The description of the earth tremor spell says:
You cause a tremor in the ground within range. Each creature other than you in that area must make a Dexterity saving throw. On a failed save, a creature takes 1d6 bludgeoning damage and is knocked prone. If the ground in that area is loose earth or stone, it becomes difficult terrain until cleared, with each 5-foot-diameter portion requiring at least 1 minute to clear by hand.
My players argued that the area affected would be the ground, and thus flying or hovering creatures wouldn't be affected by this spell. If it was meant to affect creatures over the ground, it would say 10-ft cube in its range/area description.
Does this interpretation seem right? Or there is something 'more RAW' we've missed that says otherwise?