The name "megaton" comes from the atomic bomb because we (still) measure the power of an atomic blast in megatons of TNT. (Which is a hopelessly outdated method of measuring explosions.)
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What's outdated about measuring by amounts of equivalent TNT? The energy released by blowing up TNT and by blowing up a nuclear weapon is the same as both can be measured in Joules, which is the energy expended by the force.
1 gram of TNT releases 4.184Ã?10^3 J. As you can see measuring in Joules gives pretty big numbers that aren't easily read.
1 megaton of TNT releases 1 million million times more energy than that. So it makes little sense to measure in Joules, as these number would just be ridiculous. Even megajoules wouldn't be suitable. In any case megaton is a much cooler word.
The largest nuclear weapon ever donated was the Tsar bomba (Russian), 50 megatons. A bomb big enough to waste a large city pretty easy.
But we don't hold a candle to nature.
The earthquake that caused the 2004 Tsunami released 9,560 gigatons of TNT. A couple thousand Tsar bombas.
The 6 mile wide asteroid that killed off the Dinosaurs released 100 teratones of TNT or 400 zettajoules (4Ã?10^23 joules). Creating a 100 mile wide crater in the Gulf of Mexico. Way more energy than all nuclear weapons combined.
Finally, a supernova releases 10^44 Joules or ten octillion megatons of TNT. Eat that Fallout.