- ▁or ▁syn - er upt ive ▁deg ass ing ▁of ▁a ▁separate ▁v apor ▁phase , ▁such ▁as ▁that ▁post ulated ▁for ▁the ▁Pin at ub o ▁and ▁Red ou bt ▁er u ptions . ▁The ▁ult imate ▁source ▁for ▁the ▁excess ▁vol atile ▁observed ▁during ▁the ▁ 1 9 9 1 ▁Pin at ub o ▁er u ption ▁is ▁assumed ▁to ▁be ▁sul fur - rich ▁bas alt ic ▁mag mas ▁underlying , ▁and ▁syn - er upt ively ▁intr ud ed , ▁into ▁the ▁over lying ▁f els ic ▁mag mas . ▁The ▁sul fur - rich ▁tra ch yt ic ▁and ▁tra chy and es it ic ▁mag mas ▁which ▁under lay ▁the ▁r hy olit ic ▁mag ma ▁at ▁Ch ang ba ish an ▁may ▁have ▁been ▁a ▁possible ▁source ▁for ▁excess ▁sul fur ▁accum ulation . ▁If ▁this ▁scenario ▁is ▁real istic , ▁clear cut ▁pro x ies ▁for ▁the ▁environmental ▁impact ▁of ▁the ▁er u ption ▁would ▁be ▁expected . ▁Mill en ni um ▁er u ption ▁mag mas ▁are ▁pre domin antly ▁phen oc ry st - po or ▁( ≤ ▁ 3 ▁vol %) ▁com end ites ▁plus ▁a ▁vol um etr ically ▁minor ▁late - stage , ▁more ▁phen oc ry st - rich ▁( 1 0 – 2 0 ▁vol %) ▁tra chy te . ▁S izable ▁( 1 0 0 – 5 0 0   µ m ▁diameter ) ▁glass y ▁but ▁b ubble - b ear ing ▁m elt ▁inclus