LOS LLANOS DE ARIDANE, Canary Islands—A Spanish island volcano that has buried more than 500 buildings and displaced over 6,000 people since last week lessened its activity on Monday, although scientists warned that it was too early to declare the eruption phase finished and authorities ordered residents to stay indoors to avoid the unhealthy fumes from lava meeting sea waters. The plume of ash emerging from the main vent that opened on Sept. 19 stopped in the early hours of Monday, live footage of the Cumbre Vieja range in the La Palma island broadcasted by the public Canary Islands Television showed. But the column of ash and volcanic material returned after a two-hour hiatus. “The volcano of La Palma has entered in a phase of lower activity,” the Madrid-based Institute of Geosciences, IGEO, said in a tweet. “Let’s see how it evolves in the coming hours.” The archipelago’s volcanology institute …