Eddie Van Halen used an MXR Phase 90 when recording the guitar solo for "Eruption". Eddie also used a Univox EC-80 echo box, with a different motor in it, in order for it to "go real low and delay much slower". Listen for the effect of the Univox at the ending descending growl of "Eruption".
Guitar Tab: Eruption
Apparently, the recording of "Eruption" happened spontaneously... it wasn't even supposed to be a track on the album. Eddie Valen Halen was warming up at the recording studio early one day, when producer Ted Templeman overheard the guitarist. Templeman loved what he heard, and demanded that they record the solo. Eddie quickly recorded one take of "Eruption", and they ended up putting it on the album. Says Eddie: "I didn't even play it right. There's a mistake at the top end of it. To this day, whenever I hear it I always think, "Man, I could've played it better."