IIgnimbrites are pumice-dominated pyroclastic flow deposits with subordinate ash.There are
many historic examples, most of which are restricted to valleys emanating from summit craters.
One such deposit from the 1980 eruption of Mt. St. Helens (below left) contains abundant
pumice blocks at its terminus. However, there are no historic examples of the voluminous
ignimbrite sheetflows associated with caldera formation. These extensive deposits can cover
many thousands of square kilometers. They often appear as coherent, well-compacted, often
partially welded, layers that in some cases resemble lava flows, as demonstrated by the Miocene