Compaction and creep deformation of volcaniclastic material during burial

Volcaniclastic deposits compact during accumulation and burial, causing prolonged ground deformation, and potentially affecting edifice stability. Here we present the results of laboratory compaction experiments in which volcaniclastic material (in a confining cup) was progressively loaded with 0.1 MPa/min up to 20 MPa. We tested two lithologies consisting of natural basanitic scoria and crushed hyaloclastite, each sieved to a set of two grain sizes in the ash (0.5-2 mm) and lapilli (2-4 mm) range. During experiments, axial deformation and acoustic emissions were monitored, enabling us to quantify the progressive volume reduction and material properties. Two types of experiments were performed: dynamic loading tests and static creep test. The dynamic tests show that most of the deformation takes place within the first few MPa, decreasing non-linearly with load. Ultimately, samples compacted by up to 50 vol.% at 20 MPa (here equivalent to ~1,400 m depth). We use this compaction data to build a model of compaction and material properties as function of burial depth.

In repeat experiments, static tests were undertaken at select target loads (2, 5, 10 and 20 MPa) to measure time-dependent creep deformation at these loads over 6 hours. We find that compaction continues under static load as creep occurs in stages, displaying (1) initially rapid decline in compaction strain rates, which (2) then diminish more slowly over time and (3) eventually reach stable creep strain rates in most of our tests. Moreover, both total creep strains and stable creep rates are dependent on the applied load. Stable strain rates were highest between 5 and 10 MPa for all samples. The data shows that the lithology also influences the deformation behavior as we found the hyaloclastites compacted more efficiently during initial loading compared to the scoria, but in-turn creep strain rates were nearly an order of magnitude lower due to the more efficient compaction during loading. Our results highlight the relevance of gravitational material compaction for investigations of ground deformation and volcano flank instability (e.g., measured with InSAR) and introduce new material constraints to improve the interpretation and analysis of such signals. Deposit-specific compaction data may be integrated with ground deformation monitoring to interpret flank instabilities and assess collapse hazards, particularly in eruptions where deposition of new materials rapidly shifts overburden stresses.