This is the source for the volcano being 7-B. It reads:
| But if we look at a well-known major volcanic eruption, the eruption of Mount St. Helens in 1980, we find that: "In all, Mount St. Helens released 24 megatons of thermal energy, 7 of which was a direct result of the blast. This is equivalent to 1,600 times the size of the atomic bomb dropped on Hiroshima". |
The 24-megaton figure is based on the 1980 eruption of St Helens, and according to my calculations
here, the Titan's Curse occurred in 2008. It is not the same eruption, nor is the volcano feat "calculated" to be 7-B.
Now let's see what else the article says.
| But Mount St. Helens wasn't even at the top of the scale of Volcanic Explosivity Index. It was a class 5, and the scale goes up all the way to 8, which are called "mega-colossal" eruptions. These class 8 super-volcanos erupt extremely rarely (otherwise we wouldn't be here), but when they do, more than 1,000 cubic kilometers of rock and ash are ejected, the climate of the whole planet is affected for extended periods of time, and mass-extinctions can be expected. Now that is powerful! To get anywhere close to that kind of energy release, the U.S. and Russia would have to use their entire nuclear arsenals simultaneously, and even that might not be enough to compare depending on how long the volcanic eruption lasts. |
7-B isn't the top of the scale. Those "supercolossal" eruptions causes the planet to be affected for a period of time. Obviously, Percy's volcano eruption is not that powerful, but it is obviously more than the 24-megaton figure. If you read Percy Jackson's Greek Gods, you know that when Typhon used to be imprisoned in Mount Etna and kept on causing the volcano to erupt. We can assume the same case for Mount Saint Helens. So Percy must have to produced an unknown amount more than 24 megatons to stir Typhon in his sleep.
It should also be noted that until the Last Olympian, the volcano kept on erupting until completely destroyed. In the 1980 eruption, Mount Saint Helens was nearly, but not, destroyed. Another evidence that Percy's volcano eruption is more than 24 megatons.
The above source states that all the weapons USA and Russia combined. We can do a rough calculation on that.
According to this source, that is 6550+6850 nuclear weapons, or 13 400 nukes. Assuming each nuke is at least
50 kilotons (most efficient design according to Wikipedia), that's 670 000 000 tons in total or 670 megatons. But I'm pretty sure many nukes are far more powerful than that. Tsar Bomba is 50 megatons.
tl;dr All of the weapons USA and USSR combined = supercolossal volcano eruptions > Percy Jackson's volcano eruption > Typhon's raw power while sealed inside the volcano = 24 megatons
