I am going to refer you to: The Atlas of Middle-earth by Karen Wynn Fonstad, page 146 - 147.
You can read the longer explanation there by her (she is a cartographer), but in short: Mt Doom was a composite or strato-volcano, formed of alternating layers of ash and lava, not a simple cinder cone. She goes into a detailed discussion of why, from the description Tolkien gave both in words and in his own illustrations, that she came to that conclusion.
She does say that although it stood in the Plateau of Gorgoroth in northern Mordor, a land of vulcanism, it appearst to be the only active volcano in that area.
__________________
Eldest, that’s what I am . . . I knew the dark under the stars when it was fearless - before the Dark Lord came from Outside.
|