Pyroclastic rocks formed from deposited fragmented materials generated and ejected during explosive eruptions. Examples include ash tuff, tuff breccia, pyroclastic breccia, agglomerate, lapilli tuff, lapillistone, and consolidated ignimbrites.
Materials ejected during an explosive eruption are known as pyroclast and, when deposited, tephra. However, some authors use the terms tephra and pyroclasts interchangeably.
Usually, pyroclasts have various sizes and may be juvenile, accidental, or cognate. Juvenile forms from erupting magma, and cognate are earlier formed rocks genetically related to eruption magma.
On the other hand, accidental clasts or lithic fragments are derived from pre-existing igneous, metamorphic, or sedimentary rocks.
They include those torn basement country rocks or picked by a pyroclastic flow. Also, they can be older glassy lava and pyroclasts like pumice.
However, some accidental clasts can be pyroclasts or glassy lava from old eruptions.
What are pyroclastic rocks?
Pyroclastic rocks are consolidated rocks made of at least 75% pyroclasts. The remainder may be other materials picked during eruptions. Such will include those of organic, epiclastic, authigenic, or chemical sedimentary origin.
Authigenic are materials formed in their current place or location.
On the other hand, epiclasts are fragments liberated by weathering or erosion and transported to their current location. They can be of volcanic or non-volcanic origin.
Pyroclastic rocks are either lithified or welded. Lithified means the pyroclast deposits underwent compaction and cementation into a rock.
On the other hand, welded pyroclast rocks form when deposited tephra is hot enough to sinter or fuse into a rock. Welding can occur to various degrees or extents.
Note that pyroclastic rocks are a type of volcaniclastic rocks. A broader definition will include not just those formed tephra. Instead, it will consist of rocks formed from some vent and subsurface deposits, which are pyroclasts. Examples include intrusive breccia, diatremes, and tuff dikes.
However, these rocks don’t include those formed from the lava autobrecciation during lava flows, especially aa and blocky ones.
Lastly, since they are made of individual fragments or particles and not interlocking crystals like other igneous rocks, these rocks resemble sedimentary rocks.
What are pyroclastic rocks made of?
These rocks are made of individual pyroclasts. These pyroclasts include lithic, glass fragments, and crystals or their fragments. Others are pumice or scoria fall deposits.
Also, pyroclasts may include special types like Pele’s hair, Pele’s tears, obsidian, reticulate, and sometimes cinder.
Cinder means pea to fist-sized, black, or reddish highly vesiculated rock, usually scoria.
Pyroclasts are categorized into the following categories based on their sizes.
- Volcanic ash: It refers to lithic fragments, glass, or crystals, including fragments less than 2 mm in size. Volcanic ash may include ash clusters and pellets. Those between 2mm and 1/16 mm are coarse ash, while those less than 1/6 mm are fine ash.
- Lapilli – Lapilli refers to pyroclasts measuring between 2mm and 64 mm in size. Lapilli deposits may include accretionary lapilli and some fiamme.
- Blocks: Volcanic blocks are angular to subangular, mostly lithic fragments measuring more than 64 mm.
- Volcanic bombs: Bombs are rounded, blobby, flattened, or stretched fragments of more than 64 mm. They form when molten or still plastic magma blobs are ejected during an eruption.
Composition
Pyroclastic rocks’ chemical and mineral composition depends on the magma type that erupts. They can form from ultramafic, mafic, intermediate, or felsic magma volcanism.
While naming these rocks, you can use compositional names like rhyolitic, andesitic, basaltic, carbonatitic, trachytic, or dacitic.
Usually, phenocrysts, mineral assemblage, chemical analysis, and color index may help determine the composition of various pyroclastic rocks.
How do pyroclastic rocks form?
Pyroclastic rocks are associated with explosive, phreatomagmatic, or phreatic eruptions. These eruptions generate and eject pyroclasts.
Ejected pyroclasts are transported into the air as part of the eruption column or plume and later deposited by fallout from the air as ashfalls.
Also, some are transported by pyroclastic density currents like pyroclastic surges and flows and emplaced as density current deposits.
Some deposited tephra may further be transported downslope as lahar or mudflows if it encounters or eruptions through water or ice.
Once deposited, tephra will either sinter or fuse if hot enough, forming welded pyroclastic rocks or undergoing lithification.
Lithification occurs via compaction and cementation. Also, chemical reactions from hot gases or groundwater and burial play a role in lithification.
Pyroclastic rock types or classification
Pyroclastic rocks are classified into tuff, consolidated ignimbrite, lapillistone, lapilli tuff, agglomerate, tuff breccia and pyroclastic breccia.
Here is more on each of these types:
1. Ash tuff
Tuff or ash tuff are lithified or welded pyroclastic rocks with at least 75% volcanic ash. The remaining up to 25% can be other pyroclasts and materials.
Intrusive tuffs are known as tuffisite. These rocks occur around volcanoes in diklets and dikes. Also, some occur in silica-rich lavas.
Ash tuffs usually have a few larger fragments in a sand-sized to aphanitic volcanic ash matrix. Their colors depend on composition and may include gray, tan, brown, pink, reddish, purplish, etc.
Examples of tuff rocks in the US occur in Canyon in Colorado, Lava Creek in Wyoming, Bishop in Nevada to central California, Bandelier in New Mexico, and Huckleberry Ridge tuff in Wyoming-Idaho.
2. Ignimbrite
Ignimbrites are poorly sorted pyroclastic rocks or unconsolidated deposits. They are made of mostly volcanic ash, some pumice, and a few scattered lithic fragments.
Their colors vary but are often whitish, grayish, pink, beige, brownish, or black. However, most are light-colored.
Usually, ignimbrites are deposited by pyroclastic density currents and may be welded, lithified, or loose deposits. They often pond extensive areas with individual deposits covering 20,000 km2, some as thick as 1 km.
Notable ignimbrite deposits in the USA are at Snake River Plain and Basin and Range Province. Others are at the Valley of Ten Thousand Smokes and Aniakchak National Monument in Alaska.
3. Lapilli tuff and lapillistone
A lapilli tuff is a consolidated pyroclastic rock in which 1) volcanic bombs and/or blocks make up less than 25% of the rock by volume and 2) combined lapilli and ash are less than 75%.
On the other hand, a lapillistone is a pyroclastic in which lapilli accounts for over 75% of the composition by volume.
5. Agglomerate
Agglomerates refer to pyroclastic rocks with at least 75% volcanic bombs, often cemented finer-grained matrix.
These rocks may be welded or nonwelded and include large chunks of basaltic spatter.
5. Pyroclastic breccia
Pyroclastic breccia are rocks in which volcanic blocks account for at least 75% cemented in a fine-grained matrix.
These rocks are further divided into three categories according to how they form. These categories are those from volcanic explosions like Vulcanian, pyroclastic flows, and hydrovolcanic breccia.
Volcanic explosion breccia form when volatile magma erupts, blasting blocks and other pyroclasts into the air.
On the other hand, pyroclastic flow breccia forms when pyroclastic currents fragment solidified lava or country rock.
Lastly, hydrovolcanic breccia form from either phreatomagmatic or phreatic eruptions. These eruptions involve water.
6. Tuff breccia
Tuff breccia are pyroclastic rocks in which volcanic bombs and or blocks account for 25% to 75% of the rock by volume. This means that volcanic ash and lapilli can be anywhere between 75 and 25%.
References
- Fisher, R. V., & Schmincke, H.U. (1984). Pyroclastic rocks (1st ed.). Springer-Vlg.
- Le Maitre, R. W. (Ed.) (2002). Igneous rocks: A classification and glossary of terms (2nd ed.). Cambridge University Press.
- Winter, J. D. (2014). Principles of igneous and metamorphic petrology. Pearson Education.