Gigaton
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A gigaton (or gigatonne) is a Metric Unit of mass, equal to 1,000,000,000 (1 billion) Metric tons, 1,000,000,000,000 (1 trillion) kilograms, or 1 quadrillion grams. More commonly, though, the term gigaton is used to mean a gigaton of TNT
A gigaton of TNT is more commonly used as a measure of energy equal to that released by 1 billion metric tons of TNT. A gigaton is equal to 1,000 megatons, and is from the same family of terms. However, while megatons are commonly used as a measure of explosive devices, the most powerful explosive device ever constructed had a yield of only 58 megatons (October 30, 1961 - U.S.S.R.).
The term gigaton is therefore used mostly in seismology. An earthquake measuring an 8.0 on the Richter scale releases the equivalent of approximately 1.01 gigatons of TNT; the 2004 Indian Ocean earthquake is estimated to have released the equivalent of 100 gigatons of TNT. Even in this context however, the term gigaton is not often used. It is perhaps more appropriately used in planetary science, where impact events can release thousands or even millions of gigatons of energy; the body that caused the Chicxulub crater in Mexico is estimated to have released the equivalent of as much as 191,793 gigatons of TNT.
Although the gigaton unit does not appear to be in common enough usage to warrant an official abbreviation, Gt would be the logical choice.