In many descriptions of volcanoes and their eruption histories we stumble upon the terms Tuff, Tufa or Ignimbrite. Most every self-respecting volcano has one or all of them. Generally it becomes obvious from the context that they refer to widespread deposits of volcanic materials. But what exactly is an ignimbrite?, is it different to tuff?, and where do they all come from?
Tuff and ignimbrite are clastic rocks composed of volcanic materials. Clastic meaning broken, shattered pieces of other, pre-existing rock. Pyroclastic deposits are commonly formed from airborne ash, lapilli and bombs or blocks ejected in violent explosive volcanic eruptions, mixed in with shattered country rock. The deposition of tuff and ignimbrite is often associated with caldera-forming eruptions.
To researchers, the sheets of tuff or ignimbrite are a great source of information. Their position, deposition, composition and decomposition can tell entire stories of the volcanic activity at a certain time, and the eras before and after an event.
The USGS Glossary has definitions short and pithy, but not very enlightening:
Tuff: “A general term for all consolidated (hardened and/or compacted) pyroclastic (explosive, volcanic origination) rocks. Synonymous with tuffaceous.”
Ignimbrite: “The rock formed by widespread deposition and consolidation of pyroclastic density currents (pyroclastic flows). Also known as welded tuff or ash flow tuff.”
TUFA – not volcanic
The term “Tufa” we can tick off right here: It is not a volcanic product. Tufa is a relatively soft calcareous sediment precipitated out of ambient temperature water; a non-marine carbonate rock. It should not be confused with our volcanic tuff, which is erroneously sometimes also called “tufa”. The name tufa is also often (mis)used in the building trade for any sort of light, fine-grained building material.
Let’s begin with the tuff: Tuff is a type of rock made up of the solidified debris of explosive volcanic eruptions, varying in size from very fine powder to coarse gravel. If you go by the definition for sediment, tuff can be seen as a volcanic sedimentary rock.
The original material was any tephra deposited around a volcano during explosive eruptions. Such ash layers can be tens of meters thick, or accumulate to hundreds of meters when several eruptions/eruptive periods are involved. With time these loose particles become cemented, and compacted by their own weight and by that of consecutive covers of other materials.
The results are rocks ranging from quite soft to good hardness, according to the degree of compacting and their composition. The composition of tuff can vary from mafic to felsic, according to the volcano’s magma type. The rock becomes even harder when the ash deposit was sufficiently hot when laid down: the particles would still have been soft enough to fuse and form the so-called welded tuff. Such “baking” can also occur near vents, or, at later stages, when fresh hot materials like lava flows come in contact with the tuff.
There are different tuffs according to grain size:
Ash tuff: What is generally meant by tuff is the finely grained consolidated material with at least 75% of the grains measuring from finest dust to 2 mm. Lapilli stone or lapilli tuff is a coarse rock that contains more than 75% particles of 2-64 mm diameter. Near the vent, a tuff might consist mainly of larger blocks in a volcanic ash matrix. Outcrops of tuff often show a beautiful layering, as they are built up of the deposits of various eruptions. These layers can vary in thickness, grain size and composition.
Tuff can originate from eruptions of many types, except for the purely effusive ones. The ash layers may have been produced by phreatic and phreatomagmatic explosions. Also the deposits of violent vulcanian to plinian eruptions usually lay a wide-spread ash cover. The thickness of deposits generally depends on the wind or blast direction. Tuff is usually thickest around the volcanic vent, on the downwind side of the vent, or on the side of the vent where a blast was directed.
Ignimbrite can be considered a special type of tuff.
The difference lies in the particular way of its formation. Ignimbrites form after strong volcanic explosions, when pyroclastic ash, lapilli and blocks race down the sides of volcanoes in a high-temperature gas-and-ash mix, i.e. pyroclastic density currents (PDC) or pyroclastic flows (PF). Although several mechanisms have been observed, the main one that gives rise to the pyroclastic flow, and therefore to ignimbrite, is the gravitational collapse of a vertical column of ashes and lapilli, supported by the violent emission of volcanic gas. Such ground-hugging clouds can run over considerable distances and carry a tremendous load of piping hot material. These materials are deposited along the way in a poorly sorted mixture to form the ignimbrite. Ignimbrite can also be banded, with finer ash particles settling on top of coarser material.
Ignimbrites occur worldwide and are associated with many volcanic provinces having high-silica content magma and the resulting explosive eruptions. Some ignimbrites have built up to considerable depth and would have taken years or decades to cool down completely. Ignimbrite deposits can reach a bulk volume of over 1000 km³ and a runout distance to over 100 km.
The word Ignimbrite originates from the Latin, combining ignis: fire and imber: rain. The term was originally applied only to densely welded deposits but in modern science includes non-welded deposits as well.
Ignimbrite may be loose and unconsolidated, or lithified (solidified) rock. They may be cemented by vapour phase minerals, or welded/”baked”. Some unwelded ignimbrites may be hard to distinguish from other tuff. Near the volcanic source, the ignimbrite commonly contains thick accumulations of lithic blocks, in greater distance many show meter-thick accumulations of rounded cobbles of pumice.
Ignimbrites consist of a medium-to-high percentage of volcanic glass shards (pumice) mixed with broken crystals of the current eruptive products. If the particles of pumice in the ash matrix exceed 50% of the total volume, the source event is called a pumice flow. Typically, the ash matrix also contains varying amounts of pea- to cobble-sized rock fragments (lithic inclusions). These are pieces of rock from disintegrated older lava flows, fragments of various rocks ripped from the conduit walls during ascent of the magma or sometimes from the magma chamber.
The most explosive eruptions occur in volcanoes that have silicic (felsic) magmas like rhyolite. Accordingly, most ignimbrites are of a silicic composition. Generally, an ignimbrite has the same chemical composition as the magma which caused that particular eruption. Under certain conditions volcanoes can produce pyroclastic flows of trachytic, phonolitic or andesitic composition, i.e. of magmas that are not normally associated with explosive behaviour. Some rare ignimbrites may even be formed from saturated basalt, where the ignimbrite would have the geochemistry of a normal basalt.
The colour of Ignimbrite depends on the composition and density. Most often they come in various shades of gray. Conditions during the deposition can alter components to pink, beige, brown or black; or even dark red if they contain a lot of iron minerals.
Welding is a common form of ignimbrite alteration. An important factor is that, in a PF, particles all settle very fast and in great thickness when they are still hot and sticky. They get compressed and, if hot enough, “welded” immediately. Felsic ignimbrites will weld if the temperature is at least 500 to 650°C. The softest parts get flattened and, in case of the mass still moving, streched out to form a texture called fiamme. Fiamme are small dark lenses of glassy material, mostly bits of pumice, that were softened and compressed into the tuff during a welding episode. As the surfaces of a flow cool much faster than the center, the center of a sheet is often the most densely welded, while the top and bottom are often non-welded.
To make things complicated, there are two types of welding recognized.
1. If the flow is sufficiently hot the particles will agglutinate and weld at the surface of sedimentation to form a viscous fluid; this is primary welding. 2. If during transport and deposition the temperature has already become lower, then the particles will not weld immediately. However, welding may occur later if compaction or other factors inside reduce the minimum welding temperature to below the temperature of the glassy particles. This is called secondary welding, and it is the most common in ignimbrites.
At a closer look, ignimbrites often show structures that point out the condition when deposited: gas-escape structures, columnar jointing or flow patterns. Burned ground (palaeosols) directly underlying the tuff would tell of the high temperatures. Hydrothermally altered deposits occur where the PF has run over a lake or stream.
SOME FAMOUS AND INFAMOUS IGNIMBRITE DEPOSITS
Bishop Tuff, USA
The Bishop Tuff in Inyo and Mono Counties, California, is a welded tuff that covers much of the western USA. It has been produced by the Long Valley caldera eruption of ~760 ka. Deposits of Bishop Tuff cover ~2,200 km², and range in thickness 150-200 m. In the 1980s seismic unrest at the Long Valley caldera had many people worried. USGS later stated that it was thought to have been a dike of magma being emplaced at depth.
Cappadocian Ignimbrites, Turkey
The Cappadocian volcanic plateau comprises ignimbrites that are up to 2 km thick in total. One of the last Eastern Cappadocia ignimbrites, the Valibaba Tepe ignimbrite, was linked to Erciyes volcano. This last eruption at 2.8 ma ago had a total volume of 52 km³ and was preceded by a smaller Plinian eruption covering a surface of 1,500 km² with pumice falls.
Settlers in the area soon discovered that the tuff can easily be carved. From the 4th century on they began to carve cells, sanctuaries and entire churches into the soft rock, which were richly decorated with colourful frescoes. Today they can be viewed in an open-air museum. (Also see photo on top of the post.)
Campanian Ignimbrite, Italy
This ignimbrite has been produced by the major eruption, classified at VEI 7, from the caldera of the Phlegraean Fields volcano ~40 ka. It covers most of the eastern Mediterranean and ash clouds from the eruptions reached as far as central Russia. Two thirds of Campania sank under an up to 100 m thick layer of tuff. The greater ignimbrite deposit, mostly trachytic ash and pumice, covered an area of at least 7,000 km². The column collapse that generated the widespread ignimbrite most likely occurred due to an increase of the Mass Eruption Rate.
Leaky: Yucca Mountain (Nevada)
A deep geological repository storage facility for spent nuclear fuel and other high-level radioactive waste in the U.S. is planned deep within Yucca Mountain. The mountain is composed of alternating layers of ignimbrite (welded tuff), non- and semi-welded tuff. The volcanic deposits have been tilted along faults, thus forming the current ridge line. Additionally, the ridge is criss-crossed by cracks that were caused by cooling down of the tuff layers. So far the project is laid on ice because of massive protests. (According to Wikipedia)
In Iceland, during the Halarauður eruption of Krafla caldera (110 ka), a highly unusual basaltic ignimbrite was deposited. Rooyakkers et al. described it as spatter-rich, lava-like and welded to complete coalescence, probably in an extremely hot pyroclastic current. It may have been deposited towards the end of a period of very violent silicic eruptions, when mafic magma from deep levels of the magma reservoir was squeezed through a ring-fault system by the subsiding roof block.
Even Germany has its share of ignimbrites…
Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment.
Enjoy! – GRANYIA
SOURCES & FURTHER READING
– USGS Glossary (Volcanoes)
– Sandatlas: “Ignimbrite”
– Sandatlas: “Tuff”
– Emplacement of unusual rhyolitic to basaltic ignimbrites […] Halarauður, Krafla (2020, paywalled)
– Active ignimbrite quarry in Rouperroux/France – Lithotheque de Normandie
– Types of volcanic rocks, lava, and deposits, Blog
– Armenia’s Pink City (Smithsonian Magazine)
– Climate Change and the Future of Mono Lake, Blog
– Roberto’s GeoSite