The word scoria comes from the Greek "skoria"= rust; Scoria is a highly vesicular, dark colored volcanic rock that may or may not contain crystals (phenocrysts). 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. Vesicules within scoria 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).
Applications for Scoria
Scoria has several useful characteristics that influence how it is used. It is somewhat porous, has high surface area and strength for its weight, and often has striking colors. Scoria is often used in landscaping and drainage works. It is als commonly used in gas barbecue grills. Scoria can be used for high-temperature insulation. Scoria is used on oil well sites to limit mud issues with heavy truck traffic. It is also used as a traction aid on ice and snow-covered roads.
Scoria vs. Pumice
These rocks have several similarities. They are both volcanic rocks 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. It is often glassy just as pumice.
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 is relatively low in mas, but in contrast to pumice, all scoria has a specific gravity greater than 1, and sinks in water. The dry density of scoria is only 1.115 g/cm3, much lighter than ordinary soil. It has a porosity of 57.8%, and a crushing strength of 54.8%.