This short article from Scientific American, by Peter D. Ward, provides short overview with clear illustration of the mechanism of Mass Extinction he describes in detail in his book, Under a Green Sky: Global Warming, the Mass Extinctions of the Past, and What They Can Tell Us About Our Future
Mass Extinctions have fascinated me since I was a little boy reading about Dinosaurs, Peter Ward's books are the primary way I indulge that fascination as an adult.
I recommend the pdf if you want a short concise explanation, but strongly suggest the book if your looking for a deeper understanding, or if like me your fascinated by Mass Extinctions or Paleontology.