Black Sands by Colleen Coble

A brainy—and perpetually single—volcanologist, Annie Tagama works alongside her father at Hawaii Volcanoes National Park. But despite her pedigree, Annie has lost her passion or volcanology. She's quietly grieving the mysterious loss of her brother in the Middle East.

When Annie discovers that her brother might still be alive—and in trouble—she shakes off her malaise and joins forces with Mano Oana, the unnervingly handsome family friend she had always blamed for her brother's death.

As Annie and Mano investigate—and find that they have a lot more in common than tragedy—an evil force gathers strength on the main island. An ancient cult has reemerged—and Annie's little sister is somehow involved. As the old religion bubbles to the surface like lava, the old practices are not far behind . . . including human sacrifice.

Annie and Mano are desperate to find her brother and save her family from destruction before the ancient evil smothers them all in an earth-shattering eruption.