In Pokemon Yellow, How can i get in Blaine's Gym to get the Volcano Badge?

In Cinnabar Island i surf to get in that island...I gather the Boulder Badge,Cascade Badge,Thunder Badge,Rainbow Badge,Soul Badge and MarshBadge.The next badge to get is Volcano Badge.I cant get inside of Blaine's Gym becauz the door is lock.What's the key to get inside of the Gym??PLZ tell me