Rock Deformation
Forces that move large plates are responsible for deformation of rocks into folds and faults
Interpreting Field Data - orientation of a rock layer or plane
Strike - direction of intersection of a rock layer with a horizontal surface
Dip - measured at right angle to strike is angle of inclination from the horizontal.
Constructing a Geological Map and Cross Section
Geological Map - shows the geographic location of rock outcrops as well as strike and dip of inclined layers
Cross Section - a vertical slice through the crust (cliff face or road cut)
How Rocks Deform
Compressive Forces which squeeze and shorten rocks (convergent boundaries)
Tensional Forces which stretch a body and tend to pull it apart (divergent boundaries)
Shearing forces which push two sides of a body in opposite directions (transform faults)
What Determines whether a Rock Bends or Breaks?
Ductile (bends) - smooth and continuous plastic deformation (modeling clay)
Brittle (breaks) - experiences little change until it breaks suddenly (glass)
Confining Pressure - High confining pressure favors ductile deformation
Temperature - High temperature favors ductile deformation
Composition - some rocks are more ductile than others (e.g., sedimentary rocks)
Rate and Extent of Deformation - Fast and/or Intense deformation usually causes rocks to break
How Rocks Fold - an originally planar structure is bent (usually compressive).
Anticlines - upfolds or arches. Oldest rocks at center or core.
Synclines - downfolds or troughs. Youngest rocks at center or core
Folds can be divided into
Limbs - two sides of a fold
Axial Plane - imaginary surface that divides a fold as symmetrically as possible
Fold Axis - line of intersection between Axial Plane and beds of fold
Plunging Fold - fold axis is not horizontal. Eroded plunging folds produce a zigzag pattern of outcrop.
Asymmetrical and Overturned Folds - caused by increasing deformation. One limb of the fold dips more than the other and eventually one limb tilts beyond vertical and both limbs dip in same direction. Axial plane is not vertical.
Domes - broad circular upward bulge of rock layers (anticlinal). Important in oil geology.
Basins - a bowl-shaped depression. Michigan basin.
How Rocks Fracture
Joints - a crack along which no appreciable movement has occurred. Rock breaks at flaws or weak spots.
Faults - a fracture with relative movement of rocks on both sides of it. Can occur from compression, tension or shearing.
Fault Plane - surface on which the rock fractures and slips.
Strike-Slip fault - horizontal movement. results from shearing.
Right-lateral - block is displaced to the right. San Andreas
Left-lateral - block is displace to the left.
Dip-Slip faults - movement of rock up or down the dip of the fault plane.
Normal (tension) - rocks above the fault move down relative to rocks below the fault.
Rifts - tensional forces create a long, narrow trough bounded on each side by one or more parallel normal faults. East African rift, Rio Grande Rift, Red Sea Rift
Reverse (compression) - rocks above the fault move up relative to rocks below the fault. A reverse fault with a low dip angle is called a thrust fault (Movement is mostly horizontal).
Oblique-Slip faults - components of both dip-slip and strike-slip movement.