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 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...
