Pyroclastic rocks 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. Phreatic pyroclastic deposits are a variety of pyroclastic rock formed from volcanic steam explosions and are entirely composed of accidental clasts. Phreatomagmatic pyroclastic deposits are formed from explosive interaction of magma with groundwater. Pyroclastic material has been produced during some of history's most powerful volcanic eruptions, including the 79 AD eruption of Mount Vesuvius, the 1980 eruption of Mount St. Helens, the large eruptions from the Yellowstone Caldera, and the 1991 eruption of Mount Pinatubo. The word pyroclastic comes from the Ancient Greek words pyr, meaning "fire", and klastos, meaning "broken in pieces".[1]
Description
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 process is called welding.[2]
One of the most notable 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 1,000 km3 (240 cu mi) in volume, can cover 20,000 km2 (7,700 sq mi) of land, and may exceed 1 km (0.62 mi) thick (e.g. where it is ponded within a volcanic caldera).
Classification
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.[3]
| 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) |
| < 2 mm | coarse ash | coarse ash | coarse tuff |
| < 0.063 mm | fine ash | fine ash | fine tuff |
Transportation
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),[4] 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.
Formation
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.[5] Particles fall from atmospheric eruption plumes and accumulate as layers on the ground, which are described as fallout deposits.[6]
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).[7] 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.[8] Where the density current becomes sufficiently dilute to loft, it rises into the atmosphere as a 'phoenix plume'[9] (or 'co-PDC plume').[10] These phoenix plumes typically deposit thin ashfall layers that may contain little pellets of aggregated fine ash.[11]
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[12] or 'fire-fountain'.[13] 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.[12][13]
Real-world examples
In 79 AD, Mount Vesuvius erupted, causing a large cloud of ash, pumice, and gas to rise in the sky before collapsing back onto the mountain in the form of pyroclastic flows. The pyroclastic flows rushed down the mountain side, burying nearby cities, such as Pompeii and Herculaneum, and killing thousands of people. The eruption covered the cities in pyroclastic rocks; preserving the cities underneath for centuries.[14]
In May of 1980, Mount St. Helens erupted, causing a massive landslide and a large sideways explosion, that sent ash, gas, and rock into the surrounding forests, causing substantial damage. The eruption sent a large column of ash into the sky, which then turned into pyroclastic flows that settled on the base of the mountain in the form of pyroclastic rock material.[15]
Approximately 640,000 years ago, a huge amount of magma erupted into the air, spreading ash, pumice, and other volcanic rock across thousands of miles, leaving behind a thick layer of pyroclastic material that "welded" into hard "tuff" rock. This eruption, along with two other earlier blasts, formed what is known as the Yellowstone Caldera in the Yellowstone Plateau Volcanic Field, which is one of the most studied volcanic systems in the world.[16]
On June 15, 1990, in the Philippines, Mount Pinatubo erupted, causing a large column of ash to travel into the air and pyroclastic flows to slide down the mountain and fill surrounding valleys with thick pyroclastic rock debris. The eruption blanketed huge areas, as far as the Indian Ocean, which is approximately 2,500 km away, in volcanic rock fragments and fine ash.[17]
See also
- Silicon dioxide
- Hyaloclastite – Volcanic rock consisting of glass fragments
- Peperite – Sedimentary rock that contains fragments of younger igneous material
- Scoria – Dark vesicular volcanic rock
References
- ^ "Pyroclastic - Etymology, Origin & Meaning". etymonline. Retrieved 2026-02-20.
- ^ 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 & 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&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.
- ^ Scandone, Roberto; Giacomelli, Lisetta; Gasparini, Paolo (1993-11-01). "Mount Vesuvius: 2000 years of volcanological observations". Journal of Volcanology and Geothermal Research. Mount Vesuvius. 58 (1): 5–25. doi:10.1016/0377-0273(93)90099-D. ISSN 0377-0273.
- ^ Brantley, S. R., Myers, B., & Geological Survey. (2000). Mount St. Helens, from the 1980 eruption to 2000. [publisher not identified]. http://purl.access.gpo.gov/GPO/LPS52236
- ^ Stelten, Mark E.; Thomas, Nicole; Pivarunas, Anthony; Champion, Duane (2023-09-12). "Spatio-temporal clustering of post-caldera eruptions at Yellowstone caldera: implications for volcanic hazards and pre-eruptive magma reservoir configuration". Bulletin of Volcanology. 85 (10): 55. doi:10.1007/s00445-023-01665-w. ISSN 1432-0819.
- ^ "The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines, Fact Sheet 113-97". permanent.fdlp.gov. Retrieved 2026-02-25.
Other reading
- Blatt, Harvey and Robert J. Tracy (1996) Petrology: Igneous, Sedimentary, and Metamorphic, W.H.W. Freeman & 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
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