At volcanoes like Kilauea, the bubbles are primarily CO2. The density of CO2 in a fluid inclusion is sensitive to the pressure the magma was under when the CO2 was trapped in a crystal.

The region is humbling and awe-inspiring, with the 1 million-year-old Kohala volcano rising a mile high from the sea, and Haleakala on Maui looming on the horizon. In front of Kohala Civic Center ...