As the kimberlite magma approaches the earth's surface, an explosive reaction takes place between groundwater, the molten kimberlite, and dissolved gases in the kimberlite. An explosive volcanic eruption follows, resulting in the formation of a kimberlite pipe or vent.
In the classic South African model, the kimberlite magma begins at depth as a series of dykes, sills, and blows that flare into a tapered carrot-shaped subvolcanic "diatreme" about one kilometre below the surface. Due to explosive release of pressure near the surface, a kimberlite crater is developed which is filled by shallow crater-facies deposits.
Kimberlite volcanoes have breached the crust throughout geological history with the youngest event recognized in Canada as about 50 million years before present.