This marvel of celestial configuration happens in a flash at sunset in mid-February — if the winter weather cooperates. On those days the setting sun illuminates one of the park's lesser-known waterfalls so precisely that it resembles molten lava as it flows over the sheer granite face of the imposing El Capitan.
http://news.yahoo.com/every-february...160600248.html
interesting .. I did not know about this , When I first read the headline I thought about the Caldera and how it could be venting apparently it is due to light rays off the water fall. funny how it only happens once a year though.