Lapilli are rock fragments or pyroclasts measuring 2 to 64 mm (0.08 to 2.52 inches) in diameter formed and ejected during an explosive eruption. These pyroclasts are made of mainly molten to semi-molten fresh magma or lava but can be from previously erupted magma or accidental clasts.
Most of these pyroclasts remain semi-molten when they land on the surface and may weld. However, some will fall after solidifying, creating layers of accumulated unwelded tuff.
Today, we will discuss lapilli, from definition to formation to composition. Also, we will cover accretionary and armored types.
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Geological lapilli definition
Lapilli (singular: lapillus) are pyroclasts (volcanic fragments) with diameters between 2 mm and 64 mm ejected into the air during an explosive volcanic eruption. These fragments are smaller than 2mm, known as volcanic ash; if larger than 64 mm, they are volcanic bombs or blocks.
Some strict definitions consider lapilli as spheroidal, dumbbell, button, or teardrop-shaped molten or semi-molten magma droplets hurled into the air and land while still partly molten. However, they can include 2 – 64 mm pyroclasts that land as solids.
An important thing to remember is that fragments or granules that form lapilli are not accretionary. Instead, they are molten or semi-molten lava that may cool during flight. However, we will discuss accretionary and armored types later in this discussion.
Lapilli is a Latin term that means little stones. It doesn’t indicate a certain composition, but the size of pyroclasts or tephra, which, as said, is 2-64 mm in diameter.
Lastly, their colors vary. They can be grayish, reddish, brownish, etc. It all depends on the composition of the magma.
How do lapilli form?
Lapilli and other pyroclasts form during explosive volcanic eruptions. Eruptions like Plinian and Strombolian will create and eject fragments or pyroclasts like lapilli, ash, volcanic blocks, and bombs into the air or what you may call tephra.
If what is ejected is molten and semi-molten pea to walnut-sized magma droplets, you have lapilli. These spheroidal, tear-drop, dumbbell, or button-shaped magma droplets will remain and land while semi-molten. However, some may solidify while in flight.
When semi-molten ones fall to the ground and fuse or weld, they form welded lapilli tuff (has less than 25% block and/or bombs and less than 75% ash and lapilli) or lapillistone. Lapillistone will have more than 75% lapilli.
The heat from a newly deposited material will make semi-molten material flatten and then weld, resulting in a texture like eutaxitic – characterized bands or layers. It forms when glass shards and pumice fragments compact and flatten. Also, you can discus-to-oblate-shaped plates within lapilli layers.
Another possible texture during a welding episode is fiamme (Italian for flame). This texture has an elongated lens-shaped dark material that forms most likely when pumice bits are softened and compressed as the pyroclastic rock is welded.
Lastly, if these pyroclasts remain unconsolidated (don’t fuse or lithify), you have a layer, tephra, bed, or non-welded lapilli tuff. These non-welded tuffs or accumulations are not pyroclastic rock and will erode easily.
What is their composition?
Lapilli are mainly juvenile clasts, i.e., derived from fresh magma or lava involved in the eruption. However, they can form from solidified magma that previously erupted or even have accidental clasts torn from a country rock on the vent walls.
The exact mineral composition of lapilli will vary greatly and depend primarily on the fresh magma. It can be basaltic, rhyolitic, or intermediate.
However, it is common in high-viscosity magmas like andesitic, rhyolitic, or dacitic. For instance, a base surge eruption may deposit thick dacite, rhyolitic, or andesitic lapilli layers, forming a welded tuff if semi-molted.
In basaltic pyroclastic eruption, the low viscosity of the magma will instead form Pele’s tears. Pele’s tears are glassy, teardrop-shaped lapilli that cools rapidly while still in flight or air-borne. They may have Pele’s hair attached to their ends.
Other types
Besides what we have looked at, you may have accretionary and armored types. Here is what each is:
1. Accretionary lapilli – spheroidal ash pellets or small balls
Accretionary lapilli are rounded or spheroidal pellets or balls made of aggregated volcanic ash particles. These pellets form in an eruption column or dispersing ash cloud with moisture (causes wet ash to coalesce) or by electrostatic forces.
The process will start with the volcanic ash nucleating on something – solid particles, objects – or condensing water. Afterward, volcanic ash will accrete in layers before the formed accretionary pellets fall to the ground. So, they are sometimes known as ‘volcanic hailstones’ since they form like how hailstones form from water.
The other thing to note is that accretionary lapilli form mainly phreatomagmatic eruptions. This is the kind of eruption that occurs water violently flashes to steam upon meeting molten magma.
However, these pellets can form in other eruptions, especially if sufficient atmospheric moisture allows ash to bind together, including rain falling through dry clouds.
Lastly, you can categorize these accretionary pellets into two, considering their morphology as follows:
- Rim-type lapilli – Their core is coarse-grained, and a finer-grained rim surrounds them. This rim has alternating fine and very fine ash.
- Core type – The core type doesn’t have a fine-grained rim. And as (Schumacher & Schmincke, 1991) notes, “field relationships, internal, and grain-size characteristics are specific to accretionary lapilli from different types of tephra deposits.”
2. Armored lapilli – fragment core with a volcanic ash rind
Armored lapilli are accretionary lapilli type formed wet, fine to coarse volcanic ash is plastered around a solid (crystal or lithic) core during a volcanic eruption. They will have 2-30 mm of mostly juvenile pyroclasts like pumice or a lithic center surrounded by layers of ash particles.
This armored type will only form where a significant amount of water is present, such as in hydroclastic eruptions, where cohesive ash sticks form a rind around it.
Are they volcanic hazards?
Yes. Lapilli causes damage to property or loss of life if carried downwind inside the volcano clouds. However, they are not hazardous because they will fall to the ground once the cloud cools. So they won’t cause much damage far away from the eruption vent.
If unconsolidated, these pyroclasts can feed pyroclastic flows, one of the most dangerous volcanic hazards.
References
- Selley, R. C., M., C. L. R., & Plimer, I. R. (2005). Pyroclastics. In Encyclopedia of geology (1st ed.). Elsevier.
- W., L. M. R. (2002). Igneous rocks: IUGS classification and glossary of terms (2nd ed.). Cambridge University Press.
- Schumacher, R., & Schmincke, H.-U. (1991). Internal structure and occurrence of accretionary lapilli? A case study at Laacher See Volcano. Bulletin of Volcanology, 53(8), 612–634. https://doi.org/10.1007/bf00493689
- Lapilli. (2022, August 13). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Lapilli&oldid=1104197435