LA PALMA, SPAIN—The underground conduits that feed lava from the Cumbre Vieja volcano in Spain’s La Palma island are solidifying, authorities said on Thursday, in a sign that the end of the three-month eruption could be announced by Christmas. If seismic activity and other factors remain unchanged, La Palma authorities have set Dec. 24 as the day to declare the eruption is over. “The emission of sulphur dioxide into the atmosphere, which was low yesterday, is not related to emerging magma, but to the solidification of existing magma in the vents,” Maria Jose Blanco, the director of the National Geographic Institute in the Canary Islands, said. The eruption could be ending soon, but the emergency is far from over, as lava fumes could still be dangerous and some parts of the exclusion zone around the volcano are buried in ash that needs to be cleared. Miguel Angel Morcuende, head of …