How are Automatic Alternations the synchronic, not diachronic, consequence of phonetically-motivated diachronic sound changes?

Question

Haspelmath, Sims. Understanding Morphology (2010 2 ed). p 214 Bottom - 215 Top.

  Automatic alternations are the [1.] synchronic [End of 1.] consequence of phonetically-motivated diachronic sound changes. Sound change is motivated by phonetics in the sense that it occurs because phonetic production is made

[p 215:]

easier by the change. For example, pronouncing an alveolar or velar consonant before [i] is relatively more difficult than pronouncing a palatal (or palatalized) consonant, and this explains why the diachronic change of palatalization before front vowels is so common in the world’s languages (e.g. (10.1d)). Final devoicing helps pronunciation because maintaining the vibration of the vocal chords (which is made difficult by the oral obstruction of obstruents anyway) is particularly difficult in the final position (e.g. (10.1a)). Neutralization of unstressed vowels occurs for perceptual reasons: when a vowel is not stressed, it is less loud and thus differences between vowels are harder to perceive (e.g. (10.1c)). As in German Umlaut, morphophonological alternations often result when the phonetic motivation for some automatic alternation is subsequently obscured.

[p 326:] diachronic: having to do with language change over time (Section 6.1) (cf. synchronic).

[p 343:] synchronic: having to do with language at a given point in time (Section 6.1) (cf. diachronic).

1. I don't understand 1. Oughtn't 'synchronic' be 'diachronic'?

2. 'a given point in time` means a short time period like a day. Am I correct? If not, what time period did the authors envisage?

3. Even if the authors intended 'a given point in time' to mean a year or decade, how can automatic alternations be synchronic? One year or decade isn't long enough to spur phonological changes.

Answer 1

The sound change happening during the past (i.e. diachronically) results in a certain stage of phonological patterns today (i.e. synchronic). The alternation system that currently exists with the properties that it has at this stage of development is observable and describable now, at a given point of time, and not only in view of its development over centuries.

Answer 2

I will just add a bit to lemontree's answer, which I agree with. The problem stems from the fact that the authors do not entirely accept the basis for the distinction between synchrony and diachrony. When you speak a language, you have internalized a set of rules (a grammar) by which you can generate forms in the language. There is a huge ontological difference between "what is in a particular grammar" and "what events took place in time between stage 1 and stage 2". Additionally, they are employing a misleading notion of "sound change", where "sound change" is different from "changes in grammar affecting sound systems", so it is tautological that "Sound change is motivated by phonetics". Most sound change, in the more general sense, is not about phonetics. It's a bit shocking that they also claim that the initiation of sound change is driven by production, when they know that the initiation of sound change is largely driven by perception. Ease-of-production changes like devoicing are not the general pattern.