Like the chocolate coating on the candy bar, the surface of the earth develops cracks called faults. It is one of these faults which has allowed magma to rise to the surface at Craters of the Moon. The magma underground is full of dissolved gases, like soda in a bottle of pop. It is dissolved gases that make a soda fizzy. We all know what happens when you shake up a bottle of pop and then take the top off.
Volcanoes work the same way.
The dissolved gases start to expand. Pressure builds and the volcano blows its top and erupts to release the pressure of the expanding gas. These fountains resembled the famous geyser Old Faithful but, instead of steam and water, they erupted melted rock. The frothy bits of lava shower the ground and build up large cones of cinders. The cinders are light weight and contain many air bubbles, like Rice Krispies.