LA PALMA, Spain, Dec 16 (Reuters) - Authorities on Spain"s La Palma island allowed evacuated residents to return to clear their ash-covered houses on Thursday as scientists suggested solidifying lava vents beneath the Cumbre Vieja volcano could herald an end to the three-month eruption. Deep banks of black ash had piled up to head height outside Daniel Geronimo"s whitewashed house in the village of Las Manchas, below the eruption zone. "It"s sad. It"s sad to see the houses buried like this," the 26-year-old nurse said, as his aunt swept ash from the roof. Soldiers from Spain"s emergency military unit helped them shovel debris from an outdoor corridor where it had formed thick drifts in the six weeks they had been prevented from visiting. "It"s really difficult because cleaning all this up, assuming the volcano really has shut down, will take time," he said. Underground conduits that channel magma up to the surface at Cumbre Vieja are solidifying and no seismic activity has been registered since late on Monday, fuelling optimism the longest eruption in La Palma"s recorded history could be declared over in time for Christmas. But Geronimo, who will have to leave the house again after Thursday"s clean-up, is not getting his hopes up. "It could well come back," he said. Even if the eruption has fizzled out, the emergency is far from over as existing lava deposits can still emit dangerous sulphur dioxide fumes. Miguel Angel Morcuende, head of a task force set up to monitor the volcano, warned returning homeowners, most of whom will not yet be allowed to stayovernight, to be cautious, especially those who live near the lava flows. Thousands of people have been evacuated, at least 2,910 buildings have been destroyed and the island"s econo mically vital banana plantations have been devastated.