(CNN) -- An explosion that followed a power outage left an undetermined number of people with minor injuries and prompted the closure of the UC Berkeley campus Monday. College spokesman Dan Mogulof said the power went out on campus about 4:30 p.m. local time (7:30 p.m.). Two hours later, an explosion went off on the north side of California Hall, he said. Authorities do not know what caused the explosion. The campus was closed and students and staff cleared.