Pyroclasts refer to all fragments ejected during an explosive volcanic eruption. These fragments or clasts can still be molten, partially molten, or solidified magma.
Also, these fragments may be from country rocks, i.e., accidental or cognate. Cognate refers to earlier formed clasts from magma genetically related to the erupting one.
The word pyroclasts means fire broken or shattered. It comes from two Greek words, πῦρ (pyr = fire) and κλαστός (klastós = broken or shattered).
The names ejecta and tephra may synonymously mean pyroclasts. However, some stricter definitions restrict tephra. These include fragments deposited from air fall only.
What makes up pyroclasts?
Pyroclasts are made of individual mineral crystals, glass fragments, lava blobs, and other rock fragments, i.e., accidental clasts. These fragments may be solid or vesiculated, like pumice and scoria.
Another vesiculated pyroclasts are cinders. They refer to reddish or black pea to fist-sized, highly vesiculated fragments, considered an equivalent of scoria.
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Considering particle size, we have four subtypes of pyroclastic materials:
| Name | Size |
| Volcanic ash | Fine to medium-sized grain measuring less than 2 mm |
| Lapilli | 2 to 64 cm (0.08-2.52) inches |
| Volcanic bombs | Ellipsoidal to rounded fragments measuring at least 64 mm ejected while still molten or semi-molten. |
| Blocks | Angular to subangular large fragments to boulders measuring at least 64 mm (2.52 inches) ejected when in solid form, often from country rock or older lava. |
Besides the above category that considers the size, other pyroclastic fragments include Pele’s hair, Pele’s tears, accretionary lapilli, and reticulate.
Composition
Their composition varies depending on the magma that forms them. It can be mafic, intermediate, felsic, or ultramafic.
However, unlike lava flow, it is not easy to determine the composition of pyroclasts. Reasons include having accidental clasts and sorting during transportation.
How do pyroclasts form?
Pyroclasts form during explosive volcanic eruptions, including those resulting from magma-water interaction or lava dome collapse.
First, depressurization of magma as it rises results in exsolution of volatiles into gases. These rapidly expanding gases will fragment and eject lava and country rock.
Such eruptions include Plinian, Strombolian, Vulcanian, Pelean, or even fountaining Hawaii eruptions. The largest eruption can produce thousands of km3 of pyroclasts.
They will create a volcanic pyroclastic plume rising upward from the vent and pyroclastic flow. Pyroclastic flows the deadly, fast-moving, ground-hugging hot gases and pyroclasts.
Secondly, the interaction of magma and surface water causes water to flash into steam, causing a steam or phreatomagmatic explosion.
The steam explosion will fragment and throw lava and sometimes country rock into the air or create horizontal surges.
Additionally, the collapse of lava domes may result in rapid decompression and fragmentation of magma. This will also form pyroclasts.
Lastly, all these processes may happen together, forming pyroclasts.
Transport and deposition
Buoyant eruption plumes from vent, pyroclastic flows or nuées ardentes will transport pyroclasts. Eventually, they will deposit these fragments via pyroclastic fall or current deposits.
They then form pyroclastic deposits. These are consolidated or unconsolidated assemblages with at least 75% pyroclasts. The remainder can be from other origins, including organic, chemical sedimentary, or epiclastic.
These pyroclasts form some volcanic landforms, including cinder cones, spatter cones, and hydrovolcanic cones like tuff ring and tuff cones. Also, they can be part of stratovolcanoes.
Pyroclastic rocks
Consolidation of these fragments will form various pyroclastic rocks depending on pyroclast sizes. These include agglomerate, pyroclastic breccia, tuff breccia, lapilli tuff, lapillistone and ash tuff.
Hazards
Hazards associated with pyroclasts include the formation of large tephra blankets that will suffocate or burn (if hot) plants, animals, and people. Some have poisonous gases that cause health problems.
Thick deposits may bury roads, houses, or other living things, including animals and people.
Lastly, pyroclastic surges may cause death and burn., damage, or burry properties. They represent some of the most dangerous volcanic hazards associated with eruptions.
Significance
On the geological time scale, the deposition of pyroclasts is considered an instantaneous event.
Therefore, they make good stratigraphic markers for dating fossils, past events, ancient environments, etc.
References
- Brown, R. J. & Calder, E. S. (2005). Pyroclastics. In Selley, R. C., Morrison, C. L. R., & Plimer, I. R. (Eds.). Encyclopedia of geology (Vols. 1-5, pp. 386-397). Elsevier Academic
- Filiberto, J. (2015). Pyroclastic deposits. In Hargitai, H. & Kereszturi, Á. (ed.) Encyclopedia of planetary landforms (2nd ed. pp. 1615-1672). Springer.
- Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
- Fisher, R. V., & Schmincke, H.U. (1984). Pyroclastic rocks (1st ed.). Springer-Vlg