Scoria is a highly vesiculated lava or tephra1. It is usually dark-colored and has a mafic composition. It is difficult to say for sure whether it is a rock type or not. It is tempting to say no. Scoriaceous rocks are simply dark-colored volcanic rocks with lots of variously sized and usually smooth-sided holes (vesicules) in it. It seems that “scoriaceous” simply refers to their porous texture.
However, the same is true with pumice. Pumice is usually felsic, but no strict chemical definition exists. Pumice can be rhyolitic, dacitic, phonolitic, etc. It just needs to be very porous and light-weight lava froth and yet it is still considered to be a rock type.
Scoria from a scoria cone in Tenerife, Canary Islands. Width of sample 7 cm.
Scoria and pumice have several similarities. They are both volcanic rocks (often pyroclastic) and they contain vesicules. Vesicules in pumice are usually smaller and more irregularly shaped. Pumice is a very lightweight material that usually floats in water. Scoria is lightweight also, but it sinks in water. Its vesicles can be much larger than vesicles in pumice are. Scoria is often glassy just as pumice. Fresh scoria can be distinctly pitch black and shiny. Older scoria is duller and brown or even reddish due to oxidized iron. Scoria contains much more iron than average pumice which contains more alkali metals (potassium, sodium). Iron gives black color to fresh scoria.
Vesicules within scoria (and in pumice also) form when volcanic gases are released from magma. They are released because of decreasing pressure as magma moves upward (solubility of gases in liquids is dependent on pressure — higher pressure means better solubility). Mafic magma is less viscous than felsic magma. That’s why gas bubbles can move more freely and join each other and form larger vesicules. Bubbles that form in felsic magma can not move as well. Hence, there are much more pores in pumice than scoria and they are smaller.
Both lava flows (especially their upper part) and pyroclastic fragments can be scoriaceous, but it is usually the latter that the term “scoria” is associated with. It even seems to me that sometimes the terms “lapilli” and “scoria” are used interchangeably. Small usually monogenetic volcanic cones are known as scoria cones. It is easy to make an obvious conclusion that this fragmental material these cones are mostly made of is scoria. This conclusion is not mistaken. It is scoria indeed, but not because it is fragmental. It is scoria because it is vesicular.
Scoria as a pyroclastic material (tephra) usually has a size of lapilli (2-64 mm) which is larger than volcanic ash and smaller than volcanic blocks and bombs.
Scoria from Etna, Italy. Width of sample 5 cm.
Small scoriaceous lapilli from Cumbre Vieja, La Palma, Canary Islands. The samples from Tenerife and Etna (above) look like classic scoria, but these small lapilli here are actually more typical. They are not very vesicular and vesicules are not large, but this is how scoriaceous lapilli usually look like. The two samples above are not rare or unusual, but still they are specifically chosen in the field out of many candidates to demonstrate the very vesicular nature of “true scoria”. Width of view 6 cm.
Pumice is in many ways similar rock, but it is generally lighter in both weight and color. Santorini, Greece. Width of view 4 cm.
Eroded scoria cone in La Palma. Such cones are common smaller volcanoes on the flanks of larger ones.
Field of scoriaceous lapilli in La Palma.
Pieces of scoria and pumice on the same picture. Scoria from Tenerife, pumice from Santorini.
1. Le Maitre, R. W. (2005). Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks, 2nd Edition. Cambridge University Press.