Quote from : The Elder Gods Book

Mountains could be what happens when Father Earth eats something that doesn’t agree with him. When he burps, mountains pop up." "That’s absurd," Keselo said, trying not to laugh. "If you’ve got a better theory, I’d be happy to hear it," Red-Beard said mildly. "Anyway, a burp isn’t anything but air that boils up out of a man’s stomach, so Father Earth’s mountains have chunks of empty air in the middle of them—burps that didn’t quite manage to make it to the surface, you understand.