 Complete the following phrase:  According to the Volcanic Explosivity Index, a VEI of 6 to 8 is classified as ultra-Plinian. Eruptions of this type are defined by ash plumes over high and a volume of erupted material to in size. Eruptions in the ultra-Plinian category include the Lava Creek eruption of the Yellowstone Caldera c. 640,000 years ago, VEI 8, Lake Toba c. 74,000 years ago, VEI 8, 