Ignimbrite

Ignimbrite is a pyroclastic rock formed from the deposition and subsequent lithification of pyroclastic density currents, or ignimbrite flows, that rush along the ground during explosive volcanic eruptions. These currents carry fine ash, pumice, and various rock fragments, and can blanket landscapes as broad, sheet-like deposits.

During emplacement, the hot, cohesive material may weld together to form a dense, cohesive unit known as a welded ignimbrite. If insufficiently heated, the deposit remains non-welded ignimbrite. Welded varieties often display fiamme, blade-like glassy lenses created by partial melting and welding of the pumice and ash.

Composition and textures of ignimbrites vary widely, from rhyolitic to andesitic and rhyodacitic to basaltic sources, reflecting the magma composition of the eruption. They typically contain a mix of glass shards, pumice fragments, and lithic clasts, and show different degrees of sorting and layering consistent with eruptive pulses. Thicknesses range from a few meters to several hundred meters, and deposits can cover extensive areas.

Ignimbrites are commonly linked to large explosive events, including caldera formation, and thus are important for reconstructing eruption histories and volcanic hazards. In stratigraphy, ignimbrite sheets serve as time markers and provide insights into past magma dynamics, transport distances, and regional tectonics.