RSES head cold on 'hot spots' theory By Damon Shorter A chunk of green rock thrown up by a volcano in Victoria has provided some of the best evidence yet for a geological "hot-spot" under the Australian continent, helping revive a long-standing scientific controversy. Hot spots are anomalous regions of volcanic activity thought to be caused by upwellings of hot rock from deep within the earth known as "mantle plumes". The best known example is a hot spot under the Pacific Ocean responsible for the fiery volcanoes of the Hawaiian Islands. The idea of mantle plumes has been around for years, with several ANU earth scientists leading proponents of the idea, but geological evidence to support their existence is limited. The latest discovery, reported last month in the scientific journal Nature, presents some of the best evidence yet of a mantle plume on one of the continents. Dr Takuya Matsumoto, a Japanese PhD student, made the find last year working under the supervision of Professor Ian McDougall and Dr M Honda at the Research School of Earth Sciences (RSES). While investigating a dense, green rock (called a xenolith) thrown up in a volcanic eruption in central Victoria about 25,000 years ago, Dr Matsumoto noticed parts of the rock had an unusual chemical "signature" - characteristic of rocks from deep within the earth's mantle. He concluded that some gaseous components in the rock, especially neon, had originated thousands of kilometres below the earth's surface before being carried upwards within a mantle plume. Yet not everyone is convinced by the mantle plume theory, including the Director of RSES, Professor David Green. "I'm afraid I put a dampener on hot spots," said Prof Green, who prefers to call them "wet spots". He says the enhanced volcanic activity shown at these sites is most probably caused by volatile water and carbon dioxide rather than hot plumes from the bottom of the mantle. Regardless of their cause, both parties agree the spots play a large role in shaping world geography. Prof McDougall says there is evidence for other hot spots off the east coast of Australia that have been forming chains of volcanic mountains on the sea floor for millions of years as our continental plate inches northwards. One range runs parallel to the Australian coast extending from just north of Brisbane to Gascoyne Seamount off Eden. Although this particular hot spot seems to be "running out of puff", Prof McDougall said one of the largest earthquakes in Australia's history was traced to a point where the hot spot is now predicted to be. Lord Howe Island, Middleton Reef and Elizabeth Reef were also probably formed by volcanic activity as our continental plate moved over a fixed hot spot in the Tasman Sea, he said.