Scoria
"A highly vesicular, dark-colored volcanic rock that may or may not contain crystals."
Scoria is a textural term for a volcanic rock that is highly vesicular (filled with holes or cavities). It is essentially the “basaltic version” of pumice. While pumice is light and airy, scoria is darker, denser, and coarser. It is one of the most common products of explosive eruptions involving low-viscosity magma.
Characteristics and Identification
- Color: Typically dark—black, dark gray, or deep reddish-brown. The red color often results from the oxidation of iron within the rock during eruption (rusting).
- Density: Scoria has a specific gravity greater than 1.0, meaning it sinks in water. This is a key distinction from pumice, which often floats.
- Vesicularity: The rock is riddled with vesicles (gas bubbles). However, the vesicle walls are thicker than those in pumice, giving the rock a rougher, more jagged texture.
- Composition: Usually basaltic or andesitic, meaning it is rich in iron and magnesium (mafic) and lower in silica compared to rhyolitic rocks.
Formation Process
Scoria forms during explosive eruptions or lava fountaining.
- Decompression: As gas-rich magma rises, pressure drops, causing dissolved gases (water vapor, CO₂) to exsolve and form bubbles.
- Fragmentation: The magma is torn apart by the expanding gas, creating clots of lava.
- Quenching: These clots fly through the air and cool rapidly. The rock solidifies before the bubbles can escape or collapse, freezing the “foam” structure in place.
Scoria Cones (Cinder Cones)
Scoria is the primary building material of cinder cones (also called scoria cones). In Strombolian eruptions, the accumulation of pea-to-fist-sized scoria fragments around a vent creates the classic conical shape. Because scoria is loose and jagged, these slopes are often unstable and difficult to climb.
Economic Uses
Because it is durable, porous, and lightweight relative to solid basalt, scoria is a valuable industrial material.
- Landscaping: Widely used as decorative ground cover (“lava rock”). It retains moisture for plants while preventing weed growth.
- Construction: Used as a lightweight aggregate in concrete and cinder blocks. Using scoria instead of gravel reduces the weight of concrete structures.
- Roads: In winter, crushed scoria is spread on icy roads to improve traction.
- Barbecue Grills: Used as heat retainers in gas grills, effectively radiating heat evenly.
Etymology
The word scoria comes from the Greek word skōria, meaning “rust” or “dross” (waste). This name references its rusty red color and its resemblance to the slag byproduct produced in metal smelting.
Comparison: Scoria vs. Pumice
| Feature | Scoria | Pumice |
|---|---|---|
| Color | Dark (Black, Red, Brown) | Light (White, Grey, Tan) |
| Magma Type | Basalt / Andesite (Mafic) | Rhyolite / Dacite (Felsic) |
| Density | Sinks in water | Floats on water |
| Vesicle Walls | Thick | Thin, glassy |
| Silica Content | Low (~50%) | High (>65%) |