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Benjamin R. Phillips
Research
Supercontinent cycles
The geologic record suggests that continents occasionally group together to form individual supercontinents. Familiar is Pangea, which broke up about 200 million years ago (Ma). Also hypothesized are Rodinia (~1000-700 Ma) and Columbia (~1800-1500 Ma). This series of behemoths suggests the existence of a supercontinent cycle with a time scale of several hundred million years (Myr). However, the physical mechanism behind these supposed cycles is poorly understood.
At Princeton University I worked with Hans-Peter Bunge to incorporate continents into the 3D spherical, FEM mantle convection code Terra. My work characterized how feedback between continents and the Earth's mantle can lead to the development of long wavelength convection. For a mantle with a radially stratified viscosity structure and strong internal heating via the decay of radioactive elements, a supercontinent was found to promote convection on the largest length scales. The addition of heat flux across the core-mantle boundary (CMB) tends to disrupt this feedback, as continents are less effective at organizing flow that is driven from below. In addition, smaller continents have little impact on convection regardless of mantle parameterization.
(For more information see Phillips2005.pdf)
The extent of coupling between continents and convection has implications for supercontinent cycles. In particular, the strong feedback evident in layered viscosity, internally heated models drives regular ~400 Myr supercontinent cycles for a simple model with three continents. When strong mantle plumes are introduced, this cycling is disrupted. In a more realistic model with a larger number of smaller continents, strong plumes almost entirely preclude the formation of supercontinents.
(For more information see Phillips2007a.pdf)
Flood basalts
Continental flood basalts (CFBs) are among the most voluminous of volcanic events. Many CFBs seem to mark the breakup of supercontinents. This coincidence, along with broad lithospheric uplift and hotspot tracks originating at the locus of breakup have led to the hypothesis that deep mantle plume heads are both a catalyst for rifting and the source of CFBs.
Even in the absence of mantle plumes, a supercontinent can lead to extensive warming of the mantle. By increasing the characteristic convective wavelength, acting as a thermal blanket, and creating a subduction shadow, supercontinents hinder the mantle's ability to cool itself. In work with Nicolas Coltice and others, models of supercontinents in a purely internally heated mantle yielded a subcontinental temperature increase of up to 100 C. On this basis we suggest the existence of two distinct types of CFBs, caused either by plumes or by global mantle warming.
(For more information see Coltice2007.pdf)
Magma dynamics
The transport of basaltic magma through brittle and elastic crust occurs primarily via planar dikes. Host rock composition and temperature, dike width, advective vigor, and magma supply rate all affect dike evolution. Previous models addressed these effects, typically for uniform host rock compositions appropriate to specific case studies. I am working with Scott Baldridge, Carl Gable, and Jim Sicilian at Los Alamos to expand on this view by investigating a parameter space relevant to a broad spectrum of continental volcanic regimes. We are using the code Telluride, which was originially written for metallurgy simulations. The model solves for conservation of mass, momentum, and energy for a multi-material, multi-phase system and accounts for partial melt, material mixing, and turbulent flow. We are investigating constraints on dike stability (from closing by solidification to widening by wall rock melt back) as a function of relevant parameters. Working from these bounds we will consider the implications of along-flow wall rock variations on transport characteristics in an effort to begin addressing magma dynamics in more realistic systems.
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