Magma and pressure
The mantle plume’s heat makes magma, which the deep and the shallow chamber collect. The pressure gauge shows when you erupt, and how big the VEI will be.
The heat of the mantle plume turns into magma, which collects in the deep and the shallow chamber. When the pressure reaches its critical value, the mountain erupts: it grows by a layer, and gives you tephra, the game’s currency, to spend on upgrades. You don’t build the mountain from the outside, but from below.
After an eruption a village settles at the foot, and when the alarm sounds, you decide whom to evacuate. The type of eruption changes too: lava flow, ash, pyroclastic flow, lahar, tsunami. The mountain develops through phases: caldera, hotspot migration, island chain.
