* Volcanic Explosivity Index (VEI) at USGS (U.S. Geological Survey) Volcanoes page

VEI	Type	Plume Height	Volume	Classification	Frequency	Example
8	Megacolossal	> 25 km	>1000 km3	Ultra-Plinian	>10,000 yr	Yellowstone 2 Ma
7	Supercolossal	> 25 km	>100 km3	Ultra-Plinian	>1000 yr	Tambora 1815
6	Colassal	> 25 km	>10 km3	Plin/Ultra-Plinian	>100 yr	Krakatoa 1883
5	Paroxysmal	> 25 km	>1 km3	Plinian	>100 yr	St Helens 1981
4	Cataclysmic	10-25 km	>0.1 km3	Vulc/Plinian	>10 yr	Galunggung 1982
3	Severe	3-15 km	>10.6 m3	Vulcanian	yearly	Ruiz 1985
2	Explosive	1-5 km	>105 m3	Strom/Vulcanian	weekly	Galeras 1992
1	Gentle	100-1000 m	>104 m3	Haw/Strombolian	daily	Sromboli
0	non-explosive	<100m	>1,000 m3	Hawaiian	daily	Kilauea

Source: Eruption Scale at U. North Dakota
See Also: VEI at the Smithsonian

U.S. Volcanoes since 1980

VOLCANO	ERUPTION YEAR	UNREST EPISODE
Hawaiian Volcano Observatory (HVO)
Kilauea, Hawaii	1983-present
Mauna Loa, Hawaii	1984	2002-2004, inflation and deep seismicity
Cascades Volcano Observatory
Mt. St. Helens, Washington	1980-1986, 2004-2005	1989-2003, occasional earthquake bursts, minor phreatic explosions, small mudflows
Mt. Hood, Oregon		Occasional earthquake swarms
Three Sisters, Oregon		Uplift began 1997; earthquake swarm March 2004
Medicine Lake, CA		1988-1989 earthquake swarm
Long Valley Observatory (LVO>
Long Valley, California		CA - In May of 1980, a strong earthquake swarm that included four magnitude 6 earthquakes struck the southern margin of Long Valley Caldera associated with a 25-cm, dome-shaped uplift of the caldera floor. CO2 emission from ground since 1989. Caldera unrest continues to this day, although earthquake activity within and adjacent to the caldera has remained low since mid-1999.
Yellowstone Volcano Observatory (YVO)
Yellowstone National Park		Recurrent earthquake swarms and ground deformation (uplift & subsidence), changes in hydrothermal features. In April 2004 there was an increase in earthquake activity, called a swarm, at Yellowstone National Park that drew interest from scientists and the public. There have been many swarms recorded over the past 40 years at Yellowstone.
Alaska Volcano Observatory
Redoubt, Alaska	1989-1990
Spurr, Alaska	1992	2004-2005 earthquake swarms and melt pit at summit. Overall rate of seismicity is diminishing.
Augustine, Alaska	1986
Iliamna, Alaska		1996 earthquake swarm and elevated gas emission
Veniaminof, Alaska	1983-1984, 1993-1995, 2004-2005
Pavlof, Alaska	1980, 1981, 1983, 1986-1988, 1996-1997
Shishaldin, Alaska	1986-1987, 1995-1996, 1999	2004-2005 earthquakes and tremor, thermal anomalies
Westdahl, Alaska	1991-1992
Dutton, Alaska		1984 earthquake swarm; 1988 earthquake swarm and intrusion
Shrub Mud Volcano, Alaska		1996-1999 carbon dioxide/mud eruption
Becharof Lake, Alaska		1998 earthquake swarm
Chiginigak, Alaska		1997-1998 fumarolic activity
Akutan, Alaska	1980, 1987, 1988, 1992	Intense earthquake swarm and intrusion with ground cracks in 1996.
Makushin, Alaska	1980
Bogoslof, Alaska	1992
Okmok, Alaska	1981, 1983, 1986-1988, 1997
Cleveland, Alaska	1986, 1987, 1994, 2001
Amukta, Alaska	1987, 1996?
Seguam, Alaska	1992, 1993
Korovin, Alaska	1987, 1998
Kanaga, Alaska	1993-1995
Gareloi, Alaska	1980, 1982
Kiska, Alaska	1987?
Pagan, Mariana Islands	1981-1996
Anatahan, Mariana Islands	2003-2005

Source: An Assessment of Volcanic Threat and Monitoring Capabilities in the United States: Framework for a National Volcano Early Warning System (NVEWS)

Active Volcanoes Worldwide

Kliuchevskoi, Russia
• Chikurachki, Russia
• Shiveluch, Russia
• Karymsky, Russia
• Anatahan, Mariana Islands
• Soufrière Hills, Montserrat
• Tungurahua, Ecuador
• Colima, México
• Kilauea, Hawaii, USA - Erupting since 1983.
• Mauna Loa, Hawaii, USA
• Mayon, Philippines
• Mt. St. Helens, Oregon - Erupted in 1980. Reawakened on Oct. 1, 2004. Growth of the new lava dome inside the crater of Mount St. Helens continues, accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash. Oct. 2005 code: Orange

At the USGS's An Assessment of Volcanic Threat and Monitoring Capabilities in the United States: Framework for a National Volcano Early Warning System (NVEWS), (April, 2005), they categorize volcanoes by threat score. "Volcanic threat is the combination of hazards (the dangerous or destructive natural phenomena produced by a volcano) and exposure (the people and property at risk from the volcanic phenomena)."

The NVEWS threat assessment is not a formal risk assessment of U.S. volcanoes. The latter requires that probabilities of particular hazards occurring at individual volcanoes within a set time period be calculated and that the vulnerability of people and property to the hazards be estimated as expected losses in dollars. Such an analysis for so many volcanoes is beyond the scope of this work.

Their goal is to establish monitoring levels consistent with the threat. Currently only 3 of the 18 very high threat areas are well-monitored in real-time: Mount St. Helens, Washington; Kilauea, HI; and Long Valley caldera, CA. All 18 have at least a minimal level of ground based monitoring.

Threat	Num.	Volcanoes
Very High (Red)	18	Alaska: Akutan (AI), Augustine (CI), Makushin (AI), Redoubt (CI), Spurr (CI) [Regions: Alaska Peninsula (AP), Aleutian islands (AI) Cook Inlet SW of Anchorage (CI)] California: Lassen, Long Valley Caldera, Shasta Hawaii: Kilauea, Mauna Loa Oregon: Crater Lake, Hood, Newberry, South Sister Washington: Baker, Glacier Peak, Rainier, St. Helens
High (Orange)	37	Alaska: Aniakchak (AP), Atka (AI), Churchill (Wrangell), Dutton (AP), Gareloi (AI), Great Sitkin (AI), Griggs (AP), Hayes (CI), Iliamna (CI), Kaguyak (AP), Kanaga (AI), Katmai (AP), Mageik, Martin, Novarupta, Okmok, Pavlof, Pavlof Sister, Seguam, Semisopochnoi, Shishaldin, Trident, Ugashik-Peulik, Veniaminof, Westdahl, Wrangell California: Clear Lake, Inyo Craters, Medicine Lake, Mono Craters Hawaii: Hualalai Mariana Islands: Agrigan, Alamagan, Anatahan, Pagan, Washington: Adams Wyoming: Yellowstone
Moderate (Yellow)	48	Alaska: Adagdak, Amak, Amukta, Black Peak, Bogoslof, Chiginigak, Cleveland, Dana, Denison, Douglas, Edgecumbe, Emmons Lake, Fisher, Frosty, Kasatochi, Kialagvik, Kiska, Kukak, Kupreanof, Little Sitkin, Moffett, Recheschnoi, Roundtop, Sanford, Snowy Mountain, Steller, Tanaga, Ukinrek Maars, Vsevidof, Yantarni, Yunaska, Arizona: Sunset Crater California: Coso Volc. Field, Mono Lake Volc Field, Red Cones, Ubehebe Craters Colorado: Dotsero Hawaii: Haleakala, Mauna Kea Mariana Islands: Asuncion, Farallon de Pajaros, Guguan, Sarigan Nevada: Steamboat Springs (20 km south of Reno) New Mexico: Valles Caldera Oregon: Bachelor, North Sister Field Utah: Black Rock Desert
Low (Blue)	34	Alaska: Bobrof, Buldir, Buzzard Creek, Carlisle, Chagulak, Davidof, Duncan Canal. Fourpeaked, Herbert, Ingakslugwat Hills, Isanotski, Kagamil, Koniuji, Nunivak Island, Segula, Sergief, Stepovak, St.Michael, Table Top-Wide Bay, Takawangha, Uliaga, Unnamed Idaho: Craters of the Moon, Hell's Half Acre, Shoshone Lava Field, Wapi Lava Field Mariana Islands: Esmeralda Bank, Maug Islands, Ruby Oregon: Belknap, Blue Lake Crater Utah: Markagunt Plateau Washington: Indian Heaven, West Crater
Very Low (Green)	32	Alaska: Behm Canal-Rudyerd Bay, Gordon, Imuruk Lake, Kookooligit Mountains, St. Paul Island, Tlevak Strait-Suemez Island Arizona: Uinkaret Field California: Amboy, Big Cave, Brushy Butte, Eagle Lake Field, Golden Trout Creek, Lavic Lake, Tumble Buttes, Twin Buttes Mariana Islands: Ahyi, Supply Reef New Mexico: Carrizozo, Zuni-Bandera Oregon: Cinnamon Butte, Davis Lake, Devils Garden, Diamond Craters, Four Craters Lava Field, Jackies Butte, Jefferson, Jordan Craters, Lava Mountain, Sand Mountain Field, Washington Utah: Bald Knoll, Santa Clara

Map of Volcanoes of Canada and the western USA. at the Smithsonian.

Yellowstone
A hotspot under yellowstone had created three supervolcanos;

• 2.1 million years ago created a widespread volcanic deposit known as the Huckleberry Ridge Tuff. This titanic event, one of the five largest individual volcanic eruptions known anywhere on the Earth, formed a caldera more than 60 miles (100 km) across. 5.1 cubic miles mi³ of magma.
• A similar, smaller but still huge eruption occurred 1.3 million years ago. This eruption formed the Henrys Fork Caldera.
• 640,000 years ago created the 35-mile-wide, 50-mile-long (55 by 80 km) Yellowstone Caldera.

Terms:
Ejecta: Material that is thrown out by a volcano, including pyroclastic material (tephra) and lava bombs.

Lava: Magma which has reached the surface through a volcanic eruption. The term is most commonly applied to streams of liquid rock that flow from a crater or fissure. It also refers to cooled and solidified rock.

Magma: Molten rock beneath the surface of the earth.

Pyroclastic: Pertaining to fragmented (clastic) rock material formed by a volcanic explosion or ejection from a volcanic vent.

Pyroclastic Flow (nuées ardentes): Lateral flowage of a turbulent mixture of hot gases and unsorted pyroclastic material (volcanic fragments, crystals, ash, pumice, and glass shards) that can move at high speed (100 to 500 miles an hour.) The term also can refer to the deposit so formed. Types.

Tuff: Rock formed of pyroclastic material.

Type of eruption:

Strombolian

Named after the Italian volcano, Stromboli, eject small to medium sized lava fragments to form incandescent arcs which can be traced in photographs. Cinder cones often use this eruptive style.

Vulcanian

Named for the Italian volcano, Vulcano, produce short blast-like eruptions of gas and ash. These and all such eruptions occur due to the gas content of the magma being expelled.

Plinian

Named after the guy who reported Vesuvius to the Romans (Pliny the Younger), are the most violent there are. The most famous plinian eruption for Americans is probably the eruption of Mt. St. Helens. Plinian eruptions are long duration explosions which send ash and debris billowing kilometers into the atmosphere. Fine ash can be dispersed over very large areas. They typically last hours and can expel truly massive quantities of debris. Fast-moving deadly pyroclastic flows are also commonly associated with plinian eruptions.

Vesuvian

Similar to plinian, but are not so enormous. They release large amounts of ash and gas while the cloud typically forms a sort of cauliflower shape. Vesuvian eruptions are named after the Italian Mt. Vesuvius, which destroyed the cities of Pompeii and Herculaneum.

Pelean [Peléean, Nuée Ardente (glowing cloud)]

Pelean eruptions occur when the vent is held at an angle so that the eruption shoots out laterally, not up. Named for Mont Pelee in the West Indies. Peleean eruption involve viscous magma and share characteristics with Vulcanian eruptions. The key characteristic of Peleean eruptions is the eruption of glowing avalanches of hot ash.

Hawaiian

Eruptions may occur along fissures or fractures that serve as linear vents, such as during the eruption of Mauna Loa Volcano in Hawaii in 1950; or they may occur at a central vent such as during the 1959 eruption in Kilauea Iki Crater of Kilauea Volcano, Hawaii. In fissure-type eruptions, molten, incandescent lava spurts from a fissure on the volcano's rift zone and feeds lava streams that flow downslope. In central-vent eruptions, a fountain of fiery lava spurts to a height of several hundred feet or more. Such lava may collect in old pit craters to form lava lakes, or form cones, or feed radiating flows.

Phreatic (or steam-blast)

eruptions are driven by explosive expanding steam resulting from cold ground or surface water coming into contact with hot rock or magma. The distinguishing feature of phreatic explosions is that they only blast out fragments of preexisting solid rock from the volcanic conduit; no new magma is erupted. Phreatic activity is generally weak, but can be quite violent in some cases, such as the 1965 eruption of Taal Volcano, Philippines, and the 1975-76 activity at La Soufri¸re, Guadeloupe (Lesser Antilles).

Sources: Types of Volcanic Erujptions, Basics of Volcanology, Kinds Of Volcanic Eruptions

See: Volcanic and Geologic Terms

See: Index to Volcanoes at the USGS Cascades Volcano Observatory (CVO), Vancouver, Washington
The Smithsonian Institution Global Volcanism Program : Volcanoes of the World (Region Map, Weekly World Volcanic Activity Report & Activity Map)
Volcano Live
Top Ten Earthquakes and Volcanic Eruptions
Volcanoes at MTU
Volcano Live by John Seach, Australia