Volcanoes, Eruption Types and Volcanic Hazards

LAST MODIFIED: 11 October 2022; updated entry on Arenal

MODIFIED:

POSTED: 10 June 2020

Arenal, Costa Rica

The 1657m-high (5435ft) Arenal is a moderately silicic (andesitic) stratovolcano and is one of Costa Rica's most active volcanoes. The first known eruption took place only 7000 years ago. Since about 1968, it has been active in a Strombolian eruption type, constantly rumbling, peacefully ejecting small tephra and sometimes lava flows. During its lifetime, Arenal has not always been this peaceful. Activity started 1968 with a big category VEI 3 explosion (photo from the Smithsonian Global Volcanism Program ). Until 2010, eco-tourists could watch Arenal erupting from a safe distance. The volcano has been dormant since then. The 1968-2010 activity is considered the worldwide tenth longest volcanic eruption since 1750. The class notes here introduce some of the most well known volcanoes and the hazard risks that are associated with them.

• Case Studies

Topics 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 < 500 m across, but may be as large as 1000 m), 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; may or may not be circular; typical diameters are greater than 1 km, sometimes several tens of km or even greater


• 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; Tarawera (New Zealand) 1886
6	Santorini (Greece) B.C. 1650; Vesuvius (Italy) 79; Krakatau (Indonesia) 1883; Pinatubo (Philippines) 1991; Novarupta (Alaska) 1912
7	Crater Lake/Mt. Mazama (Oregon) B.C. 5000; Tambora (Indonesia) 1815; Taupo (New Zealand) 230 AD
8	Yellowstone (Wyoming) B.C. 650,000; Toba (Indonesia) 72,000 B.C.


NB: The 230 AD Hatepe eruption on Taupo volcano (New Zealand) is considered the most violent eruption on any volcano worldwide in the last 5000 years.

The 1912 Novarupta eruption in Alaska was the largest of the 20th century. It released 30 times the volume of magma of the 1980 Mt St Helens eruption.

• 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

1. particularly benign
2. particularly violent

1. liquid, solid, gaseous
2. lava flows, pyroclastic debris, volcanic gases
3. pahoehoe lava, a'a lava, cinder
4. lahars, ash flows, lava flows

1. mudflows
2. lahars
3. pyroclastic flows
4. lava flows

1. H₂O
2. CO₂
3. O₂
4. SO₂

1. true
2. false

Appendices for the Interested

• APPENDIX 1: Examples that even "Remote" Volcanic Activity is Hazardous

• Airplanes Flying through Ash Clouds
  Mt. Redoubt in Alaska is relatively remote and is located near the southern end of Cook Inlet, about 200km southwest of the city of Anchorage. It has erupted only few times in recorded history. On Dec 15, 1989, a Dutch KLM Boeing 747 jumbo jet was on its way from Amsterdam to Tokyo. It had about 250 people on board and was at cruising altitude (about 10km) when it encountered an ash cloud of the erupting volcano. The ash clogged all four engines which lost power and the plane started a dramatic 12-min descent to 4km altitude, only 1.5 km above the mountains. The engines started up again only after the 18th attempt before the pilot could land the plane safely in Anchorage. The damage to the engines was $18 Mio. Unfortunately, such incidents are quite common and the pilots have, in general, no warning time. Examples include an equally terrifying encounter of a 747 with an ash could during the 1982 eruption of Mt. Galunggung/Indonesia (7.5km descent) and 17 flights during the 1991 Mt. Pinatubo eruption. Officials now work with scientists on a early-warning system in the Alaska region for polar flights to Eastern Asia. Recent studies suggest that weather satellites from space that could track these clouds can discriminate weather clouds from ash clouds (NB: clouds from large forest fires are also a significant hazard).



• APPENDIX 2: Some eruptions during the SIO15 2008 course

• Garbuna, New Britain Island, Papua New Guinea, 564m high: The Garbuna volcano group consists of three volcanic peaks, Krummel, Garbuna, and Welcker. They are located along a 7-km N-S line above a shield-like foundation at the southern end of the Willaumez Peninsula. The central and lower peaks of the centrally located 564-m-high Garbuna volcano contain a large vegetation-free area that is probably the most extensive thermal field in Papua New Guinea. A prominent lava dome and blocky lava flow in the center of thermal area have resisted destruction by thermal activity, and may be of Holocene age (less than 11,500 years old). Krummel volcano at the S end of the group contains a summit crater, breached to the NW. The highest peak of the Garbuna group is 1,005-m-high Welcker volcano, which has fed blocky lava flows that extend to the eastern coast of the peninsula.
  White plumes from Garbuna were emitted during 6-10 October. Deep booming noises were occasionally heard. On 7 October an explosion produced forceful emissions of dense white vapor.
  link to Global Volcanism Program


• Kliuchevskoi, Central Kamtchatka, Russia, 4835m high, stratovolcano: Kliuchevskoi is Kamchatka's highest and most active volcano. Since its origin about 7,000 years ago, the beautifully symmetrical, 4,835-m-high basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. More than 100 flank eruptions, mostly on the NE and SE flanks of the conical volcano between 500 m and 3,600 m elevation, have occurred during the past 3,000 years. The morphology of its 700-m-wide summit crater has been frequently modified by historical eruptions, which have been recorded since the late-17th century. Historical eruptions have originated primarily from the summit crater, but have also included major explosive and effusive events from flank craters.
  Fumarolic activity from Kliuchevskoi was seen during 2-6 and 8-9 October. During approximately 4-9 October analysis of satellite imagery revealed a thermal anomaly in the crater; seismic activity was above background levels. The volcanic alert level was set to orange.
  link to Global Volcanism Program


• ongoing eruptions: Bagana, Bougainville (3km high ash plumes); Chaiten, Southern Chile (3km high gas-and-ash plume; volcanic alert on red); Dukono, Halmahera (ash plume; one of Indonesia's most active volcanoes); Karymsky, Eastern Kamtchatka (one of Kamtchatka's most active volcanoes, seismic activity, 4km high ash plume, thermal anomaly in the crater, volcanic alert on orange); Kilauea, Hawaii (lava flow, ejecta, SO₂ emission rate 10 times larger than 2003-2007 average); Masaya, Nicaragua (4.6km high ash plume); Pacaya, Guatemala (multiple lava flows); Rabaul, New Britain (2.7km high steam and ash plumes; roaring noises); Shiveluch, Central Kamtchatka (seismic activity, 4km high ash plume); Soufriere Hills, Montserrat (rock falls, mudflows; hazard level remains at 3); Suwanose-Jima, Ryukyu Islands (explosions); Ubinas, Peru (6km-high ash plumes)

• APPENDIX 3: Some eruptions during the ERTH15 2006 course

• Fourpeaked Volcano, Alaska, 2105m high, stratovolcano: a poorly known volcano in the Katmai National Park. According to the Smithsonian Global Volcanism Program (GVP) website, the last eruption is not know precisely but it is thought that it was thousands of years ago. On the evening of September 17, the Alaska Volcano Observatory (AVO) received several reports of two discrete plumes rising from the Cape Douglas are, about 320km SW of Anchorage. Chemical analysis of the cloud measured high levels of SO₂ and confirmed that the cloud were of volcanic and not meteorological origin. Only little ash fall had been reported then. By early October, the volcano was still emitting large quantities of SO₂, hydrogen sulfide and CO₂. A distinct sulfur smell was evident up to 50km from the summit. Limited seismic data do not appear to indicate significant volcanic activity. The level of concern color code is currently yellow.
  link to Global Volcanism Program
  link to AVO



Photo: Smithsonian Global Volcanism Program

Mt. Mayon, Philippines, 2462m high, stratovolcano, last know eruption 2005: Mayon is the Phillipines' most active volcano. It has steep slopes (upper slope 35-40 deg). According to the Smithsonian GVP website, the volcanoes historical eruptions go back to 1616 and range from strombolian to basaltic plinian, the latter making it a dangerous volcano which is known to produce pyroclastic flows and lahars. Mayon's most violent eruption, in 1814, killed more than 1200 people and devastated several towns. A new cycle of eruptions started in July 2006 with the production of lava flows and pyroclastic flows that prompted the evacuation of about 100 families at the end of July. Seismic activity and the emission of SO2 increased and alert level went from 3 to 4 in early August. Thousands of people were evacuated but some returned by late September, after a 10-daydrop in the level of activity (see Earthwatch of September 21). The alert level was lowered from 4 to 3 on September 11. By early October, volcanic earthquakes have continued as well as intermittent discharge of incandescent lava fragments. link to Global Volcanism Program Philippine Institute of Volcanology and Seismology



• Rabaul, New Britain Island, Papua New Guinea, 688m high: A large, sustained Vulcanian eruption began on October 7 as the eruption column rose to 5km and had produced thunder and lightning. Windows rattled and doors slammed from semi-continuous air blasts. Windows in the observatory, 12km away, blew out from shockwaves. Moderately heavy ash fell in southern Rabaul town. Residents affected by heavier ash fall and air blasts self-evacuated. The eruption grew to sub-Plinian status throughout the day when thick ash plumes reached 18km altitude. Ash fall affected the entire Gazelle Peninsula.
  link to Global Volcanism Program


• something curious: A yet unknown source of pumice rafts in the Fiji Islands: according to the Smithsonian GVP website, reports have been received on September 16 when observers aboard the M/V National Geographic Endeavour noted almost continuous rows of pumice that day as they traveled about 90km east-southeast to Vatoa Island, where the pumice was present on the beaches. Large rafts of pumice were also passing through the northern Lau Group around Naitauba Island on September 19. The source of the pumice is unknown at this time.


• ongoing eruptions: Bulusan Volcano, Philippines; Karymsky Volcano on Kamchatka Peninsula, Russia; Kilauea Volcano on Hawaii; Ruapehu Volcano in New Zealand; Sakurajima, Japan; Soufriere Hills, Montserrat, West Indies; Mt. St. Helens; Suwaanosejima, Ryukyu Islands, Japan; Tall Volcano, Philippines; Tungurahua, Ecuador; Ubinas, Peru

• APPENDIX 4: Some eruptions during the 2005 ERTH15 course

• Ilamatepec, El Salvador (or Volcan Santa Ana), 2381m high, stratovolcano: October 1, 2005, killing at least 2 people and forcing the evacuation of nearby San Blas. Ejection of car-sized lava bombs, ash, lava flows. Hurricane Stan made landfall in Mexico 3 days later, causing local mudslides (at least 62 fatalities). The last eruption was in 1920. Historical activity has consisted of small-to-moderate explosive eruptions. Ilamatepec is located in a volccanic field, next to the large Coatepeque Caldera that formed after a series of explosive eruptions 50,000-70,000 years ago. A debris avalanche then swept into the Pacific Ocean to form Acajutla Peninsula.
  link to Smithsonian Global Volcanism Program
  link to NASA
  


• Erta Ale, Ethiopia, 613m high, shield volcano: an effusive eruption of lava on September 26, 2005 covered a vast area, destroying grazing land used by 50,000 nomads. The eruption was accompanied a swarm of strong earthquakes. Prior to the 2005 eruption, there was one in 2004 but the volcano has an active lava lake since at least 1967. A recent study suggests that the lake has been active for the last 90 years which would make this one of the longest known historic erutpions. Erta Ale is a huge volcano with a large caldera (100 by 15.5 km) and is located near the Afar triple junction. A recent Earthwatch feature of the San Diego Union Tribune mentioned that the last eruption was 60 years ago, which doesn't seem quite right.
  link to Global Volcanism Program
  


• Kilauea, Hawaii, 1222m high, shield volcano: ongoing eruption (more or less vigorous) that started in 1983 with impressive lava fountains at the Pu'u O'o vent. The lava from this eruption is now covering more than 100sq km, destroying nearly 200 homes and adding new land.
  link to Smithsonian Global Volcanism Program
  


• Etna, Sicily, Italy, 3350m high, shield volcano, though GVP says stratovolcano: one of world' longest documented records of historical volcanism. Mainly effusive but also explosive eruptions. The last eruption (VEI index: 0) lasted from Sep 2004 through Jan 2005. The largest recent eruption, that destroyed the Rifugio cable car station, occurred in 2002-2003 when the eruption column was seen as far away as Libya.
  link to Smithsonian Global Volcanism Program