Volcanic diapirs in the Orange Mountain flood basalt: New Jersey, USA

Diapir-shaped structures, 4–30 m high, consisting of vesicular basalt have intruded into the interior of a 50–70 m-thick subaerial Orange Mountain Basalt flow exposed at several rock quarries in northern New Jersey. The basalt flowed onto a travertine encrusted mudflat saturated with alkali salts. We propose that pressurized alkali vapors trapped under the lava created a vesicular and viscous flow bottom layer about 10 m thick. Vesicle coalescence within this layer increased its buoyancy where it locally accumulated into diapirs and displaced overlying lava. Large bubbles within the diapirs expanded upon intrusion into hot flow interiors where they explosively escaped leaving lenses of breccia. Some early diapirs reached the base of the upper lava crust. These diapirs document vapor driven convection of large blobs of contaminated lava into the lava core of the Orange Mountain flow.

► We document the occurrence of basaltic diapirs in a thick subaerial flow.
► We propose that the diapirs are products of contamination from wet sediments.
► They are highly enriched in sodium, barium, and copper.
► They contain very large partially collapsed vesicles.
► We show how volatiles can transport viscous blobs of crystal mush through a flow.