Megaton

A megaton or megatonne is a unit of mass equal to 1,000,000 metric tons, i.e. 10⁹ kg or 1 teragram (Tg). The official SI symbol for the megaton is Mt, but MT is also being used; beware that the latter is also (unofficially) used for the metric ton in some contexts. See 1 E9 kg for a comparison with similar masses.

The kiloton or megaton of TNT is used as a unit of energy, approximately equivalent to the energy released in the detonation of this amount of TNT. The megaton of TNT has traditionally been used to rate the energy output, and hence destructive power, of nuclear weapons. This unit is written into various arms control treaties, and gives a sense of destructiveness as compared with ordinary explosives, like TNT. More recently, it has been used to describe the energy released in other highly destructive events, such as asteroid impacts.

• A gram of TNT by definition for arms control purposes is 1000 thermochemical calories, which equals 4.184 kilojoules (KJ).
• A ton of TNT, (a metric ton = 1000 kg) is therefore 4.184 x 10⁹ J = 4.184 gigajoules (GJ).
• A kiloton of TNT is therefore 4.184 x 10¹² J = 4.184 terajoules (TJ).
• A megaton of TNT is 4.184 x 10¹⁵ joules = 4.184 petajoules (PJ).

(This definition is a conventional one. The actual measured output of a gram of TNT is a little less, 652 thermochemical calories = 2724 J)[1] (http://muller.lbl.gov/teaching/Physics10/chapters_Jan_2005/Chapter01.htm)

Relative explosive sizes

• The first nuclear bomb tested at the Alamagordo test site released 18.6 kilotons of TNT (Rhodes, page 677), or approximately 78 terajoules.

• The Little Boy weapon dropped on Hiroshima had a yield of approximately 13 kilotons of TNT (54 TJ). Thus, a megaton of TNT is equivalent to roughly 77 Hiroshima bombs. The Nagasaki bomb, Fat Man, released 20 kilotons of TNT = 84 TJ.

• The largest nuclear weapon ever detonated was the Tsar Bomba, which had a yield of 50 megatons of TNT (210 PJ). The most powerful nuclear weapon ever produced was a version of the Tsar Bomb that would have yielded some 100 megatons of TNT.

• Typical H-bombs today have a yield of around 1 megaton of TNT.

• The 1980 eruption of Mount St. Helens was reportedly equivalent to 27,000 Hiroshima-sized atomic bombs - which translates roughly to 350 megatons.

• The 1912 eruption of Novarupta in Alaska was ten times the size of Mount St. Helens. That is about 3500 megatons.

• The 1883 eruption of Krakatoa was about 50% larger than Novarupta. That is about 5250 megatons.

• The 1815 eruption of Mount Tambora was about seven times larger than Novarupta. That is about 24500 megatons.

• The 1650 BC eruption of Santorini was much larger than Mount Tambora.

• The Lake Toba eruption 73,000 years ago was larger even than Santorini, and is likely to have caused a mass global die-off. See Toba catastrophe theory.

• The Yellowstone Caldera was formed by a massive volcanic explosion some 640,000 years ago that was 2500 times the size of Mount St. Helens. That is about 875000 Megatons. This would have caused a mass global die-off as well.

• The impact of a roughly 15 kilometre-wide meteorite or comet with the Earth can yield upward of 100 million megatons of TNT = 4.184 x 10²³ J. Such impacts have been hypothesized to be the cause of prehistoric extinction events. This is the Nemesis hypothesis of Richard A. Muller. (Muller website) (http://muller.lbl.gov/teaching/Physics10/chapters_Jan_2005/Chapter01.htm)

• The 30 May 1998 magnitude 6.5 earthquake in Afghanistan had an energy release "equivalent to a 2000 kiloton nuclear explosion". (USGS) (http://earthquake.usgs.gov/faq/nuclear.html)

• The 2004 Indian Ocean earthquake released an estimated 2 x 10¹⁸ joules (2 EJ), or "475,000 kilotons (475 megatons) of TNT, or the equivalent of 23,000 Nagasaki bombs." (USGS (http://earthquake.usgs.gov/eqinthenews/2004/usslav/neic_slav_faq.html))

See also

• 1 E15 J for comparisons with other energies.
• gigaton

Reference

Rhodes, Richard. The Making of the Atomic Bomb, New York: Simon and Schuster, 1986.

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