▁in ▁the ▁rocks ▁with ▁less ▁than ▁ 5 3 ▁percent ▁Si O 2 , ▁while ▁it ▁is ▁absent ▁in ▁the ▁more ▁sil ica - rich ▁vol can ics , ▁character ised ▁by ▁the ▁presence ▁of ▁bi ot ite ▁phen oc ry st s . ▁The ▁ma fic ▁series ▁also ▁contain ▁tit an ium ▁magnet ite ▁and ▁the ▁tra chy bas al ts ▁are ▁domin ated ▁by ▁an orth os ite - rich ▁pl agi oc l ase . ▁Rub id ium , ▁str ont ium ▁and ▁ph osph orus ▁pent ox ide ▁are ▁especially ▁rich ▁in ▁the ▁la vas ▁from ▁Tamb ora , ▁more ▁than ▁the ▁compar able ▁ones ▁from ▁Mount ▁R in j ani . ▁The ▁la vas ▁of ▁Tamb ora ▁are ▁slightly ▁en rich ed ▁in ▁z ir con ▁compared ▁to ▁those ▁of ▁R in j ani . ▁ ▁The ▁mag ma ▁involved ▁in ▁the ▁ 1 8 1 5 ▁er u ption ▁origin ated ▁in ▁the ▁mant le ▁and ▁was ▁further ▁modified ▁by ▁mel ts ▁derived ▁from ▁sub duct ed ▁sed iments , ▁flu ids ▁derived ▁from ▁the ▁sub duct ed ▁cr ust ▁and ▁cry st all ization ▁processes ▁in ▁mag ma ▁ch amb ers . ▁ 8 7 S r 8 6 S r ▁r ati os ▁of ▁Mount ▁Tamb ora ▁are ▁similar ▁to ▁those ▁of ▁Mount ▁R in j ani , ▁but ▁lower ▁than ▁those ▁measured ▁at ▁S ange ang ▁Api . ▁Pot ass ium ▁levels ▁of ▁Tamb ora ▁vol can ics ▁exceed ▁ 3 ▁weight ▁percent ,