Title text:
Mantle plumes explain Hawaii, Yellowstone, Iceland, the East African Rift, the Adirondack uplift, the Permian extinction, the decline of Rome, the DB Cooper hijacking, and the balrog in Moria. Those little hills of sand in your yard are caused by antle plumes.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3141/
They can be both!