Scoria
Scoria is a dark, bubble-filled volcanic rock, usually basaltic or andesitic, that forms when gas-rich lava is thrown into the air and cools into a frothy cinder.
Scoria is a dark, bubble-filled volcanic rock, usually basaltic or andesitic, that forms when gas-rich lava is thrown into the air and cools into a frothy cinder. Heavier and darker than pumice, it builds the cinder cones found on volcanic fields around the world.
🪨 Geology Guide Specimen: Open Scoria in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 7 subgroups of Igneous, and test your eye in LithoClade.
Gas Bubbles: What Makes Scoria Vesicular?
Dissolved gas bubbles out of the lava as pressure drops during eruption, and the bubbles freeze in place when the rock chills. Scoria's vesicles are larger and its walls thicker than those of pumice, and its iron-rich glass is much denser, so most pieces sink, though highly vesicular ones can float.
Cinder Cones: Where Does Scoria Pile Up?
Fountains of lava during Strombolian eruptions fall back as glowing cinders, building steep cones around the vent. Sunset Crater in Arizona is a classic example, and Mexico's Paricutín grew from a cornfield in 1943 into a cone hundreds of meters high within a year.
Color and Uses: Why Is It Red or Black?
Scoria that cools quickly and stays reduced is black, while exposure to air while hot oxidizes iron to hematite and turns it red or brown. It is used as a lightweight aggregate, as landscaping rock and as filter and drainage material.
Scoria at a Glance
| Property | Value |
|---|---|
| Type / variety | Vesicular Basaltic Cinder Scoria |
| Composition | 45–52% SiO₂ • Iron-Rich Basaltic Glass & Microlites (Hematite/Magnetite) |
| Geology Guide group | Igneous Rocks (Plutonic & Volcanic) |
| Rock Classification | Mafic Volcanic (Basaltoids) |
| Mohs hardness | Mohs 5.5–6 |
| Texture / habit | Coarsely vesicular (millimeter-to-centimeter gas cavities) |
| Density | 1.20–2.20 g/cm³ (sinks in water) |
| Lustre and cleavage | Cindery matte black or oxidized brick-red |
| Geological setting | Strombolian lava fountains and cinder cones |
| Diagnostic feature (Geology Guide) | Dark charcoal-black or oxidized rust-red volcanic rock honeycombed with large spherical gas vesicles; unlike pale pumice, its thicker iron-rich walls make it dense enough to sink in water. |
| Typical composition | Basalt or basaltic andesite |
| Typical landform | Cinder cone |
See Also
- Pumice — the paler, lighter silicic froth
- Basalt — the dense lava that scoria is a foam of
- Volcanic Bomb — a larger clot of airborne lava