LUMAJANG, Indonesia—Rescuers were sifting through smoldering debris and thick mud in search of survivors a day after the highest volcano on Java island erupted with fury, killing at least 14 people with searing gas and ash. Mount Semeru in Lumajang district in East Java province spewed thick columns of ash more than 12,000 meters (40,000 feet) into the sky in a sudden eruption Saturday triggered by heavy rains. Villages and nearby towns were blanketed and several hamlets buried under tons of mud from volcanic debris. Authorities warned the thousands of people who fled the volcano’s wrath not to return during Sunday’s lull in activity. But some were desperate to check on livestock and possessions left behind. In several areas, everything—from the thinnest tree branch to couches and chairs inside homes—was caked with ash. “There’s no life there … trees, farms, houses are scorched, everything is covered in heavy gray ash,” …