Dikes
are magma intrusions into cooler country rocks. Dikes can be of
the same
or different composition as the country rock it
intrudes. Dikes propagate from a magma chamber far below
Earth's surface. Dikes move through cracks in Earth's crust
moving
towards the surface where there is less pressure and the temperature is
cooler. By looking at the xenoliths in the dikes we can see that
dikes
have been known to move up to speeds of around 1m/s (Sparks,
1977). The process of magma dikes moving through cracks is the
main way magma moves through the crust. When the dike is moving
in a crack the stress at the tip of the dike causes the rock to
fracture and create a longer crack. Once the crack is large
enough the
dike will move forward into the opening crack (Lister, 1990).
There have been studies done on the effect of topography on dike
propagation. These have shown that dikes will propagate
perpendicular to the maximum tensile stress (Cotterell and Rice, 1980),
and that dikes will propagate in the direction which will give the
greatest normal stress at the tip of the dike. The greatest
normal stress will be achieved by having a larger load on the surface
which gives off a larger compressive stress (Takada, 1989).
The stresses on the crack in which the magma is propagating causes brittle failure in the rock which is how the crack grows and allows a dike to propagate further. Brittle failure occurs in cool rocks that do not have any or little plastic flow (Tarr, 2007). All rocks in the crust have a failure point at which the stress is too great and the rock fractures. The stress that makes the rock fracture is the stress at the tip of the dike which is the normal stress. As the stress from the dike builds it weakens the rock and once the stress reaches the failure envolope the rock fractures and the dike moves forward.
picture 3: Dike at Point Lake (Boise
State University, 2007)
The stresses on the crack in which the magma is propagating causes brittle failure in the rock which is how the crack grows and allows a dike to propagate further. Brittle failure occurs in cool rocks that do not have any or little plastic flow (Tarr, 2007). All rocks in the crust have a failure point at which the stress is too great and the rock fractures. The stress that makes the rock fracture is the stress at the tip of the dike which is the normal stress. As the stress from the dike builds it weakens the rock and once the stress reaches the failure envolope the rock fractures and the dike moves forward.
picture 4: Dike Fracturing as it
propagates (Johnson, Scott)
When a dike
propagates it moves towards the maximum compressive stress and also
moves perpendicular to the maximum tensile
stress on the rocks around it (Watanabe, 2002). The compressive
stress
is the stress applied directly from the object in question. In
this case this
would be a mountain which is the volcano. This
compressive
stress is applied straight down by the mountain, this would make the
normal stress for the tip of the dike greater in the direction of the
volcano causing the dike to bend towards the volcano. The
vertical movement happens due to the fact that a dike propagates
perpendicular to the maximum tensile stress (Cotterell and Rice, 1980)
and the force applied by the magma moving upwards towards Earth's
surface. In many mountain ranges the maximum tensile stress is
applied by plate tectonics (Nakamura, 1977). This maximum
tensile stress is in a horizontal direction and the dike
propagates perpendicular to that stress, this means because of the
tensile stress the dike is forced to propagate in the vertical
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