An avalanche at Mammoth Mountain early Saturday forced the closure of the popular ski area and triggered search and rescue efforts, officials said. There were no reports of serious injuries or missing people, but emergency crews were activated, officials said. The ski patrol was conducting avalanche hazard mitigation work when the avalanche occurred about 10:15 a.m. on the upper part of the mountain near the High Five Express chairlift area, Lauren Burke, public relations manager for Mammoth Mountain, said in a statement. Such work usually involves the use of explosive devices to demolish dangerous snow piles.