YOGYAKARTA, Indonesia—Indonesia’s most active volcano erupted Wednesday with a river of lava and searing gas clouds flowing 3,000 meters (nearly 10,000 feet) down its slopes. No casualties were reported. The sounds of the eruption could be heard 30 kilometers (18 miles) away, officials said. It was Mount Merapi’s biggest lava flow since authorities raised its danger level in November, said Hanik Humaida, the head of Yogyakarta’s Volcanology and Geological Hazard Mitigation Center. She said the volcano’s lava dome is growing rapidly, causing hot lava and gas clouds to flow down its slopes. After morning rain, the ashfall turned into muck in several villages. More than 150 people, mostly elderly, living within 5 kilometers (3 miles) of the crater were evacuated to barracks set up for displaced people. Authorities in November had evacuated nearly 2,000 people living on the mountain in Magelang and Sleman districts but most have since returned. The …