Scoria rock is a vesicular, dark-colored igneous rock composed of basalt or andesite. When observed in nature these rocks are typically darker colors, including dark brown, black, and purplish red.
Scoria is relatively common in areas of recent volcanism, like the Canary Islands and near the Italian volcanoes. They have low density because of their vesicles, but they’re not as light as Pumice.
If you’re trying to determine the difference between Scoria and Pumic then you’ll want to observe larger vesicles and thicker walls. These two observations will help you correctly identify the rock as Scoria.
You’ve probably seen Scoria used in landscaping and high-temperature insulating material. Another primary use for Scoria is in the production of lightweight aggregate. The stones are crushed to desired sizes and sold for various uses.
Here’s a list of uses for Scoria:
- Ground cover
- Roofing granules
- Substrate in hydroponic gardening
- Drainage stones
- Lower-quality road metals
- Saunas and
- Heat sinks in barbecue grills
How Is Scoria Formed?
We know enough about how Scoria rocks are used, so let’s talk about how these stones are formed.
Scoria is formed when magma containing an abundance of dissolved gas flows from a volcano or gets blown out during a volcanic eruption. As these molten rocks emerge from the Earth, the pressure placed on them is reduced, and the dissolved gas begins escaping in the form of bubbles. Suppose the molten rocks solidify before the gas escapes. In that case, the bubbles will become small elongated, or rounded cavities in the rocks. These dark-colored igneous rocks with trapped bubbles are what we know as Scoria.
During some volcanic eruptions, a rush of gas will blow out of the vent, and with it comes small bits of magma that solidify as they fly through the air. This action produces a ground cover of Scoria surrounding the volcanic vent; the heaviest deposits are found on the downwind side of the volcano.
Small particles of Scoria littering the landscape around the volcano are called Lapilli, with larger particles being referred to as blocks.
What Is Scoria?
Scoria is highly vesiculated tephra or lava. The stones are usually dark-colored with a mafic composition. It’s difficult to say whether it’s a rock type or not. While it’s relatively tempting to say no, that is not the case. Some believe Scoria is volcanic glass, but it’s actually a rock. Volcanic glass is typically produced from Viscous lava, which is a bit different from the lava that produces Scoria.
Scoriaceous rocks are dark-colored volcanic rocks with many variously sized (and usually smooth-sided) holes or vesicles in them. The term Scoriaceous simply refers to the porous texture of the rock.
Scoria Rock Texture
These dark-colored igneous rocks contain an abundance of round bubble-like cavities or vesicles. Scoria ranges in color from black or dark gray to dark reddish brown. They usually have a similar composition to Basalt, but Scoria can also have a composition similar to Andesite.
Scoria has a similar texture to Pumice but is different.
Most Scoria specimens are mafic in composition, meaning their chemical composition is similar to Gabbro and Basalt. These rocks have a large concentration of dark-colored minerals, including Hornblende and Pyroxene, with relatively little Feldspar and Quartz.
Scoria isn’t usually made of proper mineral crystals. Instead, they are almost always formed as volcanic glass. While the rocks aren’t technically made of Basaltic minerals, they are made from the same chemical building blocks. This means that Scoria rocks take on the same general coloration that other mafic igneous rocks do; dark gray, black, deep red, or dark purple.
In general, the color of Scoria will align with the type of lava the stones were formed from. Basaltic lava, which is high in mafic mineral content, tends to naturally create reddish or purplish Scoria.
The term “Luster” describes how minerals appear to reflect light and how brilliant or dull the minerals are.
Scoria has a subvitreous to dull luster. Dull luster defines minerals that have a poor reflective quality, very much like unglazed porcelain. Most minerals containing a dull luster have a porous or rough surface. Subvitreous luster is somewhat vitreous, kind of a “close, but no cigar” type deal.