Most volcanism occurs along plate boundaries, along mid-ocean ridges and volcanic arcs. Broadly speaking, the causes of volcanism in these setting is understood, while the causes of the lee voluminous volcanism that occurs in plate interiors remain obscure. This project will test the novel hypothesis that intraplate volcanism along the Hawaiian Arch and along some continental margins is caused by interactions between rising plumes of mantle material and the overlying lithosphere by a combination of numerical and analog (physical) modeling. The results will shed light on the interaction of the lithosphere and the underlying mantle. This project will lead to better understanding of Hawaiian volcanism, in which there is broad public interest. Broader impacts public lectures on this topic and training of students in numerical methods and geodynamic numerical modeling.
This work focussed on what happens when a hot mantle plume, like those underlying Iceland, Hawaii, or Yellowstone, upwells and melts beneath a tectonic plate with changing thickness. We analyzed two specific cases. One was to model mantle flow and melting for a Hawaii-like situation where there was a change in the age of the ocean lithosphere crossing the plume as occurs at the Molokai fracture zone. The other was a case where the small continent of Greenland crossed over the Iceland plume, as happened between 65-55 million years ago. We find that plume melting and flow can be strongly affected by the thickness of the overlying lithosphere. Analagous to a river flowing downhill, the plume material tends to try to flow upslope the base of the ltihosphere, resulting in plume material draining towards regions of thinner lithosphere such as coastward or towards lithosphere of younger age. This results in additional volcanism on regions of younger age, due to pressure-release melting of plume material that has drained laterally from the region above its deep mantle source. We started by using a new three-dimensional code for mantle flow that includes the correct thermodynamic treatment of melting of idealized mantle rock compositions. The mantle melts to a composition, basalt, that is different from its source, and typically only melts a total of ~1-10% within an unwelling plume, meaning that 9~90-99% of the plume material remains in the mantle after the extraction and ascent of the basaltic melt that will rise to form an erupted lava at the surface. This model included the possible effects that melting makes the residue less dense This effect should occur at all conditions appropriate for plume melting, with 10% melt extraction reducing the density of the residual mantle by an amount equivalent to heating the rock by ~30-50°C. In addition, models were considered in which plume melting led to an increase in the viscosity ('stickiness') of the residual mantle. This effect, if it occurs as expected from theory and some laboratory experiments on mantle rocks, was shown to play a major role in influencing the pattern of melting and lateral drainage of plume material. Significant progress was made in better evaluated the dynamic feedbacks that occur between mantle flow, melting, and viscosity/density. So-called 'edge-driven' convection models that had been proposed as the source of massive flood basalts erupting at the edge of continents, e.g. Greenland, or the volcanism associated with the Palisades in New York, were shown to simply not work quantitatively once the models included the effects of the buoyancy changes associated with the extraction of a partial melt, and even worse once viscosity increases were also considered. We now have the tools to look at this effect in other environments such as the east coast of South America, and the east coast of Australia. This follow-up work is still in progress, but what is likely to be the hardest phase of work (figuring out how to do these experiments the first times) is now complete. We are still far from our ultimate goal, which is to couple realistic compositional models of melting to modelling techniques for mantle flow. This ability will let us ultimately explore the Earth's melting processes from a completely different perspective — not by working backwards from rocks exposed at the surface, but instead by working forwards from first principles, laboratory experiments, and observation-based thermodynamic theory to successfully predict and compare predicted melt compositions to observed lavas. These insights, once well understood, will greatly enhance our understand of how basalt, and then granites and continental crust differentiated from the primordial stuff that formed our planet at the dawn of the solar system. This work provides a useful step along this decadal path of scientific exploration.
