High-grade to extremely high-grade ignimbrites are the enigmatic deposits of a subset of particle-laden gravity currents, called pyroclastic density currents, generated by large, hot explosive eruptions. Eruptions that produce  rheomorphic ignimbrites have not been witnessed historically. Rheomorphic ignimbrites occur in a range of diverse settings from intraplate ocean island volcanoes, rifts, and continental arcs. Some ignimbrites, such as the Rattlesnake Tuff in Oregon, show a transition to rheomorphism in parts of the deposit; whereas ignimbrites like Grey’s Landing in the Snake River Plain volcanic province are more intensely rheomorphic and lava-like. We aim to better constrain the effects partially-to-fully molten ash particles have on pyroclastic density currents and on the onset, duration, and extent of rheomorphism in the ignimbrite.