Extremely high-grade, lava-like welded ignimbrites are produced by many large explosive eruptions with volumes typically 101–103 km3. However, understanding of the physical properties of these unusual deposits, and their transport and depositional mechanisms, is incomplete. The lava-like and rheomorphic Grey’s Landing ignimbrite, Idaho (western United States), provides abundant field evidence supporting the upward migration of a transient, <2-m-thick, sub-horizontal ductile shear zone at the interface between the pyroclastic density current and the deposit, through which all of the aggrading pyroclastic material passed. Here we use a combination of rheological experiments and thermo-mechanical modeling to test the syndepositional shear zone model. We show that syndepositional welding and ductile flow are achievable within a very restricted field of likely temperature–strain rate space, where rapid deformation is favored by higher emplacement temperatures (≥850 °C). The field of ductile deformation is broadened significantly by accounting for strain heating, which permits a sustained temperature increase of up to 250 °C within the shear zone and helps to explain the enormous extents of lava-like lithofacies and the intense rheomorphism recorded in extremely high-grade ignimbrites. Recognition of strain heating within rheomorphic ignimbrites suggests that large pyroclastic density currents may travel over a hot substrate, potentially hotter than the density current itself.