Volcanic Breccia
Volcanic breccia is a rock of sharp, angular volcanic fragments larger than 64 millimeters set in a finer matrix of ash or lava. It builds up where eruptions or collapses shatter solid lava.
Volcanic breccia is a rock of sharp, angular volcanic fragments larger than 64 millimeters set in a finer matrix of ash or lava. It builds up where eruptions or collapses shatter solid lava.
🪨 Geology Guide Specimen: Open Volcanic Breccia 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.
Fragmentation: How Do Volcanoes Make Breccia?
Several processes break lava into angular pieces. Explosions at a vent blast apart old and new rock, dome collapses shed hot blocks in block-and-ash flows, and the crust of a thick, slow lava flow shatters as it moves. When the debris is later moved by water or gravity, the deposit is a volcaniclastic breccia.
Angularity: What Do Sharp Edges Tell Us?
Sharp corners mean the fragments were not worn down by transport, so they were buried close to where they broke. This makes volcanic breccia a reliable indicator of the position of an ancient vent or a dome, and a contrast to the rounded gravel of conglomerate.
Vent Fills and Flow Margins: Where Is It Found?
Breccia fills the throats of volcanoes and the diatremes of explosive pipes, including the kimberlite pipes that bring up diamonds. It also appears at the margins and bases of lava flows and domes, where cooling and movement break up the rock.
Volcanic Breccia at a Glance
| Property | Value |
|---|---|
| Type / variety | Pyroclastic Block-and-Ash Breccia |
| Composition | Angular Lava Blocks (>64 mm) Cemented in Fine Ash & Lapilli Matrix |
| Geology Guide group | Igneous Rocks (Plutonic & Volcanic) |
| Rock Classification | Pyroclastic (Tephra & Tuffs) |
| Mohs hardness | Mohs 5.5–6.5 |
| Texture / habit | Chaotic angular volcanic clasts in tuffaceous matrix |
| Density | 2.30–2.65 g/cm³ |
| Lustre and cleavage | Sharp, unrounded lava fragments fused by volcanic ash |
| Geological setting | Vulcanian vent explosions and collapsing lava domes |
| Diagnostic feature (Geology Guide) | Chaotic aggregate of sharp, angular volcanic rock fragments (>64 mm) and shattered lava blocks cemented inside a compacted matrix of fine volcanic ash and lapilli. |
| Fragment size | Larger than 64 mm |
| Typical settings | Vents, domes and lava-flow margins |
See Also
- Volcanic Bomb — a rounded, molten-when-ejected fragment
- Lapilli Tuff — a rock of smaller fragments
- Sedimentary Breccia — the angular gravel rock formed at the surface