sgr ms thesis - University of Maine

through a medium, the fractal dimension D s is influenced by the intensity of
fracturing. There are several potential mechanisms that could have formed the
Shatter Zone, and three possible end-members will be discussed: 1) pre-eruptive
magma emplacement caused hydraulic fracture of wall rock, 2) caldera
subsidence produced an abrasive collapse breccia along ring faults, or 3) the
rapid volume expansion of volatiles during eruption lead to explosive fracture of
chamber walls. These three mechanisms are fundamentally different and will
therefore produce different breccias with unique D s values.
In rock mechanics studies there are two end-member mechanisms for
minimum and maximum D s : hydraulic fracture and explosion (Jebrak, 1997; Clark
and James, 2003; Barnett, 2004). Abrasive breccias tend to produce size
distributions defined by a relatively intermediate D s (Jebrak, 1997; Sammis et al.,
2007). Hydraulic breccias are well defined by Griffith fracture theory because
they form from fluid assisted (for this paper, magma and groundwater could be
considered) incremental fracture propagation driven by tensile stress loading at
the fracture tip (Goodman, 1980; Clark and James, 2003; Genet et al., 2008).
Fracture propagation is driven by the condition of pore fluid pressure (
) in the
cracks (Dutrow and Norton, 1995; Clark et al., 2006; Genet et al., 2008).
Hydraulic fractures typically form due to an increase in
by volume increase
driven by fluid flow and thermal expansion. This is generally an incremental
process, with a rate determined by the amount of fluid and interconnected
cracks, the thermal gradient, and the rate of fluid flow (Clark and James, 2003).
Cracking usually occurs by an oscillating pattern of incremental
buildup to the
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