Rock Group

Igneous (Felsic) and Pyroclastic Sedimentary

Grain Size

A fine grained (aphanitic) ash-rich matrix that hosts pumice clasts. Ignimbrite is porous as the rock cools so quickly that the gas is trapped in bubbles in the rock.

Composition

Ignimbrite is made up of volcanic ash (tephra) along with clasts of volcanic glass and pumice.

Key Features

Ignimbrite is formed during explosive eruptions from the cooling of a pyroclastic flow, where debris, volcanic ash and gas form a fast moving current of hot volcanic material. Pyroclastic flows are commonly so hot that many ignimbrite deposits weld together after deposition to become a welded tuff.