This is book 17 of the "Rick Brant" series.

This note contains spoilers for the book.

The Spindrift crew goes down to rescue an island threatened with a volcanic explosion.

Little attention is given to the fact that they're all in danger if it goes up (at Krakatoa-scale explosion is not inconceivable). And, while weather changes after Krakatoa are mentioned, no consideration of how that would be "bad" is mentioned.

One of the scientists brought in for assistance finds diamonds, and doesn't understand the risk. He's going ahead, in partnership with the lieutenant-governor, with trying to exploit the diamonds, and is interfering with the work of the scientists. Then they kidnap the governor, and the army is hoodwinked into joining them, and opposing the evacutation.

So, since it's too late for the conventional tunneling operation (to create a channel to relieve pressure, venting on the opposite side of the mountain from the city), they call in the SeaBees and have them bring a 10KT nuclear explosive. That does the job right enough!