 Complete the following phrase:  The 1991 Ultra-Plinian eruption of Mount Pinatubo was the second largest terrestrial eruption of the 20th century surpassed only by the 1912 eruption of Novarupta, and the largest eruption in living memory. The eruption produced high-speed pyroclastic flows, giant lahars, and a cloud of volcanic ash hundreds of miles across. Twenty million tons of sulfur dioxide and roughly of tephra are estimated to have been ejected in total, which corresponds to a Volcanic Explosivity Index of 6. By contrast, roughly of material was ejected in the 