Welded Tuff
Welded tuff, also called ignimbrite, is a volcanic rock formed from the hot ash and pumice of a pyroclastic flow that was still soft enough to stick together and compact after it came to rest.
Welded tuff, also called ignimbrite, is a volcanic rock formed from the hot ash and pumice of a pyroclastic flow that was still soft enough to stick together and compact after it came to rest. Large sheets of welded tuff mark some of the biggest eruptions in Earth's history.
🪨 Geology Guide Specimen: Open Welded Tuff (Ignimbrite) 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.
Welding: How Does Ash Become Rock?
A pyroclastic flow deposits ash, glass shards and pumice at temperatures high enough for glass to soften. Under the weight of the deposit the pieces sinter and flatten, and the pumice lumps squash into dark, flame-shaped lenses called fiamme. The resulting layered fabric is called eutaxitic.
Ignimbrites: What Do They Show About Eruptions?
Ignimbrite sheets can cover thousands of square kilometers and exceed hundreds of meters in thickness. They record caldera-forming eruptions, such as those of Yellowstone and Toba, in which a huge volume of magma was discharged as a ground-hugging flow of ash and gas. The deposits stack in sheets, and welding is strongest in the thick, hot interior.
Landscapes: What Do They Look Like?
Dense welded tuff forms rugged cliffs with columnar joints. Weakly welded tuff is soft enough to carve, as in the cliff dwellings of Bandelier in New Mexico, where ancestral Pueblo people hollowed rooms from the volcanic ash deposits.
Welded Tuff at a Glance
| Property | Value |
|---|---|
| Type / variety | Fiamme-Bearing Ash-Flow Ignimbrite |
| Composition | Sintered Silicic Glass Shards + Flattened Pumice Lenses (Fiamme) + Phenocrysts |
| Geology Guide group | Igneous Rocks (Plutonic & Volcanic) |
| Rock Classification | Pyroclastic (Tephra & Tuffs) |
| Mohs hardness | Mohs 5.5–6.5 |
| Texture / habit | Eutaxitic fabric (dark flattened glassy lenses in ash matrix) |
| Density | 2.10–2.45 g/cm³ |
| Lustre and cleavage | Compacted salmon-tan ash matrix with dark glassy streaks |
| Geological setting | Caldera-forming density currents (nuées ardentes) |
| Diagnostic feature (Geology Guide) | Compacted salmon-to-grey ash-flow rock streaked with dark, flame-shaped lenses ('fiamme') formed when incandescent pumice fragments collapsed and fused under the weight of an overriding eruption deposit. |
| Also called | Ignimbrite |
| Key texture | Fiamme in a eutaxitic fabric |
See Also
- Pumice — the frothy glass that collapses into fiamme
- Lapilli Tuff — a less-welded fragmental deposit
- Rhyolite — the usual magma type of large ignimbrites