcanic Material
- lava flows: cause significant structural damage but usually too slow to kill
- ash fall:
- covers ground, sometime many feet to yards deep, smothering living
organisms
- airborne ash hazardous to air traffic: i.e. to engines of large planes
flying through an ash cloud get clogged up; e.g. 1989 Mt. Redoubt eruption caused
engine failure of a Jumbo Jet; $80 Mio damage; similar problems arise on other volcanoes
(see case study 1)
- large eruptions can lead to temporary global climate change
( e.g. Mt. Pinatubo, 1991; Tambora, 1815, "the year without a summer")
- lahars: ash mixing with water form
dangerous mudflows at speeds of 50km/h (a car in fast city
traffic!)
Generation of Lahars:
- rain caused by eruption (e.g. Vesuvius, A.D. 79)
- rivers (e.g. Mt. St. Helens, 1980)
- drainage of crater lake (e.g. Mt. Kelut)
- melting ice cover (ice cap or glacier) (e.g. Nevado del Ruiz, 1985)
- post-eruption storms (e.g. typhoon after Pinatubo, 1991)
- pyroclastic flows: ash mixing with air and hot gases compose extremely
destructive fast-moving (300km/h; 3 times as much as freeway
traffic) pyroclastic flows (also called "nuee ardente", French for
glowing cloud). (e.g. Mt. Pelee on Martinique in Apr. 1902 that killed
29,000 people leaving 2 alive; Pompeii 79 A.D.).
Generation of Pyroclastic Flows:
- dome collapse (e.g. Mt. Unzen, 1991)
- overspilling of crater rim (e.g. Mt. Pelee, 1902-1903)
- directed blast (Mt. St. Helens, 1980; Mt. Pinatubo, 1991)
- collapse of eruption column (Mt. Unzen, 1991; Vesuvius 79; Mt. Mayon, 1968
Factors that kill people:
- physical impact
- inhaling superhot and toxic gas
- burns
- gas exhalations: emission of massive amounts of toxic volcanic gases leads to death; e.g. CO2Lake Nyos, Cameroon killed most people and livestock in a valley but plants survived (see case studies). Another example of volcanic gases as hazard is Popocatepetl/Mexico, a volcano that is about 100km from the world's largest city, Mexico City. Popocatepetl can emit massive amounts of CO2 (greenhouse gas, also results from traffic) and SO2 (same pollutant that results from burning coal) that further degrades Mexico City's air quality.
- erosion and mudflows caused by rainstorms (e.g. Lake Atitlan area, Guatemala, after Hurricane Stan, October 8, 2005)
Factors that increase risk:
- steep slopes (close to angle of repose)
- unconsolidated material
- little vegetation after recent eruptions
- submarine land slides, only recently discovered as hazard; if slides don't creep but happen suddenly, tsunami generation of global proportions (e.g. Canaries, Hawaii)
The Top 3 Killers
- pyroclastic flows
- Indirect (Famine)
- Tsunami
Lahars are #4 when it comes to counting fatalities. Perhaps surprisingly, lava flows are not the principle killers of all volcanic processes. In fact, they are responsible for extensive property damage but less than 1% of fatalities. The reason is that lava flows usually travel slowly enough for people to get to a safe place. Pyroclastic flows and lahars are so deadly because their fast velocities. The indirect impact comes to play after extremely large eruptions that inject enough volcanic material to temporarily change global climate. These changes can be so severe that they lead to crop failure elsewhere, and to widespread famine.
Deaths from Historical Records |
Volcanic Agent | Fatalities (%) |
Pyroclastic Flow | 29 |
Indirect (Famine) | 23 |
Tsunami | 21 |
Lahar | 15 |
Gas | 1 |
Lava Flow | <1 |
Pyroclastic Fall (bombs) | 2 |
Debris Avalanche | 2 |
Flood | 1 |
Earthquake | <1 |
Lightning | <1 |
Unknown | 7 |
|
# of fatalities are percent of total (275,000) |
(* source: Simkin et al., 2001, Science, 291, pp. 255)
Recommended Reading
- "Pompeii" by Robert Harris, Random House, 2003 ISBN: 0-67942889-5 (hardcover); 2005 ISBN: 0-81297461-1 (paperback); describes the life of people when Vesuvius erupted in 79 AD, using Pliny the Younger's letters
- "Krakatoa, Harper Collins, 2003 ISBN: 0-06621285-5 (hardcover); 2005 ISBN:0-06083859-0 (paperback); describes the eruption of Krakatau in 1883
- "Volcanoes, Crucibles of Change", by Richard V. Fisher, Grant Heiken, Jaffrey B. Hulen; Princeton Univ. Press, 1998, ISBN: 0-691-01213-X (paperback); describes the some volcanic eruptions and related hazards, including the 19889 eruption Mt. Redoubt in Alaska