After Team Magma/Aqua steals the meteorite from Professor Cozmo, you ride the Route 112 cable car to Mt. Chimney to pursue them.
Explore the Mountain
On Mt. Chimney, look behind the cable car building with your Dowsing Machine to find a hidden Zinc.
Go east and up the stairs, then go west from there. You will find the leader of Team Magma/Aqua blocked by Grunts from the opposing team.
Go west and north from that group and you will encounter a Team Magma/Aqua Grunt. In Omega Ruby, this grunt has Poochyena and Koffing. In Alpha Sapphire, the grunt has Poochyena and Grimer.
Go northwest from this grunt and you will find a small staircase. Follow the path to go down close to the volcano crater, where you find TM59 Incinerate.
Go back the way you came and go east. In Omega Ruby, you will encounter Magma Admin Tabitha, who has Koffing and Numel. In Alpha Sapphire, you will encounter Magma Admin Shelly, who has Grimer and Carvanha.
After that battle, go east, and you will reach the leader of Team Magma/Aqua. The leader talks about Mega Stones and Key Stones.
In Omega Ruby, you battle Maxie, who has Camerupt, Golbat, and Mightyena.
In Alpha Sapphire, you battle Archie, who has Mightyena, Golbat, and Sharpedo.
Afterward, the leader gets a call, and it sounds like Mt. Pyre is his team's next destination. He gives you the Meteorite, then leaves. The other leader approaches you and realizes that his opponent has found the "orbs". Then he also leaves.
Before you go, use the Dowsing Machine in the area east of the volcano crater to find a hidden Protein.
The next time you visit this area, there will be an old woman next to the cable car building who can heal your pokémon. If you want to heal your pokémon now, just leave the area and come right back.
Your next destination is Jagged Pass. If you are still using the Mach Bike, you might want to ride the cable car back down and go to Mauville City to switch to the Acro Bike. There are ledges in Jagged Pass that you can only go back up if you have the Acro Bike. If you have the Mach Bike instead, Jagged Pass is a one-way path.
When you are ready, go south from Mt. Chimney to Jagged Pass.