Criteria for the Recognition of Volcanic Breccias: Review

Abstract

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...

Field guide to geologic excursions in southwestern Utah and adjacent areas of Arizona and Nevada

“…Where the clasts in the mudflows and the lithologically similar flows are distinctive, the rock units made up of these rocks were mapped separately and given formal or informal names. Where such stratovolcano units were mapped, they commonly were separated into either vent facies or alluvial facies, following the concepts of Parsons (1969) and Smedes and Prostka (1973). The vent facies, as the name implies, consists of deposits that are concentrated near vents and high on volcano flanks, whereas the alluvial facies is concentrated lower on the flanks and beyond the flanks, becoming a grain-supported conglomerate with distance from the source.…”

Tectonic implications of early silurian thrust imbrication of the northern exploits subzone, Central Newfoundland

“…Mafic volcanic breccias are assigned a hyaloclastite origin (cf. Parsons, 1968;Sense Rittmann, 1958) because some of the angular fragments are clearly identifiable as pillow fragments.…”

The Lord Howe Rise Rhyolites

“…It should be stressed again, however, that the core diameter is only 64 mm, which happens to be exactly the size boundary between lapilli and blocks (and bombs). Parts of the core which look like lava flows may well be large blocks, which, according to Parsons (1969), can be up to several hundred feet across in volcanic breccias.…”

“…Submarine basaltic extrusions have received a great deal of attention, and the presence, for instance, of pillow lavas and palagonitic hyaloclastites seems to be diagnostic for submarine extrusion of basalts (e.g. Nayudu, 1962;McBirney, 1963;Parsons, 1969). Less information is available on submarine rhyolitic extrusions.…”

“…The more homogeneous intervals in Unit 5, with well-developed fluidal banding, may represent the unaffected interiors of flows. Rapid quenching, however, can also occur in nonmarine environments; extrusion in lakes or under ice, for example, and intrusion into wet sediments can also produce hyaloclastites (Parsons, 1969). Finally, it should be noted that no sediments have been observed intercalated within the volcanics.…”