REYKJAVIK - Iceland has shut its main airport after a volcanic eruption and other airports on the island are likely to close during the day, the civil aviation authority said on Sunday."The ash distribution forecast over the next six hours shows that the ash from the volcano will spread over Iceland today, leading to the closure of most Icelandic airports as the day goes on," the Isavia authority said in a statement."Grimsvotn is a very powerful volcano, so we're monitoringit closely, even if the last few eruptions have been harmless,"University of Iceland geophysicist Pall Einarsson was quoted assaying on the website of Morgunbladid."We do not expect this to be a big one as it's coming fromthe same crater as the last three eruptions, which were allsmall," he added.