to rise before the vision with an appearance of great or portentous size; to rear, to tower

Link to present incidents:
Photo 10: Farmer Thorarinn Olafsson tries to lure his horse back to the stable as a cloud of black ash looms overhead in Drangshlid at Eyjafjoll on April 17, 2010.

Photo 18: A dark ash cloud looms over the Icelandic south coast April 17, 2010.

boston.com/bigpicture/2010/04/more_from_eyjafjallajokull.html