Volcanic eruptions are rated on a logarithmic scale from 1 to 8. It looks like there hasn't been a formal rating assigned to Tonga yet, but it's believed to be

Volcanic eruptions are rated on a logarithmic scale from 1 to 8. It looks like there hasn't been a formal rating assigned to Tonga yet, but it's believed to be the largest of the 21st century so far, and there was already a VEI 5 in 2011. A VEI 8 is the eruption of a super volcano like the Yellowstone caldera, so obviously it would make Tonga look like a firecracker in comparison. If you consider asteroid impacts as geological events then they can be substantially larger.