Ignimbrite
Welded pyroclastic flow deposit (felsic)
A rock formed from hot pyroclastic flows, often welded, sometimes containing flattened glass lenses called fiamme.
- Mohs hardness
- 4-6 (variable)
- Color
- Grey, pink, tan to reddish-brown
- Type
- igneous
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Overview
Ignimbrite is a pyroclastic igneous rock formed from the deposits of hot, fast-moving pyroclastic density currents (ash flows) produced during explosive eruptions. The name means 'fire-cloud rock.'
Where the deposit is laid down hot, the glass shards and pumice fragments fuse together to form a dense welded tuff; cooler deposits remain soft and porous. Welded ignimbrites often contain fiamme, flame-shaped dark lenses of compressed and flattened pumice.
Ignimbrites range from grey and tan to pink and reddish-brown and can form vast sheets covering thousands of square kilometers from caldera-forming super-eruptions.
Formation & geology
Ignimbrite forms when an explosive eruption generates a pyroclastic flow, a hot, ground-hugging mixture of ash, pumice, gas, and rock fragments that races downslope. When it stops, the material settles and compacts.
If the deposit is hot enough, the glassy fragments weld and flatten, producing welded ignimbrite and fiamme. The largest ignimbrites come from caldera-forming super-eruptions. Famous examples include the Bishop Tuff in California, the deposits at Yellowstone, the Campanian Ignimbrite in Italy, and vast sheets in the Andes of Peru and Chile.
How to identify it
Look for a generally light-colored (grey, pink, tan) volcanic rock made of compacted ash and pumice, often containing angular rock fragments and, in welded varieties, dark flame-shaped fiamme lenses.
The presence of fiamme and welded, flattened pumice is diagnostic of welded ignimbrite. Hardness varies from soft and crumbly to quite hard where welded.
Look-alikes include ordinary tuff (unwelded ashfall, no fiamme), rhyolite lava (flow-banded, no pyroclastic fragments), and sandstone (sedimentary, rounded grains). The eutaxitic fiamme texture is the key clue.
Uses & significance
Ignimbrite is widely used as a building and dimension stone, especially soft, easily carved varieties; many historic cities, such as Arequipa in Peru (the 'White City'), are built from white ignimbrite (sillar). It is also used for paving, cladding, and crushed aggregate.
Geologically, ignimbrites are vital records of the largest volcanic eruptions and are studied to assess super-eruption hazards and to map ancient caldera systems. It has no gemstone value, though welded specimens with striking fiamme are collected.
Frequently asked questions
What does ignimbrite mean?
The name comes from Latin for 'fire-cloud rock,' reflecting its origin in hot pyroclastic flows from explosive eruptions.
What are fiamme?
Fiamme are dark, flame-shaped lenses of pumice that were flattened and welded while the ignimbrite deposit was still hot.
Is ignimbrite the same as tuff?
Ignimbrite is a tuff formed specifically from pyroclastic flows, and it is often welded, whereas ordinary tuff can be unwelded ashfall.
Can ignimbrite be used for building?
Yes. Soft, easily carved ignimbrite such as Peru's white sillar has been used to build entire historic cities.
Ignimbrite guides
In-depth guides for identifying, valuing, and understanding Ignimbrite.
