Volcaniclastic rocks at Coombs Hills comprise mainly (80% vol.) structureless tuff breccia and coarse lapilli tuff cut by irregular dikes and sills, within a large vent complex (>30 km²).

Steeply dipping to sub-vertical depositional contacts juxtapose volcaniclastic rocks of contrasting componentry and grainsize. These sub-vertical tuff breccia zones are inferred to have formed when jets of debris+steam+water passed through unconsolidated vent-filling deposits. Some jets and their consituent clasts probably never reached the atmosphere and most of the ejecta that did escape the debris-filled vents was rapidly recycled as vents broadened.

Recycling of water, as well as recycling of pyroclasts, was important in maintaining water supply for phreatomagmatic interactions even when aquifer rock in the vent walls lay far from eruption sites as a consequence of vent-complex widening. The proportion of recycled water increased with vent-complex size in the same way that the proportion of recycled tephra did.