The mud volcano of Unzen Jigoku is formed by white mud made from melted rocks containing high-temperature volcanic gas including hydrogen sulfide, which rises up with steam and forms a shape resembling a volcano. Volcanoes can be formed by lava that erupts from the crater and cools and hardens to create a mountain, or by the stickiness of lava that forms the mountain, so the shape varies greatly depending on the mountain. In fact, the same phenomenon occurs with mud volcanoes. Mud volcanoes are washed away by rain and each time they erupt, creating a different shape.