by Mary Caperton Morton Thursday, June 13, 2013
Yellowstone is renowned for its hot springs, geysers and for hosting one of the world’s most volatile supervolcanoes. Despite its popularity, the origin of all that volcanic activity remains poorly understood. A mantle plume seems like the most obvious source, but traditional plume models can’t explain the jumble of volcanic surface features. Now, a new study using corn syrup to replicate the mantle processes underlying Yellowstone offers a more complicated scenario: a bifurcated mantle plume, split by the Cascadia Subduction Zone.
The textbook plume model consists of a plume head where voluminous lava comes to the surface and a plume tail that produces a progressively younger volcanic track, as in the Hawaiian island chain. In the Pacific Northwest, the plume head probably produced the Columbia River Flood Basalts — a vast outpouring of lava along the borders of Oregon and Idaho and Oregon and Washington, dating to about 15 million years ago. The plume tail likely surfaces at the Snake River Plain in southern Idaho and northwestern Wyoming, which extends west to east, with the youngest rocks occurring at Yellowstone National Park.
However, some scientists maintain that the plume model doesn’t work for Yellowstone, citing the 300-kilometer north-south offset between the Columbia River Flood Basalts and the Snake River Plain, as well as the often-labeled “enigmatic” High Lava Plains in southern Oregon, which seem to be related to the other features, but display a puzzling age progression opposite that of the Snake River Plain. Instead, this group usually points to upwelling along the Cascadia Subduction Zone as the source of the region’s volcanism.
The new study, published in Nature Geoscience, combines both possibilities: a mantle plume that is heavily influenced by the nearby subduction zone.
To model the interaction between Yellowstone’s mantle plume and the complex tectonics underlying the Pacific Northwest, Chris Kincaid of the University of Rhode Island and colleagues turned to a 3-D plate tectonics model at Australian National University in Canberra.
“As far as we know, this is the first experiment looking at how a three-dimensional subduction zone interacts with a three dimensional plume,” says Kincaid, a geophysical fluid dynamicist and lead author of the study. Physical models may pre-date computer models, but they are no less sophisticated, he says.
To mimic Yellowstone’s mantle plume, Kincaid and colleagues heated high-viscosity corn syrup to a consistency so it scaled mathematically to Earth’s mantle, and then injected it into the system. The entire apparatus was held in a freezer set to zero degrees Celsius, which produced a cold skin on the surface of the syrup, mimicking Earth’s lithospheric plates. Over a matter of hours — with 1 minute equivalent to 4 million years, and a 1-centimeter per minute flow rate equivalent to 10 kilometers per million years in the mantle — the system evolved to look remarkably like the pattern of volcanism seen in the Pacific Northwest, Kincaid says.
“Right off the bat we found that if you put a plume in to the north of the symmetry axis of the subduction zone, it gets ripped apart and deformed by the flow fields being produced by the subduction zone,” he says. The head of the plume essentially split in two, with one half drawn down into the subduction zone, forming a track similar to the High Lava Plains, while the other half moved north, rising to the surface like the Columbia River Flood Basalts. The tail of the plume also formed a track similar to the Snake River Plain, he says.
“This is the first quantitative study on the interaction between the mantle plume and the subduction zone,” says Lijun Liu, a geodynamicist at the University of Illinois at Urbana-Champaign who was not involved in the new study. “Previous work has mainly focused on either a plume or subduction, not the combination of the two.”
These findings “tell us that the evolution of the plume over time is strongly influenced by circulation currents driven by the movement of the Cascadia Subduction Zone,” Liu adds. “This is an interaction that has not been well-explored before and it shows how complex thermodynamics close to a subduction zone can be.”
Yellowstone is not the only place in the world where a mantle plume sits close to a subduction zone. The plume feeding the Samoan Islands in the South Pacific sits near the Tonga Subduction Zone, where a similar interaction may take place. In the future, Kincaid and colleagues plan to create a similar model for that scenario as well. Seismic tomography, which uses seismic waves to create a map of the interior of Earth, could possibly be used to test the bifurcated plume model in the field, Liu says.
“Subduction zones are like giant attractors that pull everything toward them,” Kincaid says. “The interaction between the plume and the subduction zone makes the plume behave differently and look different than what it says in the textbook.”
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