The ∼1000-years BP explosive eruption of Caldeira Volcano (Faial, Azores): the first stage of incremental caldera formation

Abstract

The ∼1000-years BP eruption of Caldeira Volcano (Faial Island) was one of the last major explosive events recorded in the Azores. It produced a complex succession of pyroclastic deposits, known as the C11, divided into three members. At the base is the Brejo Member, a sequence of fine- to coarse-grained parallel-bedded ash layers found in the NW sector of the island. The middle part corresponds to the Inverno Member, a coarse-grained massive pumice fall deposit, restricted to the north flank of Caldeira Volcano. The top is dominated by the Cedros Member which includes massive to diffuse-stratified lapilli-ash and lithic breccias, exposed along the north and east flanks of the volcano. A minimum bulk volume of at least 0.22 km³ (>0.1 km³ dense rock equivalent (DRE)) is estimated for the C11 eruption, although a large portion may have been deposited offshore. The juvenile products are trachytic (59 wt% SiO₂) with a homogenous whole-rock composition and mineral assemblage throughout the pyroclastic succession. However, petrographic and groundmass glass analyses indicate magma mingling/mixing processes between two trachytic batches. The C11 eruption history is divided into three phases (following the member division) with distinct eruptive styles: (1) an initial phreatomagmatic phase caused by rising magma (∼950 °C) encountering a crater pond or aquifer, (2) a fall-dominated phase which established a sub-Plinian column up to 14 km high (mass eruption rate (MER) of 1.2 × 10⁷ kg/s) and (3) prolonged pyroclastic fountaining and sustained quasi-steady pyroclastic density current generation followed by summit collapse. The C11 eruption is interpreted as the first stage in the formation of an incremental caldera. This study provides valuable insights for a better understanding of small but complex explosive eruptions and their impact on ocean islands.

Appendix

Grain size, component and morphologic analyses

Twenty-five samples were sieved at 0.5 ϕ intervals (ϕ = −log₂ d, where d is grain size on mm) in the range −6 to 5 ϕ (64–0.032 mm). Coarser grain sizes (−6 to −3 ϕ) were gently hand-sieved in the field to avoid artificial breakage of pumice clasts and generation of fine ash by abrasion. The fractions were weighed to 0.1 g. Finer grain sizes (−2.5 to 5 ϕ) were dried in an oven for 24 h and split into three subsamples of 100 cm³. Each batch was mechanically sieved and weighed to 0.0001 g. Grain size parameters of Inman (1952) and Folk and Ward (1957) were calculated. To facilitate comparison among different samples, fractions coarser than −6 ϕ were not considered.

Componentry analyses were carried out on selected samples. From −6 to −3 ϕ, the components were separated in the field into pumice and lithic clasts. Finer fractions, −2.5 to 1 ϕ, were separated in the laboratory with the naked eye or under a binocular microscope into pumice clasts, lithic clasts and crystals. For each size class, a minimum of 800 grains was separated and weighed to 0.0001 g.

Morphologic analysis of fine-grained ash particles was performed by scanning electron microscope (SEM) imaging. Prior to image acquisition, ash particles from six representative samples in the range between 4 and 5 ϕ were selected and mounted onto stubs. The images were acquired with a JEOL-5410 SEM, at the Departamento de Biologia/Centro de Investigação dos Recursos Naturais of the University of Azores (Portugal), operating at secondary electron mode with an acceleration voltage of 15 kV.

Petrography, whole-rock geochemistry and mineral chemistry analyses

The petrographic features and modal analyses of eight unaltered pumice clasts were obtained by thin section observation. On each thin section, a minimum of 800 points was counted.

Whole-rock geochemical analyses of major and trace elements of six samples, representative of the juvenile products from the three members, were carried out at Activation Laboratories Ltd., Ontario (Canada) by lithium metaborate/tetraborate fusion inductively coupled plasma (FUS-ICP) and inductively coupled plasma mass spectrometry (ICP-MS) techniques (following code 4LITHO). Further information on the analytical methods is available at the Activation Laboratories website (www.actabs.com). Reproducibility for major and trace elements is commonly assessed to be better than 10 %.

Electron microprobe analyses were performed on eight samples with a JEOL JXA 8200 Superprobe, at the Dipartimento di Scienze della Terra “Ardito Desio” of the University of Milan (Italy). A spot size of 1 μm with a current of 15 nA was used for all mineral phases, except alkali feldspars (5 μm spot size and 5 nA current). Groundmass glass was analysed with a ∼10-μm-wide defocused beam and a current of 2–4 nA. Count times for major elements were 30 s on the peak and 10 s on each background. Typical detection limit for each element is 0.01 %.