Pros for Volcanism Theory
since volcanism gradually builds up, it explains perfectly the reason for the gradual decline of species at the boundary.
Ir has been discovered in the Deccan Traps and in fresh basaltic lava from Hawaii. Therefore, challenges the notion that the Ir anomaly can only be explained with a meteorite impact.
extent of volcanism would have discharged large quantities of volatiles, having an adverse effect on the environment and ultimately causing changes in the climate and environments.
Cons for Volcanism Theory
There is considerable difficulty in explaining the Ir anomalies based on the assumption of an internal source. Ir abundances found in the clay lay were considerably exaggerated to crustal abundance and even to mantle or core abundances. So it would have taken a very long to build up the Ir in the clay layer. There would have to be some consolidation mechanism involved in consolidating the Ir in the clay layer.
Currently, associated evidence for this theory points to volcanism playing only a complementary role in the extinction. It could not have been the only cause for the extinction. Perhaps, impact was the final nail in the coffin after volcanism had depleted the biota over a prolong period.