In 2009, exploratory drilling of geothermal wells in Iceland’s Krafla volcanic caldera unexpectedly struck magma. The fact that the encounter didn’t have catastrophic consequences has excited considerable interest - and an international research facility is now being set up to explore energy generation and other possibilities of closer engagement with magma. We take this event as an incitement to explore how the Earth-changing `violence’ of volcanic or igneous processes might be seen not simply as happening in time, but as both generative and destructive of time itself. We approach volcanism through the construct of a `speculative geology’ that draws on a recent return to metaphysical themes in philosophy as well as a growing interest in geological processes in the arts, humanities and popular culture. In this way, alongside cause-effect relations, we explore the more enigmatic processes through which subterranean geological forces offer an excessive potentiality from which humans and other life forms select and actualise a narrower range of creative or generative possibilities. The paper explores three significant volcanic episodes: a series of massive magma extrusions around 1.9 billion years ago linked to the ascendance of multicellular life, volcanism present in the East African Rift during pivotal phases of human evolution and the volcanic activity of the early-mid Holocene viewed as a contextual factor in the emergence of ancient practices of artisanal pyrotechnology. Our reading of the dynamic and violent interchange between the inner and outer Earth in these examples points to a non-self-identical planetary condition, on which the very structure of temporality emerges through a play of destruction and generativity. In this light, we circle back on the Krafla project to consider questions of risk, uncertainty and responsibility that attend the potential new interface with the underworld of magma.