Pyroclastic rocks (derived from the Greek: πῦρ, meaning fire; and κλαστός, meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroclasts. Pyroclastic rocks are a type of volcaniclastic deposit, which are deposits made predominantly of volcanic particles.[1][2] 'Phreatic' pyroclastic deposits are a variety of pyroclastic rock that forms from volcanic steam explosions and they are entirely made of accidental clasts. 'Phreatomagmatic' pyroclastic deposits are formed from explosive interaction of magma with groundwater.[3]
Rocks from the Bishop Tuff, uncompressed with pumice on left; compressed with fiamme on right.
Flight through a μCT-image stack of a lapillus of the volcano Katla in Iceland. Find spot: Beach near Vik at the end of road 215. Acquisition done using "CT Alpha" by "Procon X-Ray GmbH", Garbsen, Germany. Resolution 11,2μm/Voxel, width approx. 24 mm.
3D-Rendering of the above image stack, in parts transparent. Heavy particles in red.
Unconsolidated accumulations of pyroclasts are described as tephra. Tephra may become lithified to a pyroclastic rock by cementation or chemical reactions as the result of the passage of hot gases (fumarolic alteration) or groundwater (e.g. hydrothermal alteration and diagenesis) and burial, or, if it is emplaced at temperatures so hot that the soft glassy pyroclasts stick together at point contacts, and deform: this is known as welding.[4]
One of the most spectacular types of pyroclastic deposit is an ignimbrite, which is the deposit of a ground-hugging pumiceous pyroclastic density current (a rapidly flowing hot suspension of pyroclasts in gas). Ignimbrites may be loose deposits or solid rock, and they can bury entire landscapes. An individual ignimbrite can exceed 1000 km3 in volume, can cover 20,000 km2 of land, and may exceed 1 km in thickness, for example where it is ponded within a volcanic caldera.
Pyroclasts include juvenile pyroclasts derived from chilled magma, mixed with accidental pyroclasts, which are fragments of country rock. Pyroclasts of different sizes are classified (from smallest to largest) as volcanic ash, lapilli, or volcanic blocks (or, if they exhibit evidence of having been hot and molten during emplacement, volcanic bombs). All are considered to be pyroclastic because they were formed (fragmented) by volcanic explosivity, for example during explosive decompression, shear, thermal decrepitation, or by attrition and abrasion in a volcanic conduit, volcanic jet, or pyroclastic density current.[5]
Clast size
Pyroclast
Mainly unconsolidated (tephra)
Mainly consolidated: pyroclastic rock
> 64 mm
block (angular) bomb (if fluidal-shaped)
blocks; agglomerate
pyroclastic breccia; agglomerate
< 64 mm
lapillus
lapilli
lapillistone (lapilli-tuff is where lapilli are supported within a matrix of tuff)
Pyroclasts are transported in two main ways: in atmospheric eruption plumes, from which pyroclasts settle to form topography-draping pyroclastic fall layers, and by pyroclastic density currents (PDCs) (including pyroclastic flows and pyroclastic surges),[6] from which pyroclasts are deposited as pyroclastic density current deposits, which tend to thicken and coarsen in valleys, and thin and fine over topographic highs.
During Plinian eruptions, pumice and ash are formed when foaming silicicmagma is fragmented in the volcanic conduit, because of rapid shear driven by decompression and the growth of microscopic bubbles. The pyroclasts are then entrained with hot gases to form a supersonic jet that exits the volcano, admixes and heats cold atmospheric air to form a vigorously buoyant eruption column that rises several kilometers into the stratosphere and cause aviation hazards.[7] Particles fall from atmospheric eruption plumes and accumulate as layers on the ground, which are described as fallout deposits.[8]
Pyroclastic density currents arise when the mixture of hot pyroclasts and gases is denser than the atmosphere and so, instead of rising buoyantly, it spreads out across the landscape. They are one of the greatest hazards at a volcano, and may be either 'fully dilute' (dilute, turbulent ash clouds, right down to their lower levels) or 'granular fluid based' (the lower levels of which comprise a concentrated dispersion of interacting pyroclasts and partly trapped gas).[9] The former type are sometimes called pyroclastic surges (even though they may be sustained rather than "surging") and lower parts of the latter are sometimes termed pyroclastic flows (these, also, can be sustained and quasi steady or surging). As they travel, pyroclastic density currents deposit particles on the ground, and they entrain cold atmospheric air, which is then heated and thermally expands.[10] Where the density current becomes sufficiently dilute to loft, it rises into the atmosphere as a 'phoenix plume'[11] (or 'co-PDC plume').[12] These phoenix plumes typically deposit thin ashfall layers that may contain little pellets of aggregated fine ash.[13]
Hawaiian eruptions such as those at Kīlauea produce an upward-directed jet of hot droplets and clots of magma suspended in gas; this is called a lava fountain[14] or 'fire-fountain'.[15] If sufficiently hot and liquid when they land, the hot droplets and clots of magma may agglutinate to form 'spatter' ('agglutinate'), or fully coalesce to form a clastogenic lava flow.[14][15]
^Fisher, Richard V. (1961). "Proposed classification of volcaniclastic sediments and rocks". Geological Society of America Bulletin. 72 (9): 1409. Bibcode:1961GSAB...72.1409F. doi:10.1130/0016-7606(1961)72[1409:PCOVSA]2.0.CO;2.
^Schmincke, Hans-Ulrich (2003). Volcanism. Berlin: Springer. p. 138. ISBN9783540436508.
^Heiken, G. and Wohletz, K., 1985 Volcanic Ash, University of California Press;, pp. 246.
^Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. p. 73. ISBN9780521880060.
^Breard, Eric C.P.; Lube, Gert (January 2017). "Inside pyroclastic density currents – uncovering the enigmatic flow structure and transport behaviour in large-scale experiments". Earth and Planetary Science Letters. 458: 22–36. Bibcode:2017E&PSL.458...22B. doi:10.1016/j.epsl.2016.10.016.
^Sulpizio, Roberto; Dellino, Pierfrancesco (2008). "Chapter 2 Sedimentology, Depositional Mechanisms and Pulsating Behaviour of Pyroclastic Density Currents". Developments in Volcanology. 10: 57–96. doi:10.1016/S1871-644X(07)00002-2. ISBN9780444531650.
^Engwell, S.; Eychenne, J. (2016). "Contribution of Fine Ash to the Atmosphere From Plumes Associated With Pyroclastic Density Currents" (PDF). Volcanic Ash: 67–85. doi:10.1016/B978-0-08-100405-0.00007-0. ISBN9780081004050.
^Colombier, Mathieu; Mueller, Sebastian B.; Kueppers, Ulrich; Scheu, Bettina; Delmelle, Pierre; Cimarelli, Corrado; Cronin, Shane J.; Brown, Richard J.; Tost, Manuela; Dingwell, Donald B. (July 2019). "Diversity of soluble salt concentrations on volcanic ash aggregates from a variety of eruption types and deposits" (PDF). Bulletin of Volcanology. 81 (7): 39. Bibcode:2019BVol...81...39C. doi:10.1007/s00445-019-1302-0. S2CID 195240304.
^ abMacdonald, Gordon A.; Abbott, Agatin T.; Peterson, Frank L. (1983). Volcanoes in the sea : the geology of Hawaii (2nd ed.). Honolulu: University of Hawaii Press. pp. 6, 9, 96–97. ISBN0824808320.
^ abAllaby, Michael, ed. (2013). "Fire-fountain". A dictionary of geology and earth sciences (Fourth ed.). Oxford University Press. ISBN9780199653065.
Other reading
Blatt, Harvey and Robert J. Tracy (1996) Petrology: Igneous, Sedimentary, and Metamorphic, W.H.W. Freeman & Company; 2nd ed., pp. 26–29; ISBN0-7167-2438-3
Branney, M.J., Brown, R.J. and Calder, E. (2020) Pyroclastic Rocks. In: Elias, S. and Alderton D. (eds) Encyclopedia of Geology. 2nd Edition. Elsevier. ISBN9780081029084
Wikimedia Commons has media related to Pyroclastic rocks.
May 06, 2023
pyroclastic, rock, derived, from, greek, πῦρ, meaning, fire, κλαστός, meaning, broken, clastic, rocks, composed, rock, fragments, produced, ejected, explosive, volcanic, eruptions, individual, rock, fragments, known, pyroclasts, type, volcaniclastic, deposit, . Pyroclastic rocks derived from the Greek pῦr meaning fire and klastos meaning broken are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions The individual rock fragments are known as pyroclasts Pyroclastic rocks are a type of volcaniclastic deposit which are deposits made predominantly of volcanic particles 1 2 Phreatic pyroclastic deposits are a variety of pyroclastic rock that forms from volcanic steam explosions and they are entirely made of accidental clasts Phreatomagmatic pyroclastic deposits are formed from explosive interaction of magma with groundwater 3 USGS scientist examines pumice blocks at the edge of a pyroclastic flow from Mount St Helens Rocks from the Bishop Tuff uncompressed with pumice on left compressed with fiamme on right source source source source source source source source source source source source Flight through a mCT image stack of a lapillus of the volcano Katla in Iceland Find spot Beach near Vik at the end of road 215 Acquisition done using CT Alpha by Procon X Ray GmbH Garbsen Germany Resolution 11 2mm Voxel width approx 24 mm source source source source source source source source source source 3D Rendering of the above image stack in parts transparent Heavy particles in red Unconsolidated accumulations of pyroclasts are described as tephra Tephra may become lithified to a pyroclastic rock by cementation or chemical reactions as the result of the passage of hot gases fumarolic alteration or groundwater e g hydrothermal alteration and diagenesis and burial or if it is emplaced at temperatures so hot that the soft glassy pyroclasts stick together at point contacts and deform this is known as welding 4 One of the most spectacular types of pyroclastic deposit is an ignimbrite which is the deposit of a ground hugging pumiceous pyroclastic density current a rapidly flowing hot suspension of pyroclasts in gas Ignimbrites may be loose deposits or solid rock and they can bury entire landscapes An individual ignimbrite can exceed 1000 km3 in volume can cover 20 000 km2 of land and may exceed 1 km in thickness for example where it is ponded within a volcanic caldera Contents 1 Classification 2 See also 3 References 4 Other readingClassification EditPyroclasts include juvenile pyroclasts derived from chilled magma mixed with accidental pyroclasts which are fragments of country rock Pyroclasts of different sizes are classified from smallest to largest as volcanic ash lapilli or volcanic blocks or if they exhibit evidence of having been hot and molten during emplacement volcanic bombs All are considered to be pyroclastic because they were formed fragmented by volcanic explosivity for example during explosive decompression shear thermal decrepitation or by attrition and abrasion in a volcanic conduit volcanic jet or pyroclastic density current 5 Clast size Pyroclast Mainly unconsolidated tephra Mainly consolidated pyroclastic rock gt 64 mm block angular bomb if fluidal shaped blocks agglomerate pyroclastic breccia agglomerate lt 64 mm lapillus lapilli lapillistone lapilli tuff is where lapilli are supported within a matrix of tuff lt 2 mm coarse ash coarse ash coarse tuff lt 0 063 mm fine ash fine ash fine tuffPyroclasts are transported in two main ways in atmospheric eruption plumes from which pyroclasts settle to form topography draping pyroclastic fall layers and by pyroclastic density currents PDCs including pyroclastic flows and pyroclastic surges 6 from which pyroclasts are deposited as pyroclastic density current deposits which tend to thicken and coarsen in valleys and thin and fine over topographic highs During Plinian eruptions pumice and ash are formed when foaming silicic magma is fragmented in the volcanic conduit because of rapid shear driven by decompression and the growth of microscopic bubbles The pyroclasts are then entrained with hot gases to form a supersonic jet that exits the volcano admixes and heats cold atmospheric air to form a vigorously buoyant eruption column that rises several kilometers into the stratosphere and cause aviation hazards 7 Particles fall from atmospheric eruption plumes and accumulate as layers on the ground which are described as fallout deposits 8 Pyroclastic density currents arise when the mixture of hot pyroclasts and gases is denser than the atmosphere and so instead of rising buoyantly it spreads out across the landscape They are one of the greatest hazards at a volcano and may be either fully dilute dilute turbulent ash clouds right down to their lower levels or granular fluid based the lower levels of which comprise a concentrated dispersion of interacting pyroclasts and partly trapped gas 9 The former type are sometimes called pyroclastic surges even though they may be sustained rather than surging and lower parts of the latter are sometimes termed pyroclastic flows these also can be sustained and quasi steady or surging As they travel pyroclastic density currents deposit particles on the ground and they entrain cold atmospheric air which is then heated and thermally expands 10 Where the density current becomes sufficiently dilute to loft it rises into the atmosphere as a phoenix plume 11 or co PDC plume 12 These phoenix plumes typically deposit thin ashfall layers that may contain little pellets of aggregated fine ash 13 Hawaiian eruptions such as those at Kilauea produce an upward directed jet of hot droplets and clots of magma suspended in gas this is called a lava fountain 14 or fire fountain 15 If sufficiently hot and liquid when they land the hot droplets and clots of magma may agglutinate to form spatter agglutinate or fully coalesce to form a clastogenic lava flow 14 15 See also EditSilicon dioxide Hyaloclastite Volcaniclastic accumulation or breccia Peperite Sedimentary rock that contains fragments of younger igneous material Scoria Dark vesicular volcanic rockReferences Edit Fisher Richard V 1961 Proposed classification of volcaniclastic sediments and rocks Geological Society of America Bulletin 72 9 1409 Bibcode 1961GSAB 72 1409F doi 10 1130 0016 7606 1961 72 1409 PCOVSA 2 0 CO 2 Fisher Richard V Schmincke H U 1984 Pyroclastic rocks Berlin Springer Verlag ISBN 3540127569 Fisher 1961 p 1409 Schmincke Hans Ulrich 2003 Volcanism Berlin Springer p 138 ISBN 9783540436508 Heiken G and Wohletz K 1985 Volcanic Ash University of California Press pp 246 Philpotts Anthony R Ague Jay J 2009 Principles of igneous and metamorphic petrology 2nd ed Cambridge UK Cambridge University Press p 73 ISBN 9780521880060 Schmincke 2003 pp 155 176 Fisher amp Schmincke 1984 p 8 Breard Eric C P Lube Gert January 2017 Inside pyroclastic density currents uncovering the enigmatic flow structure and transport behaviour in large scale experiments Earth and Planetary Science Letters 458 22 36 Bibcode 2017E amp PSL 458 22B doi 10 1016 j epsl 2016 10 016 Schmincke 2003 pp 177 208 Sulpizio Roberto Dellino Pierfrancesco 2008 Chapter 2 Sedimentology Depositional Mechanisms and Pulsating Behaviour of Pyroclastic Density Currents Developments in Volcanology 10 57 96 doi 10 1016 S1871 644X 07 00002 2 ISBN 9780444531650 Engwell S Eychenne J 2016 Contribution of Fine Ash to the Atmosphere From Plumes Associated With Pyroclastic Density Currents PDF Volcanic Ash 67 85 doi 10 1016 B978 0 08 100405 0 00007 0 ISBN 9780081004050 Colombier Mathieu Mueller Sebastian B Kueppers Ulrich Scheu Bettina Delmelle Pierre Cimarelli Corrado Cronin Shane J Brown Richard J Tost Manuela Dingwell Donald B July 2019 Diversity of soluble salt concentrations on volcanic ash aggregates from a variety of eruption types and deposits PDF Bulletin of Volcanology 81 7 39 Bibcode 2019BVol 81 39C doi 10 1007 s00445 019 1302 0 S2CID 195240304 a b Macdonald Gordon A Abbott Agatin T Peterson Frank L 1983 Volcanoes in the sea the geology of Hawaii 2nd ed Honolulu University of Hawaii Press pp 6 9 96 97 ISBN 0824808320 a b Allaby Michael ed 2013 Fire fountain A dictionary of geology and earth sciences Fourth ed Oxford University Press ISBN 9780199653065 Other reading Edit Blatt Harvey and Robert J Tracy 1996 Petrology Igneous Sedimentary and Metamorphic W H W Freeman amp Company 2nd ed pp 26 29 ISBN 0 7167 2438 3 Branney M J Brown R J and Calder E 2020 Pyroclastic Rocks In Elias S and Alderton D eds Encyclopedia of Geology 2nd Edition Elsevier ISBN 9780081029084 Wikimedia Commons has media related to Pyroclastic rocks Retrieved from https en wikipedia org w index php title Pyroclastic rock amp oldid 1098428284, wikipedia, wiki, book, books, library,
