Lapilli tuff and lapillistone refer to a pyroclastic rock formed from predominantly lapilli-sized (2-64 mm) pyroclasts ejected during an explosive volcanic eruption. These rocks can have other pyroclasts like volcanic bombs, blocks, or ash.
Discover the difference between lapilli tuff and lapillistone, if any, composition, and how they form. We will also look at examples of associated textures – eutaxitic, fiamme, and parataxitic textures – and much more.
What is a lapilli tuff and lapillistone?
A lapilli tuff is a pyroclastic rock made predominantly of lapilli (2-64 mm), usually poorly sorted. Some definitions will consider it a pyroclastic rock with less than 25% volcanic blocks and/or volcanic bombs with lapilli and ash no more than 75% of the composition.
Lapillistone, on the other hand, refers to a pyroclastic rock with more than 75% lapilli. The rest can be volcanic ash, blocks, or bombs. However, some geologists and the International Union of Geological Sciences (IUGS) prefer to call such a rock a lapilli tuff.
Another recommendation by IUGS for poorly sorted pyroclastic rock or tephra is to consider composition and name it according to their relative amounts. For instance, an ash-lapilli-tuff suggests ash is more than lapilli in the tuff. A vice versa will be true.
Other players also have different definitions. For instance, the BGS (British Geological Survey) (Gillespie & Styles, 1999) considers a lapillistone as any pyroclastic rock with at least 75% of the pyroclasts less than 64 mm.
The BGS further clarifies that the relative percentage of ash and lapilli-sized fragments will determine if you have a lapillistone, lapilli tuff, or tuff.
Some definitions will consider tuff, lapilli tuff, or lapillistone to be any pyroclastic rocks, where pyroclast fragments less than 64 mm account for more than 75%. A lapilli tuff will have 25-75% lapilli-sized fragments, and a tuff less than 25% lapilli with more than 75% ash (less than 2mm). If the ash is more than 75%, you will have a tuff or ash-tuff.
We use a prefix to talk more about their composition. For instance, you can have andesitic and rhyolitic lapilli tuff. Also, if you consider the fragments’ granulometric state to describe these rocks further, you can use terms such as lamproitic or kimberlitic lapilli tuff. Here, these two terms are only qualifiers.
Composition
A lapilli tuff or lapillistone will have welded or unwelded predominantly lapilli together with volcanic ash (< 2 mm) and volcanic bombs and/or volcanic blocks larger than 64 mm. This will include volcanic glass shards, pumice bits, lava blobs, and accidental clasts.
However, the exact composition of lapilli tuff or lapillistone varies. It depends on the kind of fresh magma and the mineralogy of any accidental clasts or previously erupted magma.
Lapilli tuff or lapillistone often form from high-viscosity magma, especially the rhyolitic or dacitic (felsic) andesitic (intermediate). In these magmas, basal surge eruption will result in thick lapilli layers.
However, you can also have those of basaltic composition, but these are rare as their low viscosity will favor the formation of Pele’s tears.
Accidental clasts or xenoliths – torn from country rock in the vent – can be igneous, sedimentary, or metamorphic. Sometimes, you can have a xenocryst, an accidental clast made of an individual mineral.
How do they form?
These pyroclastic rocks form during explosive volcanic activities like Strombolian or Plinian. Such powerful eruptions will create and eject tephra or pyroclastic fragments into the air.
When the tephra finally falls, they form agglomerates, pyroclastic breccia, lapilli tuff, or tuff. The difference will be the relative amounts of ash (< 2 mm), lapilli (2-64 mm), block (> 64 mm), or bomb-sized (> 64mm) pyroclasts.
Also, they can be welded or unwelded. Let us discuss more:
1. Welded lapilli tuff or lapillistone
Lapilli pyroclasts are usually semi-molten magma droplets. If they fall while semi-solid, they will fuse to form a welded lapillistone or welded lapilli tuff. Such rocks are hard and will not erode easily.
Also, the high temperature and weight (pressure) of newly deposited tephra may result in the agglutination of loose glass shards, pumice, and other pyroclasts to form a welded lapilli tuff. Look at it as the transformation and sintering of initially deposited unconsolidated tephra.
A good example is the transformation sintering will result in basaltic scoriae formation from loose tephra (ash and lapilli accumulation). It happens under high temperatures and pressure (weight) from overlaying tephra from current volcanic activity.
Remember, if the temperature is high enough, or glass shards and pumice are plastic, they will sinter together. This sintering can occur during compaction or flow.
2. Unwelded
If these pyroclasts remain unconsolidated, you will have a bed, tephra, layer, or non-welded lapilli tuff. Since they have not fused or lithified, these tuffs will erode easily compared to welded ones.
Eutaxitic fabric, fiamme, and parataxitic texture
When researching lapillistone or lapilli tuff, you may come across the terms eutaxitic, fiamme, and parataxitic. What exactly do they mean? Let us look at each
1. texture or fabric
Sometimes, the heat and weight of the new tephra deposit may result in the flattening, followed by welding of semi-molten pyroclastic to form a banded or layered eutaxitic fabric or texture.
Eutaxitic texture will have flattened lapilli and discus-shaped or oblate blocks or bombs in layers. These resultant discoidal shapes or flattened bits will nearly parallel the deposition direction or show planar foliation texture.
2. Fiamme
Eutaxitic texture may also have a ragged lens or flame-shaped, flattened pumice lapilli (black bits) in a welded pyroclastic rock forming a fiamme. Fiamme is an Italian name that means flame. This term is non-genetic and only describes the lens-shaped lapilli bits.
3. Parataxitic texture
A parataxitic texture describes the highly stretched and flattened fiamme with a linear and not planar fabric.
Examples of lapillistones
It has been a long discussion so far. But we feel you need to have one or two examples of lapillistone. Here are typical examples.
1. Calciocarbonatite lapillistone
This welded lapillistone is almost exclusively composed of calcium carbonatite lapilli (tannish to brownish to grayish parts with subrounded to rounded appearance) cemented by calcite (whitish). Like other rocks with calcium carbonate (calcite), it reacts with acids, i.e., bubbles.
This extrusive rock forms when ejected molten pyroclastic droplets of calcite carbonatite fall from a lava or fountain spray and weld.
2. Olivine melilitite lapillistone
Olivine melilitite lapillistone is a very unusual extrusive igneous rock with a clastic texture. It has brownish-gray, nearly rounded to angular olivine melilite lapilli cemented by the yellowish calcite.
The porphyritic olivine melilitite lapilli has a mixture of melilite and diopside pyroxene. Melilitite is a calcium sodium magnesium aluminosilicate whose chemical formula is (Ca, Na)2(Al, Mg) (Si, Al)2O7).
Accretionary lapilli tuff
These refer to lithified accretionary lapilli. These are not true lapilli but lapilli-sized, spheroidal fragments formed when volcanic ash coalesce. This accretion commonly occurs in the presence of moisture or via electrostatic forces.
References
- W., L. M. R. (2002). Igneous rocks classification and glossary of terms (2nd ed.). Cambridge University Press.
- Gillespie, M. R., and Styles, M. T. (1999). BGS Rock Classification Scheme, Volume 1, Classification of igneous rocks: British Geological Survey Research Report, (2nd edition), RR 99–06.
- Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.