I suspect it is related to the eruption of Krakatoa in 1883... According to the OED, effusive was used in GEOLOGY in the late 19th century. One particular reference in the extended entry is enlightening: 1888 F. H. Hatch in J. J. H. Teall Brit. Petrogr. 429 Effusive, a term lately used abroad for those rocks which have been poured out at the surface, the word eruptive now being generally used for the whole group of m…
Hm, the ngram graph seems to show 'effusive' spiking well before Krakatoa, so I doubt that's the root explanation for the increase.
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.