Volcano plug welds ???....... (Mig type)

[John Buchtenkirch]

My longtime friend and swap meet buddy “Big Mike” is still banging out heavy collision work. He asked me why plugs welds (rosette welds) sometimes get a volcano effect. I told him contamination but have been second guessing myself on this all day on this. I’ve had the same effect and noticed just one pinhole in the center and no porosity on the sides so I’m kinda doubting my own initial comment.

So my first question is what are the theories on why mig welding plug welds occasionally gets this volcano effect ? And the more important second question is what is the cure ? Thank you in advance ~ John Buchtenkirch