The volcanism was detected on images taken with the Voyager I spacecraft. But as for whom? - Hmmmm. The first publication on Io's volcanism is

Strom, R.G., R.J. Terrile, H. Masursky, and C.J. Hansen, "Volcanic Eruption Plumes on Io," Nature, Vol. 280, p. 733, 1979.

So maybe you are talking about these four?

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