If you dial down the smoothing, there is a blip right before Krakatoa's rise (1884-1887). Looking at the books from that time, it seems "effusive" was general p

If you dial down the smoothing, there is a blip right before Krakatoa's rise (1884-1887). Looking at the books from that time, it seems "effusive" was general parlance appearing in other places as well, including several popular elocution guides (of which Google records many versions). The blip dies out though, so it would seem that geologic and general science reporting are likely to be the ongoing source.