BBCNews is carrying this article summarizing a recent study concerning how asteroid impacts seem to combine with massive basalt flows to trigger mass extinctions. Evidence seems to be growing against one or the other being powerful enough to cause such an event, but both together seem to do nicely.

To me, the fact that a mass extinction tends to be accompanied by both a huge impact and a basalt outflow would indicate the two were related somehow. Yet these geologists seem particularly concerned not to draw the obvious causal conclusion. I guess it's because I'm not much of a geologist.