New research has revealed the world’s longest chain of volcanoes —Rhodri Davies and colleagues are calling it “Earth’s longest continental hotspot track”—is 1,240 miles (1,996 km) long. The chain looks like four separate tracks of past volcanic activity with different geochemistry and hundreds of miles separating them. It was all about perspective. Think about how aspen trees grow in large clonal colonies, sometimes separated from a parent tree by up to 132 feet (40 meters).
However, some scientists suspected a common source: A mantle plume, or a narrow upwelling of hot rock that originates at the Earth’s core-mantle boundary.
Notably, hotspots don’t seem to be associated with the usual boundaries of tectonic plates; instead, they are formed above mantle plumes. The volcanic chains looked separate because the Australian continent is too thick in these areas between volcanic activity (81 miles or thicker) to allow the hot rock in mantle plumes to rise to Earth to melt and form magma.
Where the Earth’s lithosphere or outer layer was thinner than 81 miles, the plumes created volcanic activity.
A mineral called leucitite was associated with thin spots in the continent. The lithosphere thicknesses explained differences in the chemistry of the volcanic rocks at different locations.”If you take a mantle plume of a specific temperature and raise that to a depth of say 130 kilometres below the surface, specific minerals from the surrounding rock will enter that melt, and if the plume reaches shallower depths of say 100 kilometres, additional elements will enter the melt, changing the chemical composition,” Dr. Davies said.
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The volcanic chain has been dated to have been created over the past 33 million years, as Australia has moved north over a mantle plume. “Australia is actually the fastest moving continent on Earth, moving towards Indonesia at around seven centimetres per year,” Dr Davies said. This also explains the age progression shown in the volcanoes, with volcanoes closer to the south among the youngest in Australia.
Scientists are now trying to determine if a lack of a volcano at the plumes current location (between King Island and Tasmania) is caused by the thickness of the lithosphere in that location. They have detected seismicity in the region, so they know something is going on.
“Now that we know there is a direct relationship between the volume and chemical composition of magma and the thickness of the continent, we can go back and interpret the geological record better,” said co-author Ian Campbell from the ANU School of Earth Sciences.