0 0 0   year s ▁ago . ▁E ru ptions ▁at ▁this ▁time ▁built ▁up ▁the ▁first ▁major ▁vol can ic ▁ed ifice , ▁forming ▁a ▁strat ov ol cano ▁in ▁altern ating ▁explos ive ▁and ▁eff us ive ▁er u ptions . ▁The ▁growth ▁of ▁the ▁mountain ▁was ▁occasionally ▁interrupted ▁by ▁major ▁er u ptions , ▁leading ▁to ▁the ▁collapse ▁of ▁the ▁sum mit ▁to ▁form ▁cal der as . ▁ ▁From ▁about ▁ 3 5 , 0 0 0 ▁to ▁ 1 5 , 0 0 0   year s ▁ago , ▁Et na ▁experienced ▁some ▁highly ▁explos ive ▁er u ptions , ▁generating ▁large ▁py ro cl astic ▁flows , ▁which ▁left ▁extensive ▁ign im br ite ▁depos its . ▁Ash ▁from ▁these ▁er u ptions ▁has ▁been ▁found ▁as ▁far ▁away ▁as ▁south ▁of ▁Rome ' s ▁border , ▁ ▁to ▁the ▁north . ▁ ▁Th ous ands ▁of ▁years ▁ago , ▁the ▁eastern ▁fl ank ▁of ▁the ▁mountain ▁experienced ▁a ▁cat ast roph ic ▁collapse , ▁generating ▁an ▁enorm ous ▁lands lide ▁in ▁an ▁event ▁similar ▁to ▁that ▁seen ▁in ▁the ▁ 1 9 8 0 ▁er u ption ▁of ▁Mount ▁St . ▁Hel ens . ▁The ▁lands lide ▁left ▁a ▁large ▁de pression ▁in ▁the ▁side ▁of ▁the ▁vol cano , ▁known ▁as ▁' Val le ▁del ▁B ove ' ▁( Val ley ▁of ▁the ▁Ox ). ▁Research ▁published ▁in ▁ 2 0 0 6 ▁suggested ▁this ▁occurred ▁around ▁ 8 , 0 0 0 ▁years