How do volcanoes help scientists learn about the Earth's interior?

The deepest samples of the Earth's interior brought up from drill holes come from a depth of 12 km . . . . volcanoes provide direct samples of the Earth's interior from much greater depths . . at least 100 km, and, perhaps, as deep as the core-mantle boundary . . that is, half the distance between the surface and the Earth's center.

John Dvorak