A figure of speech that appears in the sentence 'Caroline was rendered speechless to such a degree that she talked of nothing else for the rest of the day' is irony.
When we use irony, the intended meaning of the words we use is different from their actual meaning. In other words, there is a contrast between what we say and what we mean. What the creator of this sentence actually means is that Caroline was not rendered speechless at all, as she continued talking for the rest of the day.