Green Blast Shield
The green blast door is a door that can only be destroyed by a super missile. It is seen across the whole Metroid series and has gone through several changes. It appears as an energy field in the earlier part of the series (except Metroid Fusion, as it is seen as a higher level lock). It is then seen as a metal lock in the Prime series (except Metroid Prime 3, which doesn't have super missiles). They are mainly used to keep one from going to a specific place too early, or it some times hides expansions. Unlike the normal red blast shield, that hides save stations.