Theory 38/Volcanic arcs/Threshold lens
Arcs Need a Wet Slab
The claim
Arc volcanism begins only above a threshold in slab water content and dip, so a dry shallow slab produces no arc.
01Mechanism
Fluids must reach the wedge at a depth where melting is possible; the combination of dip and water content sets whether they do.
02Prediction
Arc presence should track the threshold.
03Refutation
Arc presence tracking convergence would remove slab properties.