Understanding Scoria: A Highly Vesiculated Dark-Colored Volcanic Rock

Scoria is a mafic to intermediate highly vesiculated dark gray, black, reddish, or brown extrusive igneous rock (volcanic glass).

This vesicular textured pyroclastic rock forms during eruption explosive of volatile-rich, low-viscosity basalt or andesite magma. The expanding gas bubbles ejected lava blobs are trapped by the rapid cooling of airborne ejecta or the surface of a lava flow.

Scoria originates from a Greek word for refuse or rust, denoting its resemblance to the clinkers, cinders, or silicate slug from iron ore smelting.

Learn more about scoria, including its formation, composition, and characteristics. We will also give you some of its uses and prices.

Quick facts and properties

• Name: Scoria
• Other names: Lava rock (by landscapers and gardeners in the US), cinder, scoriaceous basalt, or scoriaceous andesite
• Rock type: Extrusive igneous that occurs mostly as volcanic glass.
• Texture: vesicular
• Colors: dark (black, dark gray, dark brown, purplish red, reddish)
• Porosity: Ranges from 30 to 80%
• Hardness: Depends on composition, but about 5-5.5 or 6 on the Mohs hardness scale but with lower compressive strength.
• Cooling rate: Fast, forming amorphous volcanic glass without crystals.
• Chemical composition: Mafic but can be intermediate (basaltic to andesitic)
• Mineral composition: Mainly glassy like obsidian and pumice since it cools rapidly so that mineral crystallization doesn’t occur. However, it can rarely be aphanitic basalt or andesite.  
• Tectonic formation environment: Divergent boundary or hot spots between oceans
• Specific gravity: Varies from >1 to 1.6. Sinks in water.

What is scoria?  

Scoria is a dark-colored, highly vesicular (has numerous bubble-like cavities or holes) extrusive igneous (volcanic) rock of basaltic to andesitic chemical composition. It is usually dark gray to black but may be dark brown, brown, reddish, or purplish red. Iron oxidation during an eruption, emplacement, or weathering causes the color to change from dark gray to black when fresh.

This mafic to intermediate rock is mostly glassy and may or may not have crystals (phenocrysts). However, it can be fine-grained (aphanitic). If it is fine-grained and has a composition like basalt, we call it basaltic scoria or vesiculated basalt. In contrast, those with an aphanitic texture and andesite composition are andesitic scoria or scoriaceous andesite.  

Therefore, it should be clear that the name scoria doesn’t refer to mineralogy but rather to the color and vesicular appearance. However, the chemical composition is mafic to andesitic.

Scoria occurs in various sizes, from lapilli larger than a soccer ball to a larger lava flow. Also, the macroscopic voids or cavities will vary in size and concentration and may be of different shapes, i.e., ellipsoidal, spherical, elongated, etc.

Pea-to-fist-sized scoria is often colloquially called cinder in the United States, while rocks with scoria fabric are scoriaceous. Gardeners and landscapers often call them lava rocks.

Reticulite is a highly vesicular scoria with 95-98% cavities and burst walls, leaving only a thin intervening delicate glass thread-like framework or filament. It forms when gases in lava melt excessively and expand, and voids are polyhedral-like shaped with joining glass threads. Some people erroneously call it basaltic pumice due to excessive expansion.

Chemical and mineral composition

The rapid cooling of glassy rocks doesn’t allow for minerals to form except for some small mineral inclusions that may be present. Using this, we can estimate that glassy scoria’s chemical composition is 42-53% silica (SiO₂) and is high in iron oxide (Fe₂O₃), calcium oxide (CaO), and aluminum oxide (Al₂O₃), low in sodium and potassium if basaltic.

On the other hand, those glassy scoriaceous andesitic ones will have a slightly higher amount of silica (55 to 65%), intermediate amounts of oxide of iron, calcium, and aluminum, and a bit more amount of potassium and sodium (alkali).

Lastly, those with aphanitic texture will have a chemical composition identical to basalt or andesite. Such will have minerals like plagioclase feldspar, pyroxene, biotite, olivine, hornblende, and quartz.

One study by Alraddadi & Assaedi (2021) on mesoporous scoria from Saudi Arabia reveals scoria to have about 45-48% silica, 15-16% silica, 6-9% calcium oxide, 11-15% iron oxide, 34-8% magnesium, 0.7-1.6% potassium, and 43-5% sodium and 2-3% titanium oxide. Also, it had smaller amounts of phosphorus pentoxide and sulfur trioxide.

This study should illustrate the chemical composition of this vesicular rock.

How does scoria form, and where?

Scoria is a pyroclastic rock. It forms from fluid-rich, low-viscosity basaltic or andesitic ejecta during an explosive volcanic eruption or lava flow. The driver is magma rising and pressure dropping (depressurization), causing rapid gas exsolution and expansion.

1. How does it form?

As a violent eruption occurs, lava fragments or tephra (ash, lapilli, bombs, or blocks) are ejected into the air, some with escaping and expanding gases. Rapid cooling and solidification of the ejecta while still airborne traps or freezes some bubbles, forming a glassy rock with vesicles.

Also, scoria can form from the remaining basaltic to andesitic lava flows, such as pahoehoe, after the initial eruption. Some bubbles will exsolve from the molten lava, rise, expand, and concentrate near the surface, creating a cavity-rich or frothy layer near the surface. Rapid surface cooling and solidification will trap some expanding and escaping gases, creating voids or vesicular texture.

This second formation scenario will result in the gradation from basalt to a bit vesicular to highly vesiculated rocks near the surface.  

2. Where does it occur?

Scoria from explosive eruptions happens in Strombolian and Vulcanian eruptions. Examples are Stromboli (Italy), Soufriere Hill (the Caribbean Islands of Montserrat), and most of the cinder cones.

However, they can also occur from Hawaiian eruptions like Kilauea, Mauna Loa, and Mauna Kea, often associated with Pele’s hairs and Pele’s tears.

Lastly, scoria can form from phreatomagmatic eruptions like in Sturtsey in Iceland (1963) and Taalian Taal in the Philippines. The rapid explosion happens when hot lava meets water, and escaping or expanding gases will make the resultant rock bubbly.

Cinder or scoria cones

Cinder cones (scoria cones) refer to conical-shaped steep-sided hills made of loose volcanic fragments (pyroclasts) like scoria, volcanic ash, and clinkers. They form during explosive volcanic eruptions or lava fountains, mostly from a single explosion (monogenetic), but can form from more eruptions (polygenetic).

Gas-charged lava will fragment into smaller pieces, and some will cool and solidify with trapped gas bubbles and fall on the side of the cone.

Most cinder cones are symmetrical and have a bowl-shaped crater and a cylindrical vent in their center. Youngly formed ones will have a slope of about 33°, corresponding to the angle of the rest of the loose scoria pyroclasts.

Related terms are scoria mounds (which lack apparent craters) like Anakies in Australia. Also, due to polygenetic eruptions, you can have nested scoria when a cone forms on top of the other.

Lastly, cinder cones are the most popular volcanos all over the world. Examples include Paricutin (Mexico), Cerro Negro (Nicaragua), Lava Butte (US) and Monte Nuovo (Italy), and Sunset Crater (USA).

How does scoria differ from pumice?

Similarities are that both scoria and pumice are highly vesicular (with subspherical cavities or voids) extrusive igneous (volcanic) rocks. Besides the vesicular texture, they also are mostly glassy. However, they have hardness, density, color, and composition differences to help you distinguish them easily.

Scoria is a darker (black, grayish, or reddish), harder, mafic, or sometimes intermediate rock with larger vesicles and thicker walls that sink in water (denser). In contrast, pumice is a lighter (whitish, light gray, or tan), softer, felsic to intermediate rock with smaller cavities that have thinner walls that float in water (less dense)

Regarding composition and magma viscosity, scoria forms from less viscous basaltic to andesitic magmas with less silica, alkali (potassium and sodium) and more magnesium, calcium, and iron, while pumice from more viscous rhyolitic, dacitic, andesitic, trachytic, or phonolitic magmas with more silica and alkalis but less iron, magnesium, and calcium.  

Why does pumice float, not scoria?

Pumice floats in water because it has many smaller, isolated, air-filled cavities with thin walls, making its specific gravity lower than water. It happens because the highly viscous magma doesn’t allow gas bubbles to move and merge.

On the other hand, scoria sinks in water because its less concentrated cavities are larger, have thicker walls, and are not isolated, making its density higher than water. The low viscosity of basaltic magma allows gas to move and fuse, with some bursting.

Uses of scoria

Scoria North America and the rest of the world come from primarily cinder cones. Some of its uses include the following:

• Gardening:  Scoria, or lava rock, helps improve aeration, drainage, nutrient retention, and soil tilth if added to the soil, just like perlite. You also use it in hydroponic gardening, mulching, and garden decoration.
• Landscaping: Its light weight, durability, and porosity make it the perfect stone or pebble for landscaping residential and commercial areas. Taco Bell uses red-colored ones.
• Concrete manufacture: A study by Ayene & Amenu (2023) shows that this stone could partially replace cement in concrete manufacturing, a good effort towards green concrete production.
• Aggregate for light construction: Scoria is quarried and used as low-grade aggregate for smaller porous country road foundation aggregate. Also, you can use it in drainage systems, rip-rap, and other construction jobs where porosity is important.
• Lightweight insulating concrete: Since it is not heavy, it can make concrete for structures that require less steel reinforcement. Also, its vesicular texture with trapped air bubbles can help provide insulation, lowering overall heating costs.  

More scoria uses include helping increase friction on icy surfaces or roads during the winter, high-temperature insulation, sauna or barbeque stones, roofing granules, earthbag houses, driveways, etc.

Frequently asked questions

References

• Huggett, R. J. (2011). Fundamentals of Geomorphology (3rd ed.). Routledge.
• Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
• Fisher, R. V., & Schmincke, H. U. (1984). Pyroclastic rocks. Springer-Vlg.
• Gill, R. (2010). Igneous rocks and processes: A practical guide. Wiley-Blackwell.
• Winter, J. D. (2014). Principles of igneous and Metamorphic Petrology (2nd ed.). Pearson Education.
• Bonewitz, R. (2012). Rocks and minerals (1st ed.). DK Pub.
• Tarbuck, E. J., Lutgens, F. K., & Tasa, D. (2017). Earth: An introduction to physical geology (12th ed.). Pearson.
• Ayene, H., Geremew, A., & Amenu, T. (2023). Potential use of scoria as a cementitious material for green concrete production. Advances in Civil Engineering, 2023, 1–11. https://doi.org/10.1155/2023/5532027
• Alraddadi, S., & Assaedi, H. (2021). Physical properties of mesoporous scoria and Pumice Volcanic Rocks. Journal of Physics Communications, 5(11), 115018. https://doi.org/10.1088/2399-6528/ac3a95