Gee, where to start?

There is another heat source that can melt sand into glass - volcanoes! And there are plenty of those in Africa...

No air blast meteor will spread glass over 300,000 square miles. The blast would have to happen very high just to get that area within the horizon, and the air is thin up there... It needs an impact, not a Tunguska air-blast.

The Shoemaker-Levy analogy has problems. Jupiter is a gas giant and meteors can penetrate a long way in the atmosphere: all of the energy is released in air-blasts. Earth is a rocky planet with an extremely thin atmosphere and a meteor of any size is going to impact the surface almost immediately; only a few small meteors are going to be able to vaporize before impacting.

In any case, before resorting one more time to the meteor excuse I'd like to see a reason why this didn't come from the other obvious source: volcanic eruptions.