Why fire come out of some mountains ?

These are volcanoes. A volcano is not a real mountain. First there is a hole in the crust, called lava, where a liquid flame from the depths of the soil gushers violently. 

After the lava flows, it becomes cold and hardens around the hole. They accumulate and make up the volcano cone.

Beneath the thin rock layer, which consists of the crust of the ground, there is a region where the substances, called magma, are more or less fluency, at temperatures above 1000 degrees Celsius. 

Enormous pressures that influence the bottom-up form deformities in the outer shell. Sometimes even, deep and wide cracks occur. From these cracks, the melted substances begin to erupate. So a volcano is born. This lava, which is usually incandescent, freezes when it flows through the craters and slopes.