LUMAJANG, Indonesia—The death toll following the eruption of the highest volcano on Indonesia’s most densely populated island of Java has risen to 13, with seven people still missing, officials said Sunday as smoldering debris and thick mud hampered search efforts. 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, and searing gas and lava flowed down its slopes after a sudden eruption Saturday triggered by heavy rains. Several villages were blanketed with falling ash. A thunderstorm and days of rain, which eroded and finally collapsed the lava dome atop the 3,676-meter (12,060-foot) Semeru, triggered the eruption, said Eko Budi Lelono, who heads the geological survey center. He said flows of searing gas and lava traveled up to 800 meters (2,624 feet) to a nearby river at least twice on Saturday. People were advised to stay 5 …