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University of Wisconsin - Milwaukee
University of Wisconsin - Milwaukee
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a push or pull that causes or tends to cause change in the motion of a body
the amount of force divided by the area where the force is applied
How do rocks respond to force and stress
If stress is small rock may reamain unchanged, if stress is greater rock may be displaced , rotated, have its shaped modified , or strained , or all four
in response to stress a rock may be moved from one place to another, rock may stay same shape or change as it moves
rock may be rotated in response to stress
a rock may respond to stress by deforming internally; change size or shape without layers breaking or rotating
at shallow levels of crust
ductile (flowing) behavior
at deeper levels where temperature and pressure are higher
Strain: a rock may respond to stress by deforming internally; change size or shape without layers breaking or rotating
brittle behavior: at shallow levels of crust
ductile (flowing) behavior: at deeper levels...
Stable: at shallow level with low pressure...
Recrystalization: at deep levels of crust w/...
Effect of Fulids
At low temperatures fluids have little effect...
Veins: may occur at low levels of crust where...
At deep levels hot water and other fluids...
at shallow level with low pressure and temperature minerals remain stable
at deep levels of crust w/ high temperatures and pressure minerals recrystallize into smaller crystals and new minerals may grow
may occur at low levels of crust where water fills fractures with minerals
the entire array of stresses applied on a point or volume of rock
a fold, generally concave and upward, with the youngest rocks in the center
when the force imposed on the rock is the same amount from all directions
pushes outward in all directions (water in pores push outward)
what deforms rocks
when a stress pushes in on rock
when stress is directed outward, pulling the rock
stresses on the edge of the block were applied in opposite directions
-tension @ shallow
can form from fractures that help the rock stretch as it is pulled apart
-shear @ shallow
usually forms a fault
a fracture along which two rock masses have slipped past one another
-compression @ deep
can squeez the rocks and form tightly squashed folds and new metamorphic structures
-tension @ deep
difficult to maintain deep in crust; high confining pressure pushes inward and generally is greater than any forces pulling rock apart; can fracture the rock and form veins
-shear @ deep
commonly is distributed across a wide zone; rocks deform and flow as weak solids
fracture that form as simple cracks representing places where the rock has pulled apart by a small amount
perpendicular to the direction of least stress
when the stress field allows the rock to be pulled apart in a horizontal direction; can form in compass direction, depending on the orientation of the stresses
form if a rock is pulled apart in a vertical direction; can occur when tectonic stresses push on the sides of the rock, which causes the vertical stress to be the smallest stress
horizontal joints: form if a rock is pulled apart in a vertical direction; can occur when tectonic stresses push on the sides of the rock, which causes the vertical stress to be the smallest stress
ways to describe faults
dip: a fault surface that is inclined in one direction; described in terms of...
strike: when water floods area of fault it will intersect or strike along a...
dip-slip faults: slip along a fault can be parallel to the dip - one block...
strike-slip fault:can occur in a horizontal direction, parallel to the strike.
oblique-slip fault: slip along a fault can also occur in a direction that is...
a fault surface that is inclined in one direction; described in terms of degrees from horizontal or use of terms such as steep, moderate, and gentle
when water floods area of fault it will intersect or strike along a horizontal line; orientation of a surface's strike relative to true norht
slip along a fault can be parallel to the dip - one block moves up or down relative to the other block
slip along a fault can also occur in a direction that is oblique - neither parallel to the dip nor the strike
the block above the fault
the block below the fault
if the hanging wall moves down relative to the footwall; forms when the rock units are pulled apart and lengthenen
if the hanging wall moves up relative to the footwall; forms as a result of horizontal compression and shortens the rock units in a horizontal direction
a reverse fault that has a gentle dip
when rocks along a fault move with a side-to-side motion, parallel to the strike of the fault surface
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