Lagour

The Flaming Mountain of Lagour is a mentioned landscape feature of Calormen. According to The Tale of Azaroth's Son, the mountain first began to burn during a war between a Tisroc and the son of Azaroth, a Calormen god. It is somewhere west of Tashbaan.

Volcanic Activity
Mount Lagour may have been one of the only active volcanoes in the world of Narnia. It was likely similar to a cinder cone volcano in structure, and could have been continuously erupting. These eruptions could have lasted for centuries, hence the indication of prolonged volcanic activity in the mountain's description.

Trivia
It is not known if the mountain was real or ledgendary. The description of the flaming mountain of lagour is similar to the biblical description of Mt. Sinai: "And mount Sinai was altogether on a smoke, because the Lord descended upon it in fire: and the smoke thereof ascended as the smoke of a furnace, and the whole mount quaked greatly". ―Exodus 19:18