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Silicic volcanic activity has long been framed as either violently explosive or gently effusive. However, recent ob- servations demonstrate that explosive and effusive behavior can occur simultaneously. Here, we propose that rhyolitic magma feeding subaerial eruptions generally fragments during ascent through the upper crust and that effusive eruptions result from conduit blockage and sintering of the pyroclastic products of deeper cryptic frag- mentation. Our proposal is supported by (i) rhyolitic lavas are volatile depleted; (ii) textural evidence supports a pyroclastic origin for effusive products; (iii) numerical models show that small ash particles !10−5 m can diffusive- ly degas, stick, and sinter to low porosity, in the time available between fragmentation and the surface; and (iv) inferred ascent rates from both explosive and apparently effusive eruptions can overlap. Our model reconciles previously paradoxical observations and offers a new framework in which to evaluate physical, numerical, and geochemical models of Earth’s most violent volcanic eruptions. 

The di!usion of water through silicate melts is a key process in volcanic systems. Di!usion controls the growth of the bub- bles that drive volcanic eruptions and determines the evolution of the spatial distribution of dissolved water during and after magma mingling, crystal growth, fracturing and fragmentation, and welding of pyroclasts. Accurate models for water di!u- sion are therefore essential for forward modelling of eruptive behaviour, and for inverse modelling to reconstruct eruptive and post-eruptive history from the spatial distribution of water in eruptive products. Existing models do not include the kinetics of the homogeneous species reaction that interconverts molecular (H2Om) and hydroxyl (OH) water; reaction kinetics are impor- tant because final species distribution depends on cooling history. Here we develop a flexible 1D numerical model for di!usion and speciation of water in silicate melts. We validate the model against FTIR transects of the spatial distribution of molecular, hydroxyl, and total water across di!usion-couple experiments of haplogranite composition, run at 800–1200 C and 5 kbar. We adopt a stepwise approach to analysing and modelling the data. First, we use the analytical Sauer-Freise method to deter- mine the e!ective di!usivity of total water DH2Ot as a function of dissolved water concentration CH2Ot and temperature T for each experiment and find that the dependence of DH2 Ot on CH2 Ot is linear for CH2 Ot K 1:8 wt.% and exponential for CH2 Ot J 1:8 wt.%. Second, we develop a 1D numerical forward model, using the method of lines, to determine a piece-wise function for DH2 Ot !CH2 Ot ; T " that is globally-minimized against the entire experimental dataset. Third, we extend this numerical model to account for speciation of water and determine globally-minimized functions for di!usivity of molecular water DH2 Om !CH2 Ot ; T " and the equilibrium constant K for the speciation reaction. Our approach includes three key novelties: (1) functions for dif- fusivities of H2Ot and H2Om, and the speciation reaction, are minimized simultaneously against a large experimental dataset, covering a wide range of water concentration (0:25 CH2 Ot 7 wt.%) and temperature (800  C T 1200  C), such that the resulting functions are both mutually-consistent and broadly applicable; (2) the minimization allows rigorous and robust analysis of uncertainties such that the accuracy of the functions is quantified; (3) the model can be straightforwardly used to determine functions for di!usivity and speciation for other melt compositions pending suitable di!usion-couple experiments. The modelling approach is suitable for both forward and inverse modelling of di!usion processes in silicate melts; the model is available as a MATLAB script from the electronic supplementary material.