Volcanism (or volcanicity) is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics and volcanic gases erupt through a break in the surface called a vent.

What is the cause of Vulcanicity?

They are caused by rainwater seeping through cracks in the rocks and draining into a deep chamber where it reaches hot rocks. This water is boiled and turns into steam. This increases the pressure in the chamber until it shoots upwards through a vent and then high in the air.

What causes a stratovolcano to form?

An eruption of highly viscous (very sticky) magma tends to produce steep-sided volcanoes with slopes that are about 30–35°. That's because the viscous volcanic material doesn't flow that far from where it is erupted, so it builds up in layers forming a cone-shaped volcano known as a stratovolcano.