| Summary: | Time-lapse drilling of cored boreholes in 1979 and 2017 at Surtsey volcano, Iceland, provides a uniquely informative record of the material and mechanical heterogeneities that develop through interdependent processes of consolidation in very young, oceanic basalt. Integration of laboratory physical property measurements with chemical, mineralogical and microstructural analyses in core segments from subaerial, submarine and subseafloor lapilli tuff deposits, a basaltic intrusion and a xenolithic block of seafloor sedimentary rock reveal heterogeneous physical characteristics within 1963–1967 Surtur crater deposits. The mass of authigenic crystalline cements (zeolites, calcium-silicate-hydrates, sulfites, carbonates) and clay mineral(s) appears similar within the suite of lapilli tuff sub-cores. Connected porosity varies from 42% in weakly-consolidated tuff in a submarine inflow zone to 21% in strongly-lithified tuff in upper subseafloor deposits near the explosively-excavated Surtur conduit. Permeability varies over six orders of magnitude, 4.45 x 10 −19 – 5.03 x 10 −12 m 2 . Uniaxial compressive strength, P-wave velocity and thermal conductivity are also highly variable: 10–70 MPa, 1.48–4.31 km·s −1 and 0.472–0.862 W·m −1 ·K −1 , respectively. Permeability, micromechanical, and thermal property modeling highlight how porosity and pore-size in eruptive fabrics – modified through diverse cementing microstructures – determine the physical and mechanical properties of the lapilli tuff deposits. The analytical results provide reference parameters for geophysical and heat transfer studies, studies of volcano stability, predictions of rates of change in the physical characteristics of pyroclastic deposits as they lithify at the decadal scale, processes controlling where and when magma erupts, and the mechanical complexities of submarine and subseafloor deposits in Surtseyan volcanoes.
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