Scoria is a textural term for any highly vesicular, dark-colored extrusive igneous rock whose composition is mafic to intermediate. This term is therefore a textural term, not a rock of a certain composition, like granite, gabbro, or basalt.
The name scoria is of Greek origin. It refers to rust or the refuse byproduct of iron ore smelting commonly called clinkers, cinders, or silicate slug.
Scoria forms during violet eruptions of volatile-rich magma with low to moderate viscosity. The numerous vesicles form when magma cools rapidly, trapping in place some of the escaping gas bubbles.
Commercially, this rock is known as lava rock. Its uses include landscaping, gardening, and making low-density concrete, among others.
Properties and description
Scoria is a highly vesicular, dark-colored extrusive igneous rock. Its Mohs hardness is 5-6, and its density ranges from 1.28 to 1.60 g/cm3.
The sizes of these rocks vary from lapilli to the size of a football or even larger. Cinders refer to scoria rocks whose size is like a fist.
Scoria rocks are usually dark gray to black, but some varieties are dark brown, brown, reddish, or reddish with a purple hint. Freshly formed ones are mostly dark gray but will turn blackish when the iron present is oxidized.
The porosity of scoria ranges from 30-80% but is higher in the reticulate variety.
Reticulate scoria is highly vesicular with 95–98% cavities and an interweaved glass filament forming polyhedral-like, joined voids. It forms when magma is high in volatiles, causing gas bubbles to excessively expand and burst. Please don’t confuse it with pumice, which is light-colored and frothy-like.
These rocks can have microscopic to macroscopic cavities that may be irregular, spherical, ellipsoidal, or elongated and are often jointed.
Microvesicular scoria is the name for those with tiny vesicles that are only visible using a microscope. You cannot see these cavities with an unaided eye.
Scoria usually has a glassy texture, but a few have an aphanitic or fine-grained texture. The fine-grained varieties are often named after the compositional rock name, such as basalt or andesite.
For instance, basaltic scoria (vesiculated basalt) has a composition like basalt, and andesite scoria (scoriaceous andesite) has a composition like andesite.
Scoria composition
Scoria is a basic to intermediate volcanic rock with 45–63% silica. It is relatively high to moderate in iron, calcium, and magnesium oxides and low to moderate in potassium and sodium oxides.
The exact content of iron, calcium, magnesium, sodium, and potassium oxides depends on the given specimen and is basic or intermediate.
Mineral composition
It is mostly a volcanic glass (amorphous solid) formed when magma quickly cools when it forms. This rapid cooling doesn’t allow time for larger mineral crystals to form and grow. However, these volcanic glasses will often have tiny, microscopic, and embryonic crystals called microlites or crystallites.
Some specimens with fine grain textures have compositions similar to basalt or andesite, depending on magma composition.
Those with basaltic composition have mainly plagioclase feldspar, augite, and a minor amount of other minerals. These minerals are olivine, quartz/feldspathoids, enstatite, and rarely hornblende or biotite.
Specimens with andesitic composition have mostly sodic plagioclase, pyroxene, and a tiny amount of hornblende. These rocks may also have alkali feldspar, quartz, and less often muscovite or biotite.
Common accessory minerals in fine-grained scoria include ilmenite, spinel, apatite, zircon, magnetite, and sometimes chromite.
How is scoria formed?
Scoria forms mostly during an explosive eruption of volatile-rich, basaltic to andesitic magma. The violent or explosive Strombolian, Vulcanian and sometimes Hawaiian style eruption will eject ash, lapilli, and bombs into the air, where cooling is rapid.
The numerous vesicles in scoria form when the ejected magma, laden with exsolving, expanding, and escaping gas bubbles, rapidly solidifies, trapping some of the gas bubbles and forming a glassy rock.
Scoria may form from basalt or andesite lava flows. Volatiles in these lavas will exsolve, rise towards the surface, and escape. However, as the gas nears the surface, the quickly cooling surface may trap some of these bubbles, forming a highly vesiculated layer.
Expect a high density of vesicles towards the upper surface if these rocks form from lava flows.
Where is it found?
Scoria is common in cinder cones like Lava Butte in Oregon and Sunset Crater in northern Arizona in the US. These rocks also occur in Mauna Loa, Kilauea, and Mauna Kea volcanoes in Hawaii, together with Pele’s tears and Pele’s hair.
Other locations are Stromboli and Monte Nuovo in Italy, Soufriere Hills in Montserrat, Paricutin in Mexico, and Cerro Negro in Nicaragua.
What is scoria used for?
Scoria is a light-weight but hard and durable rock with a Mohs hardness scale of 5–6. Although its compressive strength is relatively low, it has many uses.
Common uses of this rock include:
- Landscaping: Scoria is a perfect rock for residential and commercial landscaping and decoration. This rock is not only light weight and durable but also porous, meaning it will improve drainage wherever you use it. Taco Bell, the famous fast-food powerhouse, uses the red scoria pebble variety in landscaping.
- Gardening: If you have a garden or potted plants, you can use scoria just as you use perlite. It will improve soil tilth, nutrient retention and aeration. Additional you can mulch with these rocks or use them as a media in hydroponic gardening.
- Construction aggregate: Light construction projects that don’t require a lot of steel
- Green concrete manufacture. A recent study shows the potential use of scoria to partially replace cement in concrete manufacture. This is a big win for the environment and a shift towards green concrete manufacture.
- Lightweight insulating concrete: Light construction projects that don’t require much reinforcement can use scoria concrete to achieve better insulation. The trapped air bubbles in this rock will offer insulation against cold, reducing energy costs.
- Low quality, porous aggregate: This rock makes low-quality, porous aggregate ideal for constructing foundations for smaller countryside roads. You can also use this aggregate in driveways, pathways, or even on your patio.
More uses of scoria include making roofing granules and earthbag houses. This rock is also a perfect drainage system, riprap, sauna, and barbeque stone.
What do pumice and scoria have in common
What scoria and pumice have in common is that they are both highly vesicular and lightweight extrusive igneous rocks. Also, their names are textural terms rather than rock names.
The difference between scoria and pumice is in their densities, colors, vesicular characteristics, and composition.
Scoria rocks are darker in color, have fewer but larger, more interconnected vesicles with thicker walls, making them denser, and sink in water. In contrast, pumice rocks are lighter in color, have more but smaller isolated cavities with thinner walls, making them less dense, and float in water.
Compositionally, scoria rocks are mafic to intermediate and form from low to moderately viscous magmas like basalt and andesite. Pumice rocks are, on the other hand, mostly felsic and form from highly viscous magma like rhyolite and dacite.
Some pumice rocks are, however, of intermediate and rarely mafic composition.