This game has two modes; it can be played competetively by two players alternating modes, or by one player against a computer, or online against a worldwide pool of users.
In the first part, God Mode, you get to build a volcano. Your tools are a magma hotspot (which you can turn on and off), suspiciously
localized plate tectonics, and command over plants and weather. Use a more-or-less-realistic physics engine to go from a bare patch of land to an impressive mountain, complete (if you like) with forest, idyllic meadows, treacherous icy slopes, and glaciers. In real life this would take a few million years, but the game speeds it up for you - go ahead and run the (implied) local plaentary crust around at the speed of a jet airplane and watch it buckle! This portion is also educational, as it teaches about geology and weather patterns. Then - pick a spot, point the magma hotspot at it, and submit your design. Your goal is to make this mountain inescapable. When it blows its top - as it will very shortly - you want nothing to be able to get off the mountain alive.
In the second part, Human Mode, you are set down atop the mountain and view it in a virtual first-person-shooter style. Then the lava comes pouring out. Your goal is to make it to a desginated safe spot without being engulfed in lava and burnt to a crisp.
If playing against the computer, you can either run down predesigned mountains, or build them and let an AI run down them while you watch and cackle.