In a spin off RP of Jack of All Trades (which hasn't started yet), the opening of the game starts with Victorian London becoming a giant smoking crater.
Let's calcuate that
So, Victorian London is 2.41402 Kilometers in total length, and thereby, 1.20701 Kilometers in radius. So, using the formula for a hemisphere...
(2/3)×π×1.20701^3 is 3.68291095178609 Kilometers, or for our purposes, 3.682910951786E15 centimeters cubed.
It's entirely Vaporized, so times 25700 is....
9.465081146090E19 Joules, 22.6220868692405 Gigatons, Island level
It's not 100% stone, so, we shouldn't treat it as such.
Instead it's 45% stone, 25% water, 25% air and 5% leaves and such, and we can only really calculate water and stone.
45% of 3.682910951786E15 is 1.6573099e+15, and 25% is 9.2072774e+14, or, for the purpose of water J/KG, 920727737947 Kilograms.
Applying the J/CC of 25700 and the J/KG of 2264705, the result is...
4.2592864e+19 + 2.0851767e+18 Joules = 4.4678041e+19 Joules, 10.6783080067 Gigatons, Island level