The Volcanic Explosivity Index
    The VEI is the Vulcanologists ratings meter for eruptions.  Much like the Reichter scale in earthquakes, the VEI gives the researcher a relative chart to compare eruptions past and present.  As an added bonus, the VEI also gives you a clue as to what type of eruption occured.  In the case of Non-Explosive eruptions, clasification is actually limited by what level the eruption rated on the VEI.

The Volcanic Explosivity Index

    Any eruption that occurs anywhere will rate at least a VEI 0 on the scale.  An eruption of 0 VEI will have less than 10,000 cubic meters of Ejecta, which includes Lava and Ash.  Anything ejected from the volcanoe will not reach a height above .1 kilometers.  As for how long the eruptions last, eruptions from VEI 0 to 1 will only have as much as 1 continuous hour of blasting.
    Eruptions of VEI 1 have between 10,000 and 1,000,000 cubic meters of Ejecta.  The height of Pyroclastics will be between .1 and 1 km.  VEI 2 eruptions have between 1,000,000 and 10,000,000 cubic meters of Ejecta and Pyroclastics launched 1 to 5 km into the air.  The duration of continuous blasting in a VEI 2 eruption will generally hover around 1 or 2 hours.
    VEI 3 volcanoes have as much as 100 million cubic meters of ejecta!  At this level, the volcano can eject pyroclastics as much as 15 kms into the atmosphere.  VEI 4 eruptions belch forth up to ONE BILLION cubic meters of crud!  Also shooting ash into the upper atmosphere 10 to 25 kilometers up!
While it may seem somewhat silly to have VEI above this, there have been in recent history eruptions for which VEI's of up to 7 were applied.  Just add a zero to the amount of Ejecta for every level after VEI 4, at VEI 8 a volcano must spew out a minimum of ONE TRILLION cubic meters of ejecta (that's 1,000,000,000,000 cubic meters!!!).  Also, any eruption of VEI 5 or higher can rocket ash and cinders more than 25 kms straight up!

Non-Explosive Volcanoes

    Icelandic Volcanoes are the least explosive volcanic eruptions, they range from VEI 0 to 1 and rarely eject ash and pyroclastics.  Generally forming along a rift rather than a single tube, the Iclandic Volcanoes are responsible for flood basalts creating a volcanic plateau or a shield volcano.  Hawaiian type volcanoes always create large sheild volcanoes.  The VEI of a Hawiian type volcano is only a .1 since it rarely, if ever, ejects any ash or any violent lavaflows.  Don't let that fool you, the lava still flows, almost constantly for over a decade now and has destroyed hundreds of homes and created many square kilometers of new land.  Both Icelandic and Hawaiian type volcanoes have very low viscosity Basalt lava flows and usually form over "hot spots" in the crust or at a divergent techtonic plate margin.

Explosive Volcanoes

    The type of lava in a volcano is dependent on its temperature.  Basalt is high temperature and low in Silica DiOxide, when melted Basalt has the consistency of melted Icecream and allows gas to escape easily in a peaceful eruption.  Andesite is the halfway point in lava; cooler with more Silica DiOxide and a higher viscosity.  Lastly, Rhyolite is the coolest (but still 600 to 900 degrees celcius) and has the viscosity of toothpaste with up to 75% Silica DiOxide.  Also, it is very hard for gas to escape Rhyolite lava, and so the eruptions are usually very violent and explosive from the gas trapped in the rock.
    First up are the Strombolian type volcanoes.  These volcanoes typically form Scoria or Cinder Cones and have a low VEI of 1-3.  These volcanoes generally expell Basalt and sometimes Andesite.  Moderately explosive, Vulcanian types of volcanoes have a VEI of 2-5, produce lave from Basalt all the way up to Rhyolite and sometimes form Cinder Cones, but also create Stratovolcanoes.  A Stratovolcano is the A-typical volcano people imagine, Mt. St. Helens and Mt. Fuji are great examples of Stratovolcanoes.
    The Highly Explosive Volcanoes can actually all be seperate steps in the evolution of a Stratovolcano.  The Vesuvian type volcano, named after Mt.Vesuvious in Italy, Have violent blasts from VEI 3 to VEI 8.  They extrude highly viscous lava from Andesite to Rhyolite.  The Plinian eruption, often called the "throat clear" completely opens up the throat of the volcano and ash can be shot many kilometers into the air.  Lastly, the Pelean type eruption named after Mt. Pelee in the Caribbean Sea has an extremely violent, high viscosity, Andesite to Rhyolite eruption.  In this type of eruption the lava has become so viscous that it may completely block the throat of the volcano.  Pressure builds up from the gasses waiting to be released and eventually the entire side or top of the volcano is blow out in this most explosive kind of eruption.  The most famous type of this eruption was Mt. St. Helens in 1980.