LOS LLANOS DE ARIDANE, Canary Islands—A volcano that has already destroyed nearly 1,000 buildings on a Spanish island increased its explosive power Tuesday, roaring louder and spewing thicker lava out of its main vent. The volcanic eruption started on Sept. 19 and has forced the evacuation of over 6,000 residents of the island of La Palma in the Canary Islands archipelago off the northwest coast of Africa. Some 946 houses have been completely destroyed and nearly 100 more affected, while farmers are struggling to keep the surviving banana plantations irrigated after lava flows destroyed roads and water pipes. The Canary Islands Volcanology Institute, Involcan, said Tuesday that activity in the La Palma volcano had become “explosive, with falling pyroclasts and bombs.” A video released by the institute the night before showed a block of molten rock that, according to the institute, had hit against a wall more than 0.7 miles …