Browsing by Subject "Department of Geology and Geophysics"

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    Garnet: The Effect of Composition and Zoning On Crystal Shape
    (2009-04-08) Zahasky, Christopher
    Garnets are commonly found in metamorphic rocks. Garnets (X3Y2Si3O12) can have a variety of chemical compositions even within a single crystal. The composition of garnets can change during growth, this is known as zoning. Garnets crystallize in a number of different shapes including rhombic dodecahedron and trapezoidal. Rhombic dodecahedron is the more common shape for pyrope (Mg), almandine (Fe), and spessartine (Mn) garnets while trapezoidal is more commonly the shape of grossular (Ca) garnets. The substitutions of Ca, Mg, Fe, and Mn in the X site and its effects on certain characteristics of the garnets, specifically the shape, is the focus this research.
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    Grain Boundary Sliding in Olivine
    (2012-04-18) Stover, Dylan
    Although apparently motionless, the rocks that make up Earth are constantly moving. This motion is partially accommodated by the small grains that make up the rocks, sliding past each other. Olivine, (Mg,Fe)2SiO4, is the most prevalent mineral in Earth’s upper mantle, which is why we have chosen it for our study. The sliding of olivine along the grains’ boundaries is an important mode of movement in the mantle. However, direct measurements of the amount grain boundary sliding have not been taken and this prevents accurate modeling of the motion within Earth. Here we present the first direct measurements of the magnitude of grain boundary sliding in olivine. In our samples, grain boundary sliding dominates at low strains. As the amount of strain increases the effect of grain boundary sliding diminishes.
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    The Role of Fabric and Shear Inversion on the Development of Fractures in Shear Zones
    (2010-04-21) Zahasky, Christopher
    In this study we perform a number of shear experiments in clay in an attempt to reproduce and understand fracture patterns observed in a rock outcrop in northern Minnesota. A cake of clay is subjected to shear by movement in opposite senses of two plates on which the clay rests. The rate of displacement of the plates is held constant in each experiment. We examine the effects of anisotropy of the clay, shear-sense inversion, and rotation associated with shear, as possible mechanisms responsible for the fracture orientations that do not agree with those predicted by Coulomb theory.