Tephra is a collective name for fragmented materials ejected during a volcanic eruption. These fragments may be of any size or composition.
Also, the emplacement mechanism doesn’t matter. It may be airborne or transported over land as a hot moving flow.
The word tephra comes from the Greek word τέφρα (téphra), which means ash. However, it isn’t the same as volcanic ash, which describes rock fragments smaller than 0.08 mm (2 mm) in diameter.
Instead, tephra describes Aristotle’s account of the eruption of Vulcano in his book Meteorologica. This account implies the fragmented materials thrown into the air, not just ash.
However, some authors consider tephra to be a synonym for pyroclasts. Others view tephra as unconsolidated pyroclastic deposits or a collective term for deposited pyroclasts regardless of size.
Pyroclasts are fragmented materials ejected during an explosive eruption. They may still be molten, partially molten, solidified magma, or other rock fragments.
Britannica narrows the meaning of tephra only to include pyroclastic materials deposited by falling from the air regardless of size. This definition means airborne material except. Also, it doesn’t have pyroclastic flows.
Therefore, the definition of tephra is debatable.
Tephra classification
We can classify tephra according to the grain size. Under grain size, we have:
- Volcanic ash: Have pyroclasts with medium to fine-grained sizes, usually less than 2 mm. It is like silt or sand. You can further divide volcanic ash into fine or dust if less than 1/16 and coarse ash when 1/16 mm to 2 mm.
- Lapilli: Sizes range from 2-64 mm (0.08-2.52 inches). Their size is comparable to granular gravel or pebble gravel.
- Blocks: Volcanic blocks are angular to subangular rock fragments measuring over 64 mm (2.52 inches). Blocks are usually solid at the time of ejection. Most come from older lava bodies or country rock.
- Volcanic bombs: Volcanic blocks are rounded to ellipsoidal lava mass larger than (64 mm) 2.52 inches, just like blocks. However, they are molten to semi-molted when ejected.
Other uncommon tephra are Pele’s hair and Pele’s tears associated with the basaltic Hawaiian eruption style.
Besides the above classification, we can consider texture. Under texture, we will have the following:
Scoria and cinder:
Scoria describes dark-colored, highly vesicular pyroclastic rock. It is often of basaltic to intermediate composition and has mainly a glassy texture. However, it may be aphanitic.
Cinder, on the other hand, refers to peas as fist-sized, highly vesiculated rocks. Cinders are usually scoria.
Pumice
Pumice is a light-colored, frothy, or form-like rock with numerous vesicles separated by thin glassy walls.
It forms from explosive pyroclastic eruptions of mainly felsic magmas, especially rhyolite. However, there is rhyodacite and dacite pumice.
Composition
Tephra composition may be felsic, intermediate, mafic, or ultramafic. It depends on the host magma.
Felsic means they are enriched with silicate minerals, high silicon, oxygen, sodium, potassium, and aluminum.
Mafic, on the other hand, is high in darker silicate minerals rich in magnesium and calcium.
Intermediate falls between felsic and mafic, while ultramafic has over 90% of mafic minerals.
However, it is difficult to determine tephra composition accurately, as it is easy for lava flow.
Reasons include 1) juvenile clasts formed from erupting magma, 2) accidental or country rocks plucked during eruption, and 3) near-surface sediments.
Also, some may have undergone alteration, and sorting during transport may further affect composition.
Texture
Tephra has mainly glassy fragments formed when magma cools rapidly. This glassy particle may be vesicular, flake-like, or solid.
Also, it will have mineral components and crystalline fragments of various amounts from the country rock.
Pyroclastic rocks
Consolidated tephra will form pyroclastic rocks. These rocks are named according to the dominant pyroclast sizes, i.e., the ratio of blocks, bombs, lapilli, and ash.
These rocks include agglomerate, pyroclastic tuff, lapilli tuff, lapillistone, breccia tuff, and tuff or ash tuff.
Significance in geochronological
Since tephra formation is considered an instantaneous event on the geological time scale, it is a good isochronous same-time stratigraphic marker.
It can help in dating fossils, knowing the dates they occurred, prehistoric cultures, paleoenvironments, and ecosystems.
The scientific technique of using discrete tephra layers in sedimentary deposits to date is known as tephrochronology.
Hazards
A huge eruption will produce a tephra cloud that will expand as it incorporates more gases. As it expands, heavier particles fall and can blanket large areas, hundreds of thousands of square miles below the cloud.
Tephra blankets will suffocate plants, animals, and people. Thick ones may collapse buildings, while hot ones burn things on impact.
Some may have a high concentration of poisonous compounds like sulfur that are detrimental to plants and animals, some with long-term effects. Such can cause animal and crop diseases.
Also, ashfall may cause darkness and reduce visibility to a few centimeters.
If you consider tephra to include pyroclastic flows or nuées ardentes, these fast-moving ground-hugging pyroclast-laden flows are dangerous. They have previously killed tens of thousands and burnt or buried houses.
References
- Kusky, T. M., & Cullen, K. E. (2005). Encyclopedia of earth and space science. Facts on File.
- Fisher, R. V., & Schmincke, H.U. (1984). Pyroclastic rocks (1st ed.). Springer-Vlg.
- Okrusch, M., & Frimmel, H. (2020). Mineralogy: An introduction to minerals, rocks, and mineral deposits (1st ed.). Springer.
- Le Maitre, R. W. (Ed.) (2002). Igneous rocks: A classification and glossary of terms (2nd ed.). Cambridge University Press.