Mantle plume alternative explains Australian volcanism

by Timothy Oleson
Monday, January 5, 2015

Magma often finds its way to the surface along Earth’s crustal boundaries as tectonic plates crash together, rift apart or grind past each other. Less understood is why volcanoes sometimes emerge far away from plate boundaries. Narrow plumes of buoyant mantle rock rising from hundreds of kilometers deep have long been supposed as the source of intraplate volcanoes, but evidence for plumes is lacking in many areas. Now, in a new study, researchers have reported evidence for an alternative process, known as edge-driven convection, which appears to be driving intraplate volcanism in southeastern Australia.

Immediately west of Melbourne on Australia’s southeastern coast sits the roughly 19,000-square-kilometer Newer Volcanics Province (NVP), a region comprising more than 700 volcanoes and eruptive vents ranging in age from 4.5 million to 5,000 years old. Unlike the Hawaiian Islands, which progress from older to younger in a line from northwest to southeast and are the textbook example of mantle-plume-driven volcanism, in the NVP, volcanoes of different ages are jumbled together.

“There is no age progression” in the NVP, explains Rhodri Davies, a geodynamicist at the Australian National University and lead author of the new study, published in Geology. “The volcanism is also aligned perpendicular to the direction of plate motion” rather than parallel as in Hawaii, he says — also inconsistent with the plume hypothesis. These inconsistencies led Davies and his colleague Nicholas Rawlinson of the University of Aberdeen in Scotland to investigate what, if not a plume, might be causing magma to well up from below the plate.

One possible explanation for intraplate volcanism, called edge-driven convection (EDC), was proposed in the mid-1990s. EDC is thought to arise in the mantle where substantial changes in the thickness of the lithosphere — the crust plus the uppermost portion of the mantle — occur. “Australia, being a very old continent, has some very thick cratonic roots,” Davies notes, whereas on its eastern edge “it has very thin lithosphere.” Temperature gradients at this lithospheric step — from deeper, warmer mantle to shallower, cooler mantle — should induce convection cells that move warm mantle material to shallow depths. If a portion of this warm material melts, it could then rise to the surface and erupt, Davies says.

Using high-resolution seismic data collected in southeastern Australia, Davies and Rawlinson built a detailed 3-D profile of lithospheric depth in the region. Combining this with an understanding of the direction and velocity of the northbound Australian Plate, which moves in the opposite direction of the underlying mantle, they then modeled mantle flow beneath the lithosphere.

The modeling showed evidence for EDC along Australia’s southeastern margin. And beyond that, “the highest upwelling velocities were restricted to [an area right below] the NVP,” Davies says, suggesting another part of the reason for the volcanism there. From their lithospheric depth profile, the researchers noticed that the NVP is located above a long, narrow notch, or salient, of relatively thin lithosphere that is mostly surrounded by thicker lithosphere. In the model, the salient’s orientation with respect to the direction of plate motion appears “to focus [mantle] flow in the region,” Davies says, further increasing upwelling velocities and allowing warm mantle rock to reach shallower depths where it can decompress.

“It’s a combination of needing thin lithosphere to get the hot material to low enough pressures to melt, and needing high upwelling velocities to essentially refill the melt zone with fresh material,” Davies says. Just how thin the lithosphere and how high the velocities must be for melting to occur are unclear, he adds, and may depend on factors such as the particular petrology of the local mantle or how much water is present.

The new findings suggest an answer to a longstanding question about EDC, Davies says: Why, if EDC occurs in some places, do we not see surface evidence of it everywhere in the world where substantial changes in lithospheric thickness occur, including all the way around Australia? It may be that EDC does occur in many parts of the mantle, but that the sort of localized, 3-D focusing mechanism seen in the team’s modeling is necessary for it to manifest as volcanism at Earth’s surface. This “probably explains why you [would] only get it in isolated locations elsewhere,” he says, adding that intraplate volcanic regions on Africa’s west coast could be other surface expressions of EDC.

“It’s a very nice study,” bringing the tomography and modeling together, “and then being able to actually test that model,” says Scott King, a geophysicist at Virginia Tech University who was part of the team that developed the EDC theory but was not involved in this study. There “appears to be a compelling argument for this type of mechanism to apply in” the NVP, King says. Integrating these results with a more detailed view of the underlying petrology is probably “the next significant step forward.”

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