Can Degassing-induced Undercooling and Crystallisation Control Eruptible Magma Volume?

Assessing the likelihood, intensity, style, and magnitude of eruptions is vital for societies living near active volcanoes worldwide. The intensity and magnitude of volcanic eruptions are controlled by multiple factors, but magma degassing upon decompression plays a critical role, causing growth of crystals that eventually lock up the magma. H₂O–CO₂ fluid composition modulate magma undercooling and crystallisation with H₂O degassing increasing the liquidus temperature and CO₂ degassing decreasing it. Using published experiments, we correlate magma undercooling with the crystal volume fraction and evaluate empirically the conditions that favour ascent of crystal-poor versus crystal-rich magmas. Magma crystallinity and undercooling are then examined for previous mafic and felsic eruptions with known erupted volumes, crystallinity of erupted tephra, and released excess SO₂. The latter parameter is suggestive of excess fluid in the subvolcanic reservoir prior to eruption. We observe that H₂O-rich systems with crystal volume fractions > 0.2 and undercooling > 100 °C tend to erupt ≤ 5 km³ of magma, whilst CO₂-rich systems with crystal fractions < 0.2 and undercooling < 110 °C erupt > 10 km³ of magma. Our results suggest that the composition of magmatic fluids exercises an important control on eruptible volumes by suppressing or enhancing decompression crystallisation.