▁to ▁the ▁most ▁alter ation ; ▁especially ▁in ▁the ▁sum mit ▁area ▁and ▁between ▁the ▁Ang ulo ▁and ▁Az uf rera ▁summ its ▁lie ▁sul fur ▁rich ▁tal us ▁depos its . ▁The ▁central ▁part ▁of ▁the ▁complex ▁is ▁heavily ▁alter ed ▁by ▁f umar ole ▁activity . ▁H ydro ther mal ▁alter ation ▁may ▁be ▁driven ▁by ▁the ▁formation ▁of ▁a ▁deep ▁mag ma ▁res er voir ▁and ▁resulting ▁hydro ther mal ▁circul ation ▁in ▁over lying ▁rocks . ▁ ▁E rupt ive ▁history ▁ ▁E ruption ▁rates ▁increased ▁ 6 ▁million ▁years ▁ago , ▁coinc iding ▁with ▁mag mas ▁becoming ▁more ▁uniform ▁in ▁composition ▁and ▁the ▁on set ▁of ▁strong ▁hydro ther mal ▁alter ation . ▁It ▁is ▁possible ▁that ▁solid ification ▁of ▁mag mas ▁below ▁the ▁vol cano ▁ins ulated ▁the ▁feed ing ▁channels ▁from ▁heat ▁loss ▁and ▁caused ▁the ▁temper atures ▁in ▁the ▁system ▁to ▁increase . ▁Activity ▁w an ed ▁again ▁ 2 ▁million ▁years ▁ago , ▁with ▁mag ma ▁and ▁included ▁cry st als ▁being ▁drawn ▁from ▁depth s ▁of ▁ ▁and ▁forming ▁the ▁A uc an qu il cha ▁vol cano ▁proper . ▁The ▁ 1 0 - mill ion - year ▁duration ▁of ▁activity ▁of ▁the ▁Al ti pl ano – P una ▁vol can ic ▁complex ▁systems ▁is ▁compar able ▁to ▁the ▁duration ▁of ▁A uc an qu il cha ▁activity . ▁The ▁age ▁of ▁the ▁la va ▁flows ▁ranges ▁from ▁heavily ▁gla ci ated ▁and es ite ▁flows ▁over lying ▁less - er