Tuff or ash tuff is a pyroclastic rock with at least 75% volcanic ash. Volcanic ash are particles no more than 2 mm ejected during a volcanic eruption.
On the other hand, pyroclast rocks form from lithified or welded materials ejected during volcanic eruptions.
These materials are called tephra or pyroclasts. They include lapilli, volcanic bombs, blocks, and, of course, volcanic ash.
Note that tuffs can have the other pyroclasts. However, volcanic ash must account for at least 75% of pyroclasts. Rocks with between 25 and 75% volcanic ash are known as tuffaceous.
When used alone, tuff usually means a pyroclastic rock with at least 75% volcanic ash. Other lithified or welded pyroclasts can use the name too.
However, you must include a modifier, such as the pyroclasts or textural. For instance, you can have breccia or lapilli tuff.
Lastly, the origin of tuff can be sedimentary or volcanic. We usually study its petrology as volcanic rock. However, we can use some sedimentary terms when describing it.
Rhyolite tuff rock from Smith Rock State Park, Oregon, Photo credit: Jstuby, CC0, via Wikimedia Commons.
Appearance and Characteristics
Tuffs are relatively soft rocks, some with vesicles. However, welded tuffs are harder because they fuse during formation.
These rocks have fewer larger angular fragments in sand-sized to fine-grained. The angular fragments vary in size from lapilli to volcanic blocks or bombs. However, they only account for less than 25% of the rock volume.
Ash tuff colors depend on composition. They may be dark brown, tan, or dark to light gray. Also, some can have shades of colors like yellow, pink, green, or purple.
Usually, tuffs are thicker near their vents, some hundreds of meters thick and thinner as you move away.
Also, they may show grading as you move away from the vent. Some may also vertically grade.
Lastly, they are often bedded. Additionally, some may have sedimentary features like antidunes and dunes.
What is tuff made of?
As noted, tuff has at least 75% volcanic ash. The rest can be lapilli, which measures 2-64 mm, and volcanic bombs or blocks that measure more than 64 mm. Also, it may have any other material picked during a volcanic eruption.
Usually, volcanic ash may have fragments of minerals, volcanic glass, and lithic fragments. Lithic fragments are rock pieces torn from the country rock surrounding the vent.
On the other hand, mineral fragments come mostly from the erupting magma. However, a few may be plucked from country rocks. Such are known as xenocrysts.
Lastly, glass shards or fragments form when magma cools rapidly. They may include shattered pieces from walls of bubbles formed as lava exsolved and rapidly expanded. Their shapes vary from irregular to triangular and have convex sides.
Classification
We can further subdivide according to particle or grain size. Coarse tuff will have volcanic ash particles between 2 mm and 1/16 mm.
On the other hand, dust or fine tuff will have volcanic ash grains less than 1/16 mm.
Another way to classify these rocks is according to the dominant fragments in volcanic ash. Under this classification, you will have three categories, namely:
- Vitric tuff: It has predominantly glassy or vitric fragments. These usually form from the erupting magma.
- Crystal tuff: This type is dominated by crystal fragments.
- Lithic tuff: This rock has mostly rock fragments that are not from the erupting magma.
- Transition: These have two dominant fragments. Examples include crystal-lithic or crystal-vitric stuffs.
Besides these two ways, tuff can be classified according to its deposition environment. This category will have subaerial, lacustrine, and submarine tuff.
Another way to classify these rocks is by considering transportation means. This method gives fallout tuff formed from ashfall, ash-flow tuff, or ignimbrite, representing those from pyroclastic flow or surge currents.
That is not all. We can further subdivide ignimbrite depending on how the rock forms. This classification gives you the lithified and welded tuffs. Each shows different characteristics.
Lastly, reworked tuff represents those that form from volcanic ash that erodes and redeposits elsewhere. We include the transport agents in their name. This gives you aeolian tuff when transported by wind action or fluvial tuff transported by rivers or streams.
Tuff composition
Tuff rock composition represents the entire volcanic magma range from ultramafic to mafic to intermediate to felsic.
Usually, we use compositional names to describe these rocks. For instance, you have carbonatite, basaltic, rhyolitic, andesitic, or dacitic tuff.
Let us look at each in detail.
1. Felsic – dacitic and rhyolitic tuffs
They form from viscous and high silica (> 63 wt. % SiO2) volcanism and include dacitic and rhyolitic.
Dacitic and rhyolitic tuffs have volcanic ash, mainly with pumice fragments, a few scattered lithic pieces, and other pyroclasts. Also, they can have a small amount of scoria.
Common crystals these rocks have include quartz, alkali feldspar, and sometimes biotite.
Examples occur in Iceland, Hungary, New Zealand, the US, and Lipari in Italy. These include Lava Creek Tuff, Wyoming and Basin and Range in the USA, and North Island in New Zealand, which covers 700 km2.
Lastly, older deposits have opal, chalcedony, or quartz resulting from silicification. Also, most will have undergone devitrification.
2. Intermediate composition – andesitic tuff
Andesitic tuffs are the most common with intermediate composition. They are usually reddish, brownish, grayish, greenish, or other colors and will have crystals like plagioclase, hornblende, and pyroxenes.
Andesitic tuffs are widespread in the Andes, American Cordilleras, Japan, New Zealand, and the West Indies.
Recent deposits are in Cotopaxi in Ecuador, Krakatoa in Indonesia, and Mt. Pelé in the French island of Martinique,
Also, parts of Great Britain have ancient rocks of this composition with red or brown scoriae fragments of various sizes. Their secondary cavities have calcite, epidote, chalcedony, quartz, and chlorite.
3. Mafic composition – basaltic tuff
Mafic magmas are highly fluid and low silica (45-52 wt.% SiO2). They often erupt with little or no ash unless phreatomagmatic eruptions form hydrovolcanic landforms like tuff cones, rings, and maars.
However, basaltic tuffs with mostly basaltic glasses like tachylite and sideromelane exist. Such may have crystals like augite, plagioclase, olivine, and sometimes hornblende.
Unfortunately, this glass rapidly alters to palagonite, a yellow or orange-gray material, via hydration and eventually to laterite. Palagonite helps it in lithification.
Most of the weathered ones have calcite, chlorite, and serpentine. Also, they have zeolites if their magmas have leucite or nepheline.
Examples include the Diamond Head tuff ring and the 15-meter-thick Pahala ash of Hawaii.
Older black, dark green or red basaltic tuff deposits occur in Skye, Mull, Antrim, Derbyshire, Lake District, and other places in the UK.
Lastly, recent basaltic tuffs occur in the Faroe Islands in the United Kingdom of Denmark, Jan Mayen in Norway, Sicily in Italy, and Samoa.
4. Ultramafic
Although very rare, ultramafic vulcanism will produce kimberlite, carbonatite, and komatiite tuffs. Such are abundant in olivine, sometimes altered to serpentine. Also, they consciously lack quartz or feldspar.
Examples occur in kimberlite maars in South Africa. Also, komatiitic ones occur in greenstone belts of South Africa and Canada.
Lastly, tuff Kerimasi Tanzania, Kaiserstuhl, Germany, and one in Cape Verde Island are carbonatite examples.
5. Alkaline magma eruptions
Alkaline volcanism involves explosive eruption of slightly silica-saturated to undersaturated magmas.
Trachyte tuff is a typical example. These types have mostly sanidine and anorthoclase or sometimes oligoclase feldspar with little to no quartz. They may also have augite, biotite, or hornblende.
Weathering turns them into soft reddish or yellowish claystones. These claystones are rich in kaolin and secondary quartz.
Examples in Rhine and Naples, Italy. Also, the East Africa Rift has Trachyte-carbonatite tuff and Ro de Janeiro Brazil alkaline crystal tuffs.
How does tuff form?
Tuff forms from the deposition and consequent lithification or welding of tephra. This tephra has mostly volcanic ash that may form from explosive, phreatomagmatic, and phreatic eruptions.
Deposited volcanic ash will form a uniform blanket that may cover huge areas. This ash deposition can occur from ashfall, i.e., settling of ash blow into the air or pyroclastic flow deposits.
Pyroclastic flows are dangerous, rapidly moving, ground-hugging hot gases laden with pyroclasts. They form when eruption columns or lava domes collapse.
Ashfalls will create well-sorted deposits. In contrast, pyroclast deposits are poorly sorted and will form ignimbrites. Also, they show sedimentary features like dunes and antidunes, seen on high-velocity flows.
Volcanic ash after deposition may also flow downslope as mudflows or lahars if it mingles with water or ice. Water may be from rainfall or if an eruption occurs through water.
Once deposited, volcanic ashes and other pyroclastic fragments will either weld to form a welded tuff or undergo lithification.
1. Welded tuff
If deposited volcanic ash is hot enough, usually more than 600 °C (1,112 °F), it may sinter or fuse, forming a welded tuff. Such welded rocks are common in rhyolitic volcanic ash deposits but can occur in other types.
The compaction and deformation during welding will create a eutectic texture. Eutectic describes a layered or banded texture. It forms from compaction and flattening of pumice and glass fragments surrounding undeformed crystals.
Welded tuff is like welded pyroclastic rocks are commonly associated with fiamme.
A cross-section of welded tuff may show variation in welding degree, with some sections not welded. Also, compaction and porosity will vary.
For instance, the lowermost section will not be welded as it touches cold ground.
However, the welding degree and action of secondary fluids will increase toward the center. This middle part is well insulated. Thus, it will remain hotter for long. Also, overlaying material will cause compaction.
Towards the upper surface or thinner layers, the welding level will decrease. This happens due to quick cooling. Also, these rocks will be less compact and more porous.
Expanding gases may also cause irregular, disc-like lithophysae in compacted tuffs. These are spherulitic structures with cavities at their centers and concentric chambers.
2. Lithified tuffs
Sediments not hot enough will, with time, compact, cement, and recrystallize to form a rock.
Usually, the higher glass content makes lithification happen quickly. Why? Because volcanic glasses are unstable. Thus, it will react with ground or seawater. This will release calcium and alkali metals like sodium and potassium and crystallize new minerals.
Newly crystallized minerals like clays, calcite, and zeolite will help cement the ash into tuff.
Tuff rock alteration
Tuff rocks are more vulnerable to chemical weathering than other pyroclastic rocks. This happens because of their high volcanic glass content.
Factors such as composition, structure, environment, geologic age, and physical or chemical conditions affect the extent or rate at which it happens.
Also, water pH can affect the alteration rate and influence the products formed.
The felsic/acidic volcanic glass will alter to smectites (especially bentonites), opal, cristobalite, chalcedony, and zeolite in fresh and seawater. This alteration occurs slowly.
On the other hand, basic/mafic tuffs, which mainly have basaltic glass, will alter fast into palagonite.
Naming
Using the name tuff may be incomplete. You should include modifiers such as compositional name, texture, dominant fragments, etc.
For instance, texture can be fine or coarse, and the dominant fragments can be lithic, vitric, or crystals.
On the other hand, the compositional name is based on color index, minerals of crystal fragments, or chemical analysis.
Lastly, you can also state whether it is welded or lithified.
Occurrence
Tuffs occur anywhere with explosive volcanism, including phreatic and phreatomagmatic eruptions.
In the US, they are common in the western states like New England, Texas, Wyoming, Idaho, Nevada, Michigan, and Virginia.
Notable examples in the US include Fish Canyon (Colorado), Bandelier (New Mexico, Lava Creek (Wyoming), Messa Fall (Idaho), Bishop (Nevada and central California), and Huckleberry Ridge tuff in Wyoming-Idaho.
What is tuff used for?
Tuff is relatively easy to work with, being soft unless welded. Also, it resists weathering.
Some ancient uses of tuff include making roads, railroad stations, bridges, buildings, and other construction projects. These included castles, cathedrals, large public buildings, and private homes.
Also, it made pozzolanic cement, sculptures, statues, and hydraulic mortar and served as dimensional stone, among other uses. Other uses were monuments, birdbaths, and carved door or window frames.
These uses were widespread among the Romans, Germans, Armenians, and others in Europe. Also, people in South America, the US, and many other places used this for various purposes.
Modern tuff uses include concrete mix and as a construction stone. Also, the Rapa Nui people of Easter Island in Chile still use it to make most of their famous moai statues.
Lastly, it serves as a nuclear waste respiratory unit with ignimbrite at Yucca Mountain nuclear waste repository in Nevada, USA.
Uses in geochronology
Volcanic ash that forms tuff is deposited instantaneously on a geological time scale and on an extensive scale. It is, therefore, a useful stratigraphic marker.
Using chemical composition and phenocrysts assemblage, it is possible to correlate individual tuff beds over a wide area or associate them with known events.
Frequently asked questions
Ignimbrite pyroclastic rock with poorly sorted pumice fragments and other scattered lithic and volcanic debris cemented in a volcanic ash matrix. In contrast, tuff refers to any pyroclastic rock with over 75% ash.
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
- Romaine, G. (2020). Rocks, gems, and Minerals (3rd ed.). Globe Pequot Pr.
- Winter, J. D. (2014). Principles of igneous and Metamorphic Petrology (2nd ed.). Pearson Education.
- Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
- Bonewitz, R. (2012). Rocks and minerals (1st ed.). DK Pub.