An ignimbrite or ash-flow tuff is a type of volcanic rock formed from pyroclastic flow deposits. It has poorly sorted pumice fragments, scattered lithic fragments, and other volcanic debris in a volcanic ash matrix. The volcanic ash matrix or groundmass has crystal fragments and glass shards.
Pyroclastic flows or density currents are hot gases, ash, rock debris, and other volcanic materials traveling rapidly down-slope. These materials are what explosive eruptions eject. We collectively call them tephra, while pyroclasts are fragment portions.
Usually, pyroclasts flow under the influence of gravity, being heavier than surrounding air. They can be hazardous, and when they finally settle, consolidate, or weld, they form various pyroclastic rocks. Rocks formed depend on the pyroclasts present. These will include tuff, tuffaceous, lapilli tuff, lapillistone, etc.
A New Zealand geologist, Patrick Marshall (1869–1950), coined the term ignimbrite. It comes from two Latin words: igni, which means fire, and imbri, rain.
Although it initially meant a kind of welded tuff. It no longer does. Also, this name does not refer to ignimbrites’ thickness, lateral extent, or local distribution.
Appearance and Characteristics
Ignimbrites are very poorly sorted rocks. They have mainly pumice fragments, scattered lithic fragments, and volcanic debris in volcanic ash. If the volcanic ash is compacted and cemented (lithified), It is known as tuff.
Usually, they are massive (not layered) but may be layered. Also, they can be unconsolidated (loose), consolidated, or welded. Some authors called the lithified ignimbrites welded lapilli tuff.
Color, thickness, extent, and hardness will vary. Some will show various types of grading. Here is more:
1. Colors
Ignimbrites are usually whitish, grayish, beige, pink, reddish brown, brown, or black, with most being light-colored.
Usually, the given hand specimen will depend on its composition, density, and whether they are welded or consolidated.
For instance, less welded will be gray to white, and those intensely welded will be dark gray to black. Also, those with mafic composition are darker. However, the darker varieties with heavily welded volcanic glass are less common.
Lastly, high-temperature iron oxidation of fine-grained ignimbrite will reddish the upper parts.
2. Thickness and extent
Their thickness varies from a few decimeters to hundreds of meters. Some, like the Cappadocian volcanic plateau, are up to 2 km thick.
Usually, thicker ones tend to be massive. However, some thick deposits may show distinctive stacked flow or cooling units. Also, deposits closer to the vent are thicker than those far away, and very thick deposits may have columnar jointing due to contraction from cooling.
On the other hand, deposits can have a bulk volume of 0.1 to over 1,000 km3. For instance, the Fish Canyon Tuff erupted at a volume of 5,000 km3 of ignimbrite. These deposits will cover areas from a few to thousands of kilometers,
Lastly, the distribution of deposits depends on topography. Thicker deposits will occur in low areas and thinner on ridges. Some sites will have extensive flat sheets.
3. Ignimbrite texture
Consolidated ignimbrite will have a fine-grained or aphanitic texture. In contrast, welded will have a eutaxitic texture (a layered or banded texture). It forms from the fusing of pumice fragments and glass shards.
4. Hardness and density
Hardness and density will vary with composition, welding, or thickness. For instance, thicker deposits cool slowly, causing inner deposits to fuse and compact. This will form tough, medium-density rocks.
On the other hand, thinner, unwelded deposits or those closer to the surface will be less dense and tough.
5. Grading and compositional zonation
Usually, finer ash particles settle on coarse material. However, some may show inverted grading. Also, lateral grading occurs.
On the other hand, vertical and lateral composition variations may occur, including crystal-poor and rich zones. It may happen when supereruptions tap from different magma sources. However, mechanical mixing of crystal phases may occur in the eruption column.
What are ignimbrite rocks made of?
Ignimbrites are very poorly sorted. They have mainly volcanic ash matrix, some pumice fragments, and a few scattered lithic fragments. Also, it may have any other ejected fragments.
The volcanic ash matrix has glass shards and crystal fragments. Crystal fragments are mostly broken phenocrysts. They form part of the tephra blow during an explosive eruption and turbulent flow. However, a few of these crystals are xenocrysts.
Phenocrysts are larger crystals that grow in magma, while xenocrysts don’t. Usually, xenocrysts originate from other magma, igneous, or country rocks.
Lithic fragments or inclusions are pea to cable-sized rock pieces. These are older solidified volcanic debris entrained from the eruption conduit or land surface. Clasts are rarely from the erupting magma.
Lastly, emplacement close to the eruption vent will have a thick, lithic block accumulation. Those far away will have a meter-thick collection of rounded pumice cobbles.
Composition
Most ignimbrites are felsic (rich in quartz and feldspar) with more than 65 wt. % silica (SiO2). These are usually silica-rich dacitic or rhyolitic.
Resultant ignimbrite chemistry, phenocrysts mineralogy, and population depend on how dominant elements potassium, sodium, calcium, and lesser dominant like iron or magnesium vary.
However, some will have intermediate composition, i.e., andesitic. Also, a few may be basaltic or mafic (iron and magnesium-rich), especially the volatile saturated.
Mineral composition
The mineral composition of ignimbrite depends on the original magma. These rocks usually have quartz, biotite, and alkali feldspar phenocrysts like sanidine. Also, it may sometimes have hornblende and rarely pyroxenes.
Most of these phenocrysts come from erupting magma, i.e., are juvenile. However, a few may be xenocrysts from older igneous rocks, country rocks, or other magmas.
However, phonolite tuffs will have feldspathoid phenocrysts like nepheline and leucite instead of quartz. Phonolite is a type of trachyte with feldspathoids instead of quartz. It is the extrusive or fine-grained equivalent of nepheline syenite.
Lastly, welded ignimbrites and breccia may have cristobalite and tridymite. These are high-temperature quartz polymorphs or forms. These two appear as an alteration after the eruption. Therefore, they are not primary minerals from cognate magmas.
Ignimbrite formation
Ignimbrite forms when ground-hugging, fast-moving pyroclastic flows from explosive or violent eruptions like Plinian or Volcanian settle. These eruptions may form ignimbrite calderas. Compaction may occur on their weight, and if it is hot, welding will occur.
Also, exsolving volatiles after emplacement may create vesicles and alter the surrounding groundmass.
Alteration
With time, glass in ignimbrite may undergo devitrification. This involves changing its structure from an amorphous solid without crystals into a crystalline solid.
Also, chemicals in hot fluids may alter these rocks when large, hot ignimbrites blanket wet soil or cover rivers and water courses. This may form pockets of kaolin-altered rocks and chimney-like structures.
Also, as in the Novarupta Tuff eruption, blanketing can form fumaroles or geysers, some running for years.
Welded ignimbrite
Welding is an alteration of ignimbrite. It occurs when temperatures are high enough, at least 535-650°C, to make glassy ash particles fuse or sinter.
Silence-is-infinite, CC BY-SA 3.0, via Wikimedia Commons.
Also, pressure from overlaying material can make the compressible, highly vesicular juvenile pumice fragments collapse to form a fiamme. A fiamme has a darker-colored lens-shaped surface formed by collapsed pumice.
Welded ignimbrites will have a eutaxitic texture, and the welding may be primary or secondary. Primary occurs during transport and emplacement when temperatures are hot enough to cause sintering. It forms flow-banding, as seen in felsic lava flows. However, this layering isn’t the same as laminar fluid flow in some pyroclastic flows.
On the other hand, secondary afterward under favorable conditions that favor sintering.
Usually, depth can affect the extent of welding, with interiors of thick deposits retaining heat for longer. Also, such rocks will be denser and not vesicular due to heat. Those on the surface or thinner deposits will be vesicular and less dense.
Lastly, some thick ignimbrite deposits may have a glassy top, bottom, and crystalline center. This results from heat insulation at the center that makes rocks cool slowly.
Deposition
Deposition occurs via en masse freezing or progressive aggradation. In en masse freezing, a flow section halts abruptly on its entire depth. However, fronts may continue flowing.
On the other hand, according to a study, the progressive aggradation model considers the depositional boundary layer continuously supplying material. This can form inversely graded ignimbrites.
Besides the models, rheomorphic flow occurs in ignimbrites with temperatures exceeding welding or solidus temperatures, i.e., high grade. Fusing happens after deposition, and ductile flow will deform layering, vesicles, and clasts.
Where are ignimbrites found?
Ignimbrites occur in many volcanic provinces with silica-rich magma across the globe—the high silica results in explosive eruptions that form pyroclastic flows.
In the US, notable ignimbrite deposits are in 1) Basin and Range Province and 2) Snake River Plain. The Basin and Range Province occur mainly in Nevada, Southern Arizona, Western Utah, and in the North and Central parts of New Mexico.
Also, it occurs in the Rattlesnake Formation in Oregon, the Aniakchak National Monument, and the Valley of Ten Thousand Smokes in Alaska. The other place is Volcanic Tableland, part of Bishop Tuff’s outflow sheet in California.
Elsewhere, it occurs in Australia at the Hunter Region in New South Wales and New Zealand in the Coromandel region and Taupō Volcanic Zone.
More places are the Sierra Madre Occidental in Western Mexico, the Canary Islands at Tenerife, and Gran Canaria. Also, it occurs at Campanian ignimbrite eruptions in Italy and Cappadocian in Turkey.
Uses of ignimbrite?
Some of the uses of ignimbrite are cladding or decorative wall building and making aggregate for road building and other construction jobs. Also, you can use it for paving, landscaping, etc.
For instance, layered ignimbrites are split to make flagstones for paving or garden edging stones. In Zealand, the Hinuera Stone is a welded ignimbrite for cladding.
Besides these uses, one unconventional use of this rock and tuff is a nuclear waste respiratory at Yucca Mountains.
Frequently Asked Questions (FAQs)
The tertiary ignimbrite flare is a mid-Cenozoic (40-25 mya) widespread and intense volcanic eruptions of ash-flow tuff and intermediate to silicic lava in the western United States. Its eruptions produced 5×105 km3 of ash tuff flow and 5×106 km3 lava flow.
Hardened ignimbrite will form erosional landscapes resembling granitic, including domes, tors, nubbins, tafonis, inselbergs, and grammas. Examples are in Sierra de Lihuel Calel, in Argentina. The joining system of these rocks may affect these landforms.
Further reading
- Branney, M. J., & Kokelaar, B. P. (2003). Pyroclastic density currents and the sedimentation of ignimbrites. Geological Society.