ards ; ▁and , ▁" warning ", ▁if ▁a ▁highly ▁ha z ard ous ▁er u ption ▁is ▁under way ▁or ▁im min ent . ▁These ▁levels ▁reflect ▁conditions ▁at ▁a ▁vol cano ▁and ▁the ▁expected ▁or ▁on going ▁ha z ard ous ▁vol can ic ▁phen omena . ▁When ▁an ▁alert ▁level ▁is ▁assigned ▁by ▁an ▁observ atory , ▁accomp any ing ▁text ▁will ▁give ▁a ▁full er ▁explanation ▁of ▁the ▁observed ▁phen omena ▁and ▁clarify ▁ha z ard ▁imp lications ▁to ▁affected ▁groups . ▁ ▁Sum mary ▁of ▁Vol can ic ▁Activity ▁Alert ▁Notification ▁System ▁ ▁A viation ▁color ▁codes ▁ ▁Ear lier ▁vol cano ▁warning ▁schemes ▁for ▁the ▁United ▁States ▁ ▁Prior ▁to ▁October ▁ 2 0 0 6 , ▁three ▁parallel ▁Vol cano ▁warning ▁schemes ▁were ▁used ▁by ▁the ▁United ▁States ▁Ge ological ▁Survey ▁and ▁the ▁vol cano ▁observ ator ies ▁for ▁different ▁vol cano ▁ranges ▁in ▁the ▁United ▁States . ▁They ▁each ▁have ▁a ▁base ▁level ▁for ▁d orm ant - qu ies cent ▁states ▁and ▁three ▁gr ades ▁of ▁alert . ▁ ▁Color ▁Code ▁Cond itions , ▁Long ▁Valley ▁Cal der a ▁and ▁Mon o - In yo ▁C rat ers ▁Region , ▁California ▁Develop ed ▁in ▁ 1 9 9 7 ▁to ▁replace ▁a ▁previous ▁ 5 - level ▁system ▁de vised ▁in ▁ 1 9 9 1 . ▁ ▁Level ▁of ▁Conc ern ▁Color ▁C odes ▁for ▁vol cano es ▁in ▁Al aska ▁ ▁The ▁Al aska ▁Vol cano ▁Observ atory ▁( AV O )