This is the Volcanic Explosivity Index.
Rating 0: Weak Eruption of mainly lava and smoke. Little or no serious explosions occur from this eruption.
Rating 1: Minor/Moderate eruption of lava, smoke and hot blocks. This eruptions can cause intermittent explosions with/without debris fall-out and lava bombs.
Rating 2: Moderate Eruption. This eruption can cause an Ash cloud that can extend at least couple of miles from the volcano. Lava and Hot blocks can be generated by this level of eruption type.
Rating 3: Moderate/Considerable Eruption. This type of eruption can cause an ash cloud that may extend as far as Ten Miles or More from the main eruptive vent. Weak pyroclastic flows can sometimes be generated by these eruptions. Lava can be generated but at this raring, it would start getting thicker as the eruption level goes up.
Frequency: Every Couple of months to a year
Rating 4: Considerable/Severe Eruption This type of eruption can cause ash clouds that extend Ten, Fifteen or More miles from the vent. Hot blocks, pumice and ash are the main events of this rated eruption. Some lava can occur as well as weak pyroclastic flows.
Frequency : Every year or so.
Rating 5: Severe eruption. This eruption happens more frequently than 6+. Volcanoes like Mount St. Helens have been rated this level. The ash cloud of this level of eruption can extend up to 50 or more Miles from the main eruption. Devastating pyroclastic flows can be generated, even some lava flows.
Frequency : every 5 to 95 years.
Rating 6: Severe/Serious This eruption can cause an explosion that can be heard for miles. This eruption can devastate land miles away from the volcano. Ash clouds can extend over 75 miles from the volcano. Mount Pinatubo is an example of such an eruption.
Frequency : Every 10 to 3,000 years.
Rating 7: Serious/Major This eruption can cause devastation for up-to 100 miles from the main vent. The ash cloud can extend up to 125 miles or More from the vent. This eruption can produce lava, pumice, hot blocks and Pyroclastic Flows. Climate can be affected a good amount by the eruption. Mount Tambora is an example volcano of a sort of eruption rating.
1 or 2 small secondary vents may/can Possibly erupt along with the main vent.
Frequency : Every 50 to 12,500 years
Rating 8: Super Eruption This eruption is the highest rating. If such an eruption occurred, areas in a radius as far as a few miles to the radius size of Texas would be flattened. If such an eruption happened, it could unfold in a series of explosive eruptions/vents and turn into one, or it could just erupt as one single massive vent. After such an eruption, a caldera would form after such an eruption. The ash cloud could extend anywhere from 100 miles to 2,500 miles. Ash, pumice and hot blocks as likely to occur from this eruption. As well as pyroclastic flows and some lava.
Frequency: 500 Years or Greater.