▁The ▁latter ▁estimate ▁published ▁in ▁ 2 0 1 2 ▁is ▁based ▁on ▁arg on ▁d ating ▁of ▁the ▁first ▁pre - cal der a ▁la va ▁flows . ▁The ▁formation ▁of ▁Tamb ora ▁dra ined ▁a ▁large ▁mag ma ▁chamber ▁pre - ex isting ▁under ▁the ▁mountain . ▁The ▁Mo jo ▁is let ▁was ▁formed ▁as ▁part ▁of ▁this ▁ge ological ▁process ▁in ▁which ▁S ale h ▁Bay ▁first ▁appeared ▁as ▁a ▁sea ▁bas in ▁about ▁ 2 5 , 0 0 0 ▁years ▁B P . ▁ ▁A ▁high ▁vol can ic ▁cone ▁with ▁a ▁single ▁central ▁vent ▁formed ▁before ▁the ▁ 1 8 1 5 ▁er u ption , ▁which ▁follows ▁a ▁strat ov ol cano ▁shape . ▁The ▁diameter ▁at ▁the ▁base ▁is ▁. ▁The ▁vol cano ▁frequently ▁er upt ed ▁la va , ▁which ▁desc ended ▁over ▁ste ep ▁s lop es . ▁Tamb ora ▁has ▁produced ▁tra chy bas alt ▁and ▁tra chy and es ite ▁rocks ▁which ▁are ▁rich ▁in ▁pot ass ium . ▁The ▁vol can ics ▁contain ▁phen oc ry st s ▁of ▁ap at ite , ▁bi ot ite , ▁clin opy ro x ene , ▁le uc ite , ▁magnet ite , ▁ol iv ine ▁and ▁pl agi oc l ase , ▁with ▁the ▁exact ▁composition ▁of ▁the ▁phen oc ry st s ▁varying ▁between ▁different ▁rock ▁types . ▁Orth opy ro x ene ▁is ▁absent ▁in ▁the ▁tra chy and es ites ▁of ▁Tamb ora . ▁Oliv ine ▁is ▁most ▁present