 Tephra volumes measured in this way must then be corrected for void spaces (vesicles – bubbles within the pumice, empty spaces between individual pieces of pumice or ash) to get an estimate of the original volume of magma erupted.This correction can be made by comparing the bulk density of the tephra deposit with the known density of the original gas-free rock-type that makes up the tephra.The result is referred to as the dense-rock equivalent of the erupted volume.