A 13‐month flank eruption of Fayal volcano occurred in 1957 and 1958. Intermittent violent explosions (phreatomagmatic) and ejection of ash characterized the eruption during its first 8 months. Following a 2‐day intense seismic period, Capelinhos volcano entered an effusive phase which continued during the last 5 months of activity; Strombolian activity occurred in the crater vent, and a number of lava flows issued from the base of the cone. An outer, semicircular cinder cone, an inner spatter cone, and associated tephra deposits added 2.5 km2 of new land to Fayal Island. Total energy dissipated during the eruption was estimated to be 4×1021 ergs. Differential movements or warping of west Fayal occurred during the seismic crisis. Tephra and lava composition is olivine basalt. Lava viscosity was computed to be between 3×104 and 5×106 poises. Eruption sounds and flashing arcs were tape recorded and analyzed by an audio and subaudio spectrograph; Strombolian‐type eruptions produced essentially broadland sound, explosions had maximum energy below 50 cps, and the dominant recorded energy of flashing arcs was between 30 and 50 cps.
Criteria for the recognition of different types of pyroclastic and related rocks, especially breccias, in the vent or cone-complex facies of volcanic provinces are reviewed. These criteria include structural features, character of the fragments, and the composition and texture of the groundmass of the volcanic rocks. Autobrecciation of lava flows produces monolithologic autoclastic volcanic breccias with angular, lithic, unsorted fragments, usually with a central zone or lens of nonbrecciated material. Underground brecciation of previously consolidated material and subsequent extrusion as breccia flows deposit thick, unstratified, unsorted, heterolithologic, nonglassy, nonvesicular, chaotic volcanic breccias. Pyroclastic flows (nuee ardentes) are responsible for coarse blocky breccias, pumice breccias, and ash-flow tuffs, all of which are unsorted, unbedded, and monolithologic. Their fragments contain much glassy vesicular material, especially pumice fragments of all sizes. The crumbling of domes and spines produces a type of glowing avalanche deposit of limited lateral extent and with a larger proportion of dense, angular fragments to vesicular fragments. The deposits of volcanic mudflows or lahars are unsorted, unbedded, heterolithologic, chaotic tuff breccias of great lateral extent. Many pyroclastic rocks are formed by the explosive ejection of tephra with subaerial deposition. Vulcanian eruptions produce poorly stratified, unsorted, heterolithologic, chaotic tuff breccias as a part of volcanic cone structure. These breccias contain angular, accessory, and accidental fragments and subrounded juvenile materials. Strombolian and lava fountain eruptions form the common cinder and spatter cones of the world. These deposits are sorted, have graded and mantle 263 on July 27, 2015 memoirs.gsapubs.org Downloaded from 264 IGNEOUS AND MET AMORPHIC GEOLOGY bedding, and are monolithologic. Their fragments are subrounded, vesicular, cindery, and contain breadcrust and other bombs; the resulting breccias include the true agglomerates. Phreatic eruptions with deposition on land produce breccias similar to vulcanian-eruption deposits if no new magma is present. With new magma these eruptions are phreatomagmatic and build saucer-shaped tuff cones of graded, mantle-bedded, essential, glassy granules, usually palagonitized. Subaqueous breccia flow deposits may be formed from subaerial eruptions in which deposition only is subaqueous or both eruption and deposition may be subaqueous. Terrestrial eruptions with subaqueous deposition result in chaotic tuff breccias with poor stratification, low initial dips, and no sorting in coarse-grained beds, but with interbedded stratified lenses of tuffs or volcanic sandstones which show the minor structures of turbidity-current deposits. Phreatic submarine eruptions produce breccias with very similar structural relations but with a much larger amount of glassy granular material, usually palagonitized. Brecciation of lava flows under water produces pillow breccias or hyaloclastites. These gl...
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