Wikipedia credits Bourbaki with coining it, but doesn't provide a source. Does anyone happen to know the motivation for using this term?



What is the origin of the term magma?
The second definition in <http://www.larousse.com/en/dictionnaires/francais/magma/48543> would seem to answer your question and agrees with the wiki page. 


Update: here is what the link said to avoid broken links as YCor pointed out. 



> 
> magma  
> 
>  nom masculin
>  (latin magma, résidu, du grec magma)
> 
> 
> Mélange formant une masse pâteuse, épaisse : Magma informe.
> 
> 
> Mélange confus, inextricable de choses abstraites : Ces propositions constituent un magma incohérent.
> 
> 
> Matériau constitué de liquide et de cristaux en proportions variables, qui se forme à l'intérieur de la Terre et qui, en refroidissant, forme une roche.
> 
> 
> 


