LA PALMA, Canary Islands—A massive cloud of ash prevented flights in and out of the Spanish island of La Palma on Sunday as molten rock continued to be flung high into the air from an erupting volcano. No flights arrived or departed, despite emergency workers clearing the ash from the airport runway. Islanders faced a mixed picture of good and bad news with some evacuees allowed to return to their homes amid low seismic activity while authorities took stock of the damage caused. Around 430 buildings have so far been destroyed in the countryside. The volcano on La Palma, which is part of the volcanic Canary Islands off northwest Africa and is home to about 85,000 people, erupted on Sept. 19. The prompt evacuations of more than 6,000 people helped avoid casualties. Life on the rest of La Palma, which is roughly 35 kilometers (22 miles) long and 20 kilometers …