Ignimbrite
Ignimbrite Tuff from Cumbria
A fragmental, pyroclastic, igneous rock of felsic (and sometimes intermediate) composition, formed by explosive eruption and deposited as a pyroclastic flow. Forms as a deposit from a rapidly moving, turbulent, ignited cloud of gas called a nuee ardente. Associated with especially violent eruptions, producing clouds of incandescent gas and lava drops. These flow from volcanic eruptions at great speed, close to the ground.