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Today’s list____________ Ch15: Rock Deformation Mapping geologic structures How rocks get deformed 3) Folds. Study questions____________ Ch15: Rock Deformation · What type of tectonic forces makes a normal fault? A reverse fault? A strike slip fault?
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Today’s list____________ • Ch15: Rock Deformation • Mapping geologic structures • How rocks get deformed • 3) Folds
Study questions____________ Ch15: Rock Deformation · What type of tectonic forces makes a normal fault? A reverse fault? A strike slip fault? What is the difference between brittle and ductile deformation? What are the strike and dip angle of a deformation? What is the shape of an anticline, syncline, and what is the relative age of rocks exposed? What type of deformation is the San Andreas Fault? The mid-ocean ridge?
rock structures out crops 1) Mapping geologic structures Deformational processes create faults, folds, joints Visible when rock beds are exposed at surface
1) Mapping geologic structures • Strike and dip TSP 15.8
1) Mapping geologic structures • Strike and dip • Compass bearing of a rock layer relative to North • Angle of tilt of a bed from a horizontal plane Strike Dip Fig.11.4
2) How rocks become deformed Confining pressure: force equal in all directions Differential stress: force not equal in all directions
2) How rocks become deformed Response to differential stress 1. Elastic deformation – the rock returns to nearly its original size and shape when the stress is removed 2. Brittle deformation rocks crack/fracture 3. Ductile deformation rocks flow Shallow in crust Deeper down
2) How rocks become deformed Fault definition: a fracture where displacement has occurred: rocks on either side of fault have moved relative to each other.
2) How rocks become deformed Fault definition: a fracture where displacement has occurred: rocks on either side of fault have moved relative to each other.
Slip happens on faults! Joints Fractures where no slip has occurred Not same as a fault! Why?
2) How rocks become deformed- brittle materials Differential stress: force not equal in all directions Compressive forces: shortening
2) How rocks become deformed- brittle materials Differential stress: force not equal in all directions Tensional forces: stretching
2) How rocks become deformed- brittle materials Differential stress: force not equal in all directions Shearing forces: shearing
2) How rocks become deformed - brittle materials Fig. 11.13 • Rift valleys are the result of tensional forces in the crust, creating a series of normal faults. • The downfaulted blocks result in a Rift Valley.
2) How rocks become deformed - brittle materials • St. Andreas fault is an example for a strike-slip fault • Because it is a plate boundary, it is also a transform fault Fig. 11.10
2) How rocks become deformed- ductile materials Differential stress: force not equal in all directions Compressive forces: shortening Tensional forces: stretching Shearing forces: Shearing/bending
3) How rocks fold • series of wavelike undulations of once-flat rock layers • many sizes: microscopic to 100’s of meters • compressive stresses: shorten+thicken crust
3) Folds Synclines fold downward… Anticlines fold upward… Fig. Story 11.16
3) Folds horizontal and plunging anticline Plunging fold: axis of fold penetrates into the ground Fig. 11.16
Plunging anticlines and synclines See Fig. 11.17
3) Folds • Types of folds • Other deformations Symmetrical, asymmetrical, overturning, plunging….. dome - circular upwarping (anticlinal structure) basin - circulardownwarping (synclinal structure) Fig 11.19