▁bas alt ▁( M OR B ) ▁is ▁a ▁th ole i it ic ▁bas alt ▁commonly ▁er upt ed ▁only ▁at ▁ocean ▁rid ges ▁and ▁is ▁character ist ically ▁low ▁in ▁in compatible ▁elements . ▁E - M OR B , ▁en rich ed ▁M OR B ▁N - M OR B , ▁normal ▁M OR B ▁D - M OR B , ▁de ple ted ▁M OR B ▁▁ ▁High - al um ina ▁bas alt ▁may ▁be ▁sil ica - und ers atur ated ▁or ▁- o vers atur ated ▁( see ▁norm ative ▁min eral ogy ). ▁It ▁has ▁greater ▁than ▁ 1 7 % ▁al um ina ▁( Al 2 O 3 ) ▁and ▁is ▁intermediate ▁in ▁composition ▁between ▁th ole i it ic ▁bas alt ▁and ▁alk ali ▁bas alt ; ▁the ▁relatively ▁al um ina - rich ▁composition ▁is ▁based ▁on ▁rocks ▁without ▁phen oc ry st s ▁of ▁pl agi oc l ase . ▁ ▁Al k ali ▁bas alt ▁is ▁relatively ▁poor ▁in ▁sil ica ▁and ▁rich ▁in ▁sod ium . ▁It ▁is ▁sil ica - und ers atur ated ▁and ▁may ▁contain ▁f eld sp ath oid s , ▁alk ali ▁f eld s par ▁and ▁ph log op ite . ▁ ▁Bon in ite ▁is ▁a ▁high - m agn es ium ▁form ▁of ▁bas alt ▁that ▁is ▁er upt ed ▁generally ▁in ▁back - arc ▁bas ins , ▁distinguished ▁by ▁its ▁low ▁tit an ium ▁content ▁and ▁trace - element ▁composition