She is petite, trim, almost birdlike. He’s big, slightly lumbering, closer to a bear. Her hair is short, kept in a shaggy pixie cut. His resembles an unruly, curly mop — a Chia pet in full sprout. Both of them have very large ears. No one can remember exactly how they met; it might have been on a bench at a university, or at a movie screening, or perhaps on a blind date. What we do know is that Katia and Maurice Krafft bonded over a common obsession: volcanoes. And by the time they’d married in 1970, these two scientists had already started trotting the globe together to study these beautiful, violent natural phenomenons whenever and wherever eruptions occurred. “Certain colleagues see us as weirdos,” Katia admitted. The first couple of volcanology, however, simply recognized each other as soulmates.

It’s a...