If you are in Mauville city, just go up right next to where the pokemon center is, and if you keep going for awhile (there will be a lot of trainers) there will be this little building. Go inside and talk with the lady and she will send you up to a place called Mt. Chimney. After you fight some grunts you will have to fight Maxie team magma's leader.
he is hard use revive to your pokemon....................
Maxie will be in the tunnel in Mt Chimney for Emerald.
you have to go to team magmas base and deafeat there leader in a battle.
You use it to get groundon activated
To get past the boulders you have to let one of your Pokemon to learn the move strength.
you have to catch groundon
you can find him at the top of the volcano a couple gyms after you fight the leader of team magma
after getting into team magmas hideout
a master ball
Use Strength.
Go through the jagged path and go through the cave opening u should be in team magmas base
it is at mt chimney