Abstract
The thermal-mechanical response of rock at shallow to medium depth
beneath the earth's surface has been modeled as a magma body
ascends toward
it. The overall stress field is calculated by considering the
country rock
as a viscous fluid, a thermal-elastic material, or as an
elasto-plastic
material that fractures when its strength is overcome.
The stress field within and around a spherical magma body
surrounded
by a homogeneous, Newtonian fluid has been evaluated and can be
used at
deeper levels in the earth's crust where the viscosity of the
country rock
is relatively low. With decreasing depth wall rock material
becomes more
viscous which results in diminishing stress magnitudes. In a
highly viscous
material shear stress and tangential stress have negligible
magnitudes.
They become more important by considering a spherical magma
chamber rising
within a bigger spherical container.
In the thermal-elastic model the current pressure inside the magma
body and the stress field within the host rock are determined. A
pressure
increase of the magma chamber is induced by crystallization of
anhydrous
minerals associated with exsolution of an aqueous phase. This
results in
magma chamber expansion and pressure increase since the elastic
deformability
is limited.
Thermal stress due to heating of country rock material is the most
important stress component and is sufficient to fracture brittle
country
rock. The temperature distribution within the wall rock has a
fundamental
influence on the fracturing process and its associated stress
field. Four
regions in the elasto-plastic host rock can be distinguished. From
the
magma outward, they are: (1) a cataclastic region with shear
fractures
more or less parallel to the chamber's margin, (2) a
thermal-elastic zone
with preexisting fractures, (3) a fractured region containing
shear fractures
with high angles to the interface between magma and host rock, and
(4)
an almost intact elastic outer region. The fluid pressure of a
porous host
rock enhances the fracturing process, but the fluid pressure of
the magma
hinders the development of the cataclastic region. Ascent of a
magma body
surrounded by a fractured material can occur by stoping, in which
disengaged
wall rock fragments sink in less dense magma.
Arz, C., 1992. Thermal-mechanical response to an intruding magma
chamber.
Unpublished MSc. thesis, State University of New York at Albany.
129 pp.,
+xiv
University at Albany Science Library call number: SCIENCE
Oversize
(*) QE 40 Z899 1992 A79
thesis (scanned
text) - 4.3MB pdf file
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Geological
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