See how to measure volcanic eruptions based on The Volcanic Explosivity Index (VEI).
Volcanic eruptions vary in size and explosiveness. Volcanologists take a number of factors into account when they determine the "bigness" of a volcanic eruption. As you can see from the chart below, small volcanoes occur more frequently and truly colossal volcanoes don't happen very often (whew). For a large eruption, it takes much time for gas pressures to build up.
The Volcanic Explosivity Index is determined by using one of more of the following critieria: Volume of ejecta, Height of the eruptive column, Qualitative descriptions ("gentle", "effusive", "explosive", "cataclysmic", etc.), Style of past activity, and Height of spreading of the eruptive plume head (in troposphere or stratosphere). The VEI numbers below correspond with the following eruption characteristics:
Click to expand below VEI chart for better readability.
Did you know? When Krakatoa, a volcano on the Indonesian island of Rakata erupted, the explotions heard in the eruption remain the loudest noise on human record. The sound was heard across the Indian Ocean as far away as Rodriguez Island and Australia.