A fire that destroyed a canvas yurt in Sisters exploded with a 10- to 15-foot fireball as Sisters-Camp Sherman firefighters arrived on scene Wednesday to control the blaze, officials said Thursday. Fire district crews were dispatched around 3 p.m. to the reported brush fire in the 200 block of East Black Crater Avenue that quickly escalated to a grass fire with structural involvement, Public Information Officer Julie Spor said. The fireball erupted and was visible over the garage roof as the first units arrived. They found a 25-foot diameter fire burning in debris on the ground and extending to a wooden fence on the property line, Spor said. It was later determined the burning debris were the remnants of a 16-foot diameter canvas yurt and its contents. Spor said it's believed the explosion and fireball resulted from an oil heater that over-pressurized when flames reached it. A nearby outdoor trampoline and two fence panels also burned.