What does magma mean?
All I know about sontag is the "disease as metaphor" stuff. so I know she is interested in using material situations (hi5/tb). so is it totally unpredictable that she'd use geology as metaphor?
I tried searching for what you might be alluding to and found like this page where sontag is incidentally in the side bar: Sappho and the Fevered Heart: Anne Carson on Jealousy –
This, indeed, is the raw nature of jealousy — beneath the narrative, beyond the magma of feeling, it is a projection, a self-construction, a self-response that reveals more about our relationship to love itself, which springs from our relationship to ourselves, than about any object of desire.
And a few other similar hits... if you are saying that Sontag is wrong to use "magma" as "foundation"... I would have to say on what basis you claim that?