Scoria Rock Type, Composition, Formation, Occurrence & Uses

Scoria Rock
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Scoria is a porous and dark-colored igneous rock that may or may not contain crystals. The usual color of scoria is dark brown, black, and dark purple with a hint of red in it.

Scoria is often confused with pumice. Pumice is another famous porous rock with large vesicles and thicker vesicle walls and is denser than scoria. The difference in both rocks is due to lower magma viscosity, allowing quick volatile diffusion, bubble growth, coalescence, and bursting.

Scoria’s small particles found on the landscape are known as “lapilli” if the size is 2 – 64 millimeters. Particles bigger than 64 millimeters are known as blocks.


Scoria vs. Pumice

Scoria particles are often called cinders because many petrologists and geologists believe that small pieces of scoria look like the ash produced on a coal surface. The small volcanoes that erupt scoria are also known as cinder cones.

Scoria and pumice also differ in colors. Scoria is black, dark gray, or reddish brown, while pumice is usually white, light gray, or tan. The color difference is due to the difference in their composition. Scoria forms from basaltic magmas, and pumice is formed from rhyolitic magmas. Rhyolitic magmas contain more gas than basaltic magmas.

A close inspection with a hand lens shows tiny mineral crystals in scoria, whereas pumice has a glassy texture similar to obsidian. Pumice forms after rapid magma cooling. The process is so quick that atoms cannot arrange themselves into ordered crystalline structures.

Rock Composition

Scoria is usually composed of approximately 50% silica, 10% calcium oxide, and lower contents of potash and soda. The major minerals found in scoria include plagioclase, pyroxene, and olivine. Apatite, biotite, hematite, hornblende, ilmenite, magnetite, and quartz may be present in minor amounts.

Scoria Formation

Scoria is formed when rising magma encounters lower pressures, allowing dissolved gases to escape and make bubbles on the rock surface. The vesicles are trapped when magma cools and solidifies. The vesicles are mostly small and spherical. They do not overlap but open into one another with little distortion.

Where is Scoria Found?

Scoria is found in regions with volcanic activity. It gathers around the vents of a volcano. The cone-shaped hill formed by scoria deposits is known as an ash cone.

Scoria Uses

The biggest commercial use of scoria is to prepare lightweight aggregate. The rocks are crushed into desired sized to be used for construction. A concrete slab with scoria is 35% lighter than concrete slabs with sand and gravel. This low weight allows engineers to use less structural steel in buildings and homes. The air inside the scoria also improves concrete’s insulation properties, resulting in lower heating and cooling bills.

Powdered scoria is also used as roofing granules, landscaping ground cover, hydroponic gardening substrate, and drainage stone. Small amounts of scoria are often used as a sauna rock and heat sink in BBQ grills.

Scoria is often used on oil well sites to limit mud problems due to heavy traffic. It is used as a traction aid on snow-covered roads.

Scoria has horticulture uses too. It holds water in vesicles and porous spaces between grains. It is used to improve the capacity of soils surrounding plants to hold moisture. Specific scoria blocks can stop pests such as termites due to their ragged surface.

Scoria Substitutes

Scoria substitute can be produced by heating shale in a rotating kiln under controlled conditions. Only particular shale types will produce results similar to scoria. This man-made material is sold under expanded clay, grow rocks, or expanded aggregate.

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