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The primary eruption characteristic used to determine the volcanic explosivity index is the volume of pyroclastic material ejected by the volcano. Pyroclastic material includes volcanic ash, tephra, pyroclastic flows, and other types of ejecta.
Full Answer. Magma that cools and hardens above ground creates extrusive igneous rock, whereas magma that cools underground forms intrusive igneous rock. Extrusive igneous rock hardens quickly and tends to be very small. Examples of this type of rock are obsidian, pumice and basalt.