Halliday And Matthiessen (2014: 673-4):
From an ideational point of view, the difference in meaning relates most directly to the question of what is construed as a quantum of change in the flow of events. The examples in Table 10-4 form a scale extended between two poles. At one pole, the experience of the flow of events is construed as two distinct quanta of change, realised by two independent clauses that are related cohesively but not structurally (She didn’t know the rules. Consequently, she died.). At the other pole, the experience of the flow of events is construed as a component part of a quantum of change – a participant that can itself be an element of some other quantum of change (cf. the quite incredible ignorance on which it is based must be a cause for grave concern), realised by a single nominal group (her death through ignorance of the rules). Intermediate between these two poles are various manifestations that represent a move from two distinct quanta of change via two interdependent ones to a single one.
The scale is thus one of degree of integration of two quanta of change. This scale of integration is based on the rank scale. At one pole, the sequence of cohesively related clauses transcends the rank scale, and, at the other, the nominal group is located at the rank below that of the clause. These two poles are thus connected by a move down the rank scale. At the same time, this move involves a shift in metafunction: textual – logical – logical + experiential – experiential. Here the meaning of expansion changes with the change of metafunctional manifestation. For example, the manifestation of cause changes from rhetorical relation (textual: consequently) via logico-semantic relation (logical: so, because) to process or minor process and even participant (experiential: cause, through; cause). This means that the category meaning of ‘cause’ changes; so while, for example, consequently and through share the meaning of cause, they differ in the category meanings they assign to it. (We shall see below that some of these realisations of cause are, in fact, ideational metaphors within the domain of the grammar.)