https://www.google.com/?gws_rd=ssl#q=effusive
What happened in 1866 that made this word popular? And why did growth hit a plateau around 1900?
Is there something we could learn about virality just by studying word popularity?
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 massive rocks.
As noted, Krakatoa erupted in 1883. It was massive in sheer destructive force (though not quite as bad as Tambora in 1815, which remains the deadliest in recorded history). Still, it was a global phenomenon.
Adding "krakatoa" to ngram reveals that it may just be tied to the spike, though perhaps this is just correlation.
https://books.google.com/ngrams/graph?content=effusive%2Ckra...
The blip dies out though, so it would seem that geologic and general science reporting are likely to be the ongoing source.
1. The data is smoothed. Set the smoothing to 0 to see the real data.
2. The data is not absolute, but relative (note it's %) to the entire body of data Google has available for that year. The further back you go, the less overall data is available. I suspect strongly that the spikes you see are merely years where very few books overall are available.
I think this is what you are talking about though:
https://books.google.com/ngrams/graph?content=effusive&case_...
Serendipitously, someone submitted a similar story just 3 minutes before: https://news.ycombinator.com/item?id=9573862
https://books.google.com/books?id=41RDAAAAYAAJ&pg=PA10&dq=%2...