Different prononunciations of "consummate"
Whenever the stress in such -ate words is not on the final syllable, then generally the last syllable will be:
for verbs: /eɪt/ [long vowel]
for adjectives: /ət/ [short schwa vowel]
The speaker may use a glottal stop in either case as an allophone of the /t/.
When the word stress does fall on the final syllable, however, the word will end in /eɪt/ regardless, even if it is an adjective:
- irate /aɪˈreɪt/
If the word is a verb, then usually the suffix -tion can be used to convert it into a noun. You will note in this case that the /eɪ/ sound will not change, as it will be full length in this position. This is because it will take the main stress in the word. The -tion suffix actually requires the preceding syllable to be stressed.
In other words if the word is a verb it should have the same vowel sound as the -tion noun. It's not really a case of a rationale, though. It's more just how the language has evolved - not how it's been designed!
Consummate is one of a fairly large number of -ate words in English whose adjective and verb forms are always, usually, or often pronounced differently. Here are 23 other words that follow this pattern:
aggregate
animate
appropriate
approximate
concatenate
correlate
degenerate
deliberate
designate
deviate
duplicate
enervate
incarnate
legitimate
obligate
precipitate
predicate
predominate
regenerate
reticulate
separate
striate
terminate
This list includes the following endings: two -cate, two -gate, three -iate, two -late, three -mate, five -nate, four -rate, one -tate, and one -vate.
According to Merriam-Webster's Eleventh Collegiate Dictionary (2003), the adjective form of designate is more often pronounced like the verb form than not, but the only multisyllable -ate words I could find that the dictionary says are always pronounced the same in both their adjective and verb forms are these two:
consecrate
serrate
Another word that may be an exception is this one:
reprobate
Unfortunately, the Eleventh Collegiate doesn't bother to provide a pronunciation for this word as an adjective; but I believe that it is usually (if not always) voiced the same as the verb form. The adjective fenestrate would constitute a fourth exception, if the Collegiate Dictionary listed a verb form of that word as well, but the closest it comes to such a verb is defenestrate, which doesn't have a matching adjective form.
The most striking thing about the -ate words listed here is that all but two of them (serrate and striate) are at least three syllables long. In English, most two-syllable -ate verbs ("collate, create, debate, deflate, dilate, locate, mandate, migrate, relate, truncate, etc.) do not have identical adjective forms, and most two-syllable -ate adjectives (oblate, ornate, ovate, palmate, etc.) have no matching verb.