First Advisor

Maxwell Lutman Rudolph

Date of Award

2015

Document Type

Thesis

Degree Name

Bachelor of Science (B.S.) in Geology and University Honors

Department

Geology

Subjects

Mantle plumes -- Mathematical models, Thermal conductivity, Plate tectonics

DOI

10.15760/honors.200

Abstract

We quantified the fixity of mantle plumes relative to the mantle in high Rayleigh number numerical thermal convection with radially- and temperature-dependent viscosity structures in a 3D spherical shell geometry. The relative velocities between plumes and the mantle increased with increasing Rayleigh number. Net migration rates of plumes vary from ~0.016-0.053 °/Myr, and differential plume velocities can be up to ~25% of past- and present-day rates of net lithospheric rotation. Individual plumes migrate on average at ~0.0617-0.223 °/Myr for Rayleigh numbers between 1·107 and 2·108. We compared the ratio of plume velocity to horizontal average velocity (a) at the surface and (b) at mid-mantle for experiments with Rayleigh number between 5·107 and 2·108, which were (a) ~0.15 and (b) ~0.35. Additionally, we plotted the mean migration rates of plumes against Rayleigh number and found that a linear fit had a greater R2 value than a power law fit. There is a transition in the long-wavelength structure of convection from degree-2 to degree-1 poloidally dominant motion between Ra = 1·107 and Ra = 2·108 where thermal upwellings change from dominantly sheet-like to dominantly plume-like. Future studies should investigate whether the scaling of plume velocities with vigor of convection depends on the mode of convection. We find that all cases produce long-lived plume-like features which generate from a thermal boundary layer at the core-mantle boundary, and persist for more than 80 Myr. On average, the lifespan of mantle plumes decreased with increasing Rayleigh number. Our studies suggest that hotspots may be an imperfect proxy for a lower mantle reference frame.

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Persistent Identifier

http://archives.pdx.edu/ds/psu/16099

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