0   km ▁and ▁contains ▁four ▁major ▁strat ov ol cano es : ▁Mount ▁O ku , ▁Mount ▁Bab ank i ▁ 1 5   km ▁SW ▁of ▁O ku , ▁Ny os ▁and ▁N k am be . ▁Ro cks ▁in ▁the ▁mass if ▁have ▁ages ▁from ▁ 2 4 . 9 ▁to ▁ 2 2 . 1 ▁million ▁years ▁ago , ▁but ▁more ▁recent ▁activity ▁has ▁occurred . ▁▁ ▁The ▁mass if ▁is ▁composed ▁of ▁r hy olit ic ▁and ▁tra ch yt ic ▁rock , ▁and ▁contains ▁many ▁ma ars ▁and ▁bas alt ic ▁c inder ▁con es . ▁ ▁The ▁O ku ▁Vol can ic ▁Field ▁includes ▁two ▁cr ater ▁la kes , ▁Lake ▁Ny os ▁to ▁the ▁north ▁and ▁Lake ▁Mon oun ▁to ▁the ▁south . ▁On ▁ 1 5 ▁August ▁ 1 9 8 4 , ▁an ▁earth qu ake ▁and ▁lands lide ▁triggered ▁a ▁major ▁release ▁of ▁carbon ▁dio x ide ▁from ▁Lake ▁Mon oun , ▁killing ▁several ▁people . ▁ ▁Lake ▁Ny os ▁is ▁inside ▁a ▁maar ▁formed ▁by ▁an ▁explos ion ▁about ▁ 4 0 0 ▁years ▁ago , ▁and ▁is ▁about ▁ 1 , 8 0 0   m ▁wide ▁and ▁ 2 0 8   m ▁deep . ▁There ▁is ▁a ▁low - temper ature ▁res er voir ▁of ▁free ▁carbon ▁dio x ide ▁below ▁the ▁lake ▁bottom , ▁which ▁see ps ▁into ▁the ▁lake ▁through ▁a ▁vol can ic ▁pipe . ▁On ▁ 2 1 ▁August ▁ 1 9 8 6