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At Kawah Ijen (Indonesia), vigorous SO 2 and HCl degassing sustains a hyperacid lake (pH ∼0) and intensely alters the subsurface, producing widespread residual silica and advanced argillic alteration products. In 1817, a VEI 2 phreatomagmatic eruption evacuated the lake, depositing a widespread layer of muddy ash fall, and sending lahars down river drainages. We discovered multiple types of opaline silica in juvenile low-silica dacite pumice and in particles within co-erupted laharic sediments. Most spectacular are opal-replaced phenocrysts of plagioclase and pyroxene adjacent to pristine matrix glass and melt inclusions. Opal-bearing pumice has been found at numerous sites, including where post-eruption infiltration of acid water is unlikely. Through detailed analyses of an initial sampling of 1817 eruption products, we find evidence for multiple origins of opaline materials in pumice and laharic sediments. Evidently, magma encountered acid-altered materials in the subsurface and triggered phreatomagmatic eruptions. Syn-eruptive incorporation of opal-alunite clasts, layered opal, and fragment-filled vesicles of opal and glass, all suggest magma-rock interactions in concert with vesiculation, followed by cooling within minutes. Our experiments at magmatic temperature confirm that the opaline materials would show noticeable degradation in time periods longer than a few tens of minutes. Some glassy laharic sedimentary grains are more andesitic than the main pumice type and may represent older volcanic materials that were altered beneath the lake bottom and were forcefully ejected during the 1817 eruption. A post-eruptive origin remains likely for most of the opal-replaced phenocrysts in pumice. Experiments at 25 • C and 100 • C reveal that when fresh pumice is bathed in Kawah Ijen hyperacid fluid for 6 weeks, plagioclase is replaced without altering either matrix glass or melt inclusions. Moreover, lack of evidence for high-temperature annealing of the opal suggests that post-eruption alteration of pumice is more likely than pre-eruption envelopment of Lowenstern et al. Opal in Glassy Pumice euhedral opal-replaced phenocrysts in dacitic melt. At Ijen and elsewhere, the ascent of magma into hydrous acid-altered mineral assemblages (e.g., opal, kaolinite, alunite) could induce rapid dehydration of hydrous minerals and amorphous materials, generating considerable steam and contributing to magmatic-hydrothermal and phreatomagmatic explosions.

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Silicification is the most important process of fossilization resulting in the preservation of internal tissues in plants, thereby providing essential information on the anatomy, life history, and evolution of land plants. However, fundamental knowledge of silica uptake, precipitation, and contribution to in situ plant fossilization is limited. To identify the cellular pathway of aqueous silica and subsequent fluid–wood interaction processes, we investigated upright standing young trees of lodgepole pine (Pinus contorta) in the hot‐spring environment of Cistern Spring, Yellowstone National Park, USA. Our multi‐method analytical approach using x‐ray diffraction, Raman spectroscopy, scanning electron microscopy, and electron probe microanalyses shows that the surficial and internally co‐precipitated silica–halite–gypsum assemblage traces the flow of silica‐rich fluid and documents fluid retention after short‐term hot‐spring water immersion and evaporation. Element distribution maps reveal systematic 2D and 3D differences between silica quantities deposited in earlywood and latewood. The distribution of inorganic impurities in cell walls traces the anatomical structure of the wood and indicates rapid migration of homogeneous fluid into the waterlogged organic substrate. Our results show that the preferential pathway of silica‐rich fluid into the above‐ground wood was through the decorticated periphery which took place during a short‐term flooding, ranging from days to weeks, of hot‐spring fluid. In conclusion, the fluid retention capability in cellular pore space controls the in situ silicification process. The silicification of trees in growth position is a rapid process in which the in vivo transport of silica‐rich fluid upward through the secondary xylem plays an insignificant role.