Halliday & Matthiessen (2014: 715-6):
When a sequence is realised metaphorically by a clause, this means not only that it is mapped onto the transitivity patterns of the clause but also that it falls within the domain of the Theme + Rheme organisation of the clause and also, by extension, that of the Given + New organisation of the information unit. Thus the following sequence –
Displacement along these faults caused failure of the Baldwin Hills Reservoir in 1963
– is realised metaphorically as a ‘relational’ clause, and the figures making up the sequence are realised metaphorically as nominal groups serving as elements of the clause. Since they function within the clause, these nominal groups can be assigned textual statuses: the nominal group displacement along these faults serves as Theme, and the nominal group failure of the Baldwin Hills Reservoir in 1963 serves as New: see Figure 10-12. The metaphoric ‘relational’ clause thus creates a textual pattern of Theme: figure ‘displacement’ + New: figure ‘failure’.
This textual patterns is an effective resource in the rhetorical development of scientific discourse (see Halliday, 1988; Halliday & Martin, 1993). There is thus a gain in textual meaning in the shift from the congruent mode of realisation to the metaphoric mode. In this way, the ideational metaphor accommodates the textual metafunction: the experiential configuration of Identified + Process + Identifier structures the sequence into two textual quanta, one figure as Theme followed by another as New. In some cases, this kind of textual accommodation may be the main reason behind the ideational metaphor.
