Skip to Main Content

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 (nuée 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 sub-aerial deposition. Vulcanian eruptions produce poorly stratified, unsorted, hetero-lithologic, 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 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 sub-aqueous. 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 glassy pyroclastics produce a group of chaotic, unsorted, unstratified, pillow breccias with angular, glassy, palagonitized material in their groundmass. Underwater slumping and reworking by currents of the finer material from such growing extrusive piles deposit stratified, sorted tuffs and tuff breccias intermixed with marine sediments and fossils. Submarine eruptions of rapidly vesiculating magmas result in quick quenching of materials in a submarine eruptive column which settle back to the ocean floor and are spread laterally by turbidity-current slides to build partially sorted, graded-bedded layers of lapilli tuffs with pumice fragments, crystal fragments, and shattered glass shards.

You do not currently have access to this chapter.

Figures & Tables





Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal