ash-flow tuff

Ash-flow tuff, also known as ignimbrite, is a type of rock formed from volcanic ash and other volcanic fragments that have been deposited by a pyroclastic flow. A pyroclastic flow is a fast-moving, ground-hugging cloud of hot rock fragments, gases, and air that can reach speeds over 100 miles per hour. These flows originate from explosive volcanic eruptions where magma is shredded into particles.12

Tuff is generally a soft, porous rock created when volcanic ash or dust is compacted and cemented. The composition can vary, including glass (vitric), crystal fragments (crystal), or pre-existing rock debris (lithic). Some ignimbrites are formed from very hot, welded tuffs that have compacted into hard rock. The Mazama Ash, a deposit from Mount Mazama's eruption that formed Crater Lake, is an example of widespread volcanic ash useful as a time marker in archaeological studies.23