▁than ▁those ▁of ▁other ▁common ▁ igne ous ▁rocks . ▁ ▁The ▁majority ▁of ▁th ole i it ic ▁bas al ts ▁are ▁formed ▁at ▁approximately ▁ 5 0 – 1 0 0   km ▁depth ▁within ▁the ▁mant le . ▁Many ▁alk ali ▁bas al ts ▁may ▁be ▁formed ▁at ▁greater ▁depth s , ▁perhaps ▁as ▁deep ▁as ▁ 1 5 0 – 2 0 0   km . ▁The ▁origin ▁of ▁high - al um ina ▁bas alt ▁continues ▁to ▁be ▁controvers ial , ▁with ▁disag re ement ▁over ▁whether ▁it ▁is ▁a ▁primary ▁m elt ▁or ▁derived ▁from ▁other ▁bas alt ▁types ▁by ▁fraction ation . ▁ ▁Ge och em istry ▁ ▁Rel ative ▁to ▁most ▁common ▁ igne ous ▁rocks , ▁bas alt ▁compos itions ▁are ▁rich ▁in ▁M g O ▁and ▁Ca O ▁and ▁low ▁in ▁Si O 2 ▁and ▁the ▁alk ali ▁ox ides , ▁i . e ., ▁Na 2 O ▁+ ▁K 2 O , ▁consistent ▁with ▁the ▁T AS ▁classification . ▁ ▁Bas alt ▁generally ▁has ▁a ▁composition ▁of ▁ 4 5 – 5 5 ▁w t % ▁Si O 2 , ▁ 2 – 6 ▁w t % ▁total ▁alk alis , ▁ 0 . 5 – 2 . 0 ▁w t % ▁Ti O 2 , ▁ 5 – 1 4 ▁w t % ▁Fe O ▁and ▁ 1 4 ▁w t % ▁or ▁more ▁Al 2 O 3 . ▁Cont ents ▁of ▁Ca O ▁are ▁commonly ▁near ▁ 1 0 ▁w