Caldera size modulated by the yield stress within a crystal-rich magma reservoir

The largest volcanic eruptions in the geologic record have no analogue in the historical record. These eruptions had global impacts1,2, but are known only through their eruptive products. They have left behind calderas that formed as the surface collapsed when eruption evacuated magma chambers at 5–15 km depths3,4. It is generally assumed that calderas reflect the spatial dimensions of underlying magma reservoirs. Here we use a numerical model of conduit flow and dynamic magma-chamber drainage to show that caldera size can be affected by the material properties of crystal-rich silicic magma. We find that magma in the chamber can experience a rheological transition during eruption. This transition causes magma near the conduit to behave as a fluid, whereas magma farther away behaves elastically and remains locked. The intervening surface—the yield surface—expands through the chamber as eruption progresses. If a yielding transition occurs, calderas can form before complete mobilization of the entire reservoir. The resulting distribution of eruption volumes is then bimodal, as observed in the geologic record. We suggest that the presence or absence of a magma yield stress determines whether caldera size reflects the true spatial extent of magma storage. Magma chambers function both as repositories for melt rising through the crust and as reservoirs that feed individual volcanic eruptions. During silicic caldera-forming eruptions, these functions occur on vastly different timescales as many cubic kilometres of magma gradually assembled and maintained at high crystal fraction5 in the crust are probably erupted in hours to days2,6. Such eruptions have global impacts and leave behind calderas 10–100 km in diameter as evidence of contiguous magma chambers at depth4. Petrologic evidence from supereruptions (>500 km3 erupted1) as well as smaller recent eruptions such as Pinatubo indicates that these magma reservoirs are incrementally assembled over timescales that vary from 102 to 105 years6–8. Mobilization of this reservoir and eruption triggering may be caused by the injection of hot, volatile-rich,maficmagma to the base of the locked crystal mush9,10. However the relationship between the initiation of eruption and the pre-eruptive development of these systems remains a challenge to constrain. Our study is motivated by the observation that many magmas from caldera-forming eruptions, such as the Fish Canyon tuff4 and Atana ignimbrite3, are crystal rich. Crystal fractions in these ignimbrites approach the maximum packing limit11 where a rheological (yield stress) transition from liquidto solidlike behaviour occurs. Many other smaller deposits have crystal fractions in the 10–30% range, in which connected networks of crystals may impart the suspension with an effective yield stress on eruptive timescales12,13. We focus on caldera-forming silicic eruptions because these events have a significant impact on other Earth systems14 and because large erupted volumes minimize