Why are volcanoes called "Windows into the Earth's Interior?"

reader

The deepest mines extend about two kilometers into the Earth. The deepest drill holes extend about 12 km into the Earth. And so, our artificial means of penetrating the Earth extend way less than one per cent toward the core. Volcanoes, however, erupt material from tens to hundreds of kilometers into the Earth . . and so the deepest samples of the Earth's interior are volcanic rocks . . . which provide a window into the Earth's interior.

Sincerely,
John Dvorak

Other Categories Other Questions
To VolcanoWorld