Halliday & Matthiessen (2014: 499):
The finite variant is illustrated by examples such as the applause [[ × when she finished singing]], the scar [[ × where the bullet entered]], the difference [[ × since I started taking Brandex]]. These are condensed variants of an embedded nexus consisting of an elaborating clause with an enhancing clause dependent on it:
the applause [[ = which erupted || × β when she finished singing ]]
the scar [[ = which has formed || × β where the bullet entered ]]
The items when and where are structural conjunctions rather than relative adverbs; they do not have the sense of preposition + which: we cannot say e.g. the scar at which the bullet entered. Contrast Some may precipitate directly from sea water in areas [[where volcanism releases abundant silica]]. Here where is a relative adverb; and it is related to the prepositional phrase in which: ... in areas in which volcanism releases abundant silica.