Around 80 million years ago, China’s largest continental plate began to creep northeast. As it drifted, it passed over a stationary plume of hot, fluid rock in the mantle. The plume broke through weak parts in the crust and formed a linear anomalous zone. At least, that’s the scenario proposed in a new study, which used multiple lines of evidence to support the existence of the crustal footprint of the Hainan plume and puzzle out how the volcanoes formed.
The idea that volcanic chains such as the Hawaiian Islands arise when tectonic plates drift over hot spots in the mantle, the partly viscous layer of Earth that runs about 2,900 kilometers deep and makes up more than 80% of our planet’s interior, is about half a century old. It remains the most widely accepted explanation for volcanoes that occur far from plate boundaries, where magma can passively ooze through cracks to the surface. But the mantle plume theory is not universally accepted, in part because the mantle is so hard to access for study.
To explore whether a mantle hot spot could have formed the Hainan Island volcanoes and could have left a track on the overriding plate, Liu et al. combined a joint geophysical imaging technique with supercomputer modeling. The technique simultaneously uses seismic surface waves from ambient noise analysis and gravity measurements from the Gravity Recovery and Climate Experiment (GRACE) satellite mission to image the mantle’s 3-D structure, similar to a computerized axial tomography (CAT) scan.
Once the team had delineated the structure of the plume’s track, they ran a computer simulation of how the plume would have likely interacted over millions of years with the continental plate, known as the South China Block. They found that a weak lower crust causes the mantle plume to break through and produce the linear corridor of high seismic velocity anomalies, thinned crust, and high heat flow anomalies. After that, the model suggests, the plate kept traveling northeast at a rate of about 1.8 centimeters per year from the age progression of volcanic rocks sampled along this linear corridor.
The study can’t conclusively prove that this interaction produced the Hainan Island volcanoes, but it does bolster the mantle plume hypothesis, the authors write. Their approach could be applied to places where the link between a mantle hot spot and volcanic activity remains unclear, they say. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2017JB014712, 2018)
—Emily Underwood, Freelance Writer