The Whorneyside Tuff Formation is an extensive pyroclastic sheet within the Ordovician Borrowdale Volcanic Group of the English Lake District. It marks a change in the eruptive style of a calc-alkaline continental arc volcano from multicentered eruptions of mostly basic and intermediate lavas to violent explosive activity associated with the development of a subaerial dacite-rhyolite caldera complex.
The lower part of the formation consists of ignimbrite, deposited from hot pyroclastic flows that inundated the early-formed shield-like lava field. The pyroclastic flow deposits pass upward into a laterally extensive, shower-bedded phreatomagmatic tuff that is about 30 m thick. The change records access of large volumes of surface water to the erupting magma, and it was probably facilitated by volcanotectonic subsidence. The phreatomagmatic tuff is characterized, by parallel bedding and lamination defined by variations in grain size and in abundance of crystal fragments. Abundant accretionary lapilli, block sags, and beds that drape irregular topography indicate its pyroclastic origin. The fine stratification records fallout from a vast umbrella cloud in rapidly successive localized showers of damp ash. The original grain size distribution and grain morphology of the phreatomagmatic tuff are not completely known, but the ubiquitous fine grain size even near the source, the sheet-like geometry, and the unusual thickness of the deposit all indicate an exceptionally large-magnitude phreatoplinian eruption.
The phreatomagmatic tuff was deposited mainly on a subaerial ignimbrite plain and was subjected to minor gullying and redistribution by surface run-off water during the eruption. In proximal areas, however, the ash fell into shallow ephemeral lakes and was reworked by waves and redeposited in ash-rich turbidity currents and debris flows. While still wet, the near-vent deposits were intruded by degassed andesitic magma, forming peperitic sills.
The subaerial phreatomagmatic tuff underwent widespread, intense soft-state sliding and slumping caused by seismicity and faulting during caldera subsidence, and it was preserved only because it was buried under intracaldera welded ignimbrites immediately after this deformation.