Deformation of rock and geologic structure - chapter 10

 Before we can appreciate the distinctions between different kinds of structures and the processes that cause them to form, we need first to understand some basic principles about the physical properties of rock and the ways that rock may respond to different kinds of stress. These principles are most familiar to engineers who study the mechanical properties of solid materials. Of course the combinations of temperature and pressure associated with folding and faulting of rock generally occur at some depth within the earth's crust and are not easy to replicate in the laboratory, but there is enough experimental observation to provide a basis for interpreting the geologic structures observed in the field. The physical properties of the rocks also determine the extent to which they resist weathering and erosion; for this reason we frequently observe that the underlying shape of a geologic structure does not necessarily match the form of the land surface following an extended period of weathering and erosion. Thus, for example, the structure at Sideling Hill in western Maryland is a syncline or trough-shaped fold, its limbs pointing upward like the sides of a bowl; but the center of the "bowl" is made of hard, resistant rock. Therefore the center of the synclinal structure also is associated with the crest of the ridge.

Some key points about deformation of rock and geologic structure are outlined below in question form.

• Two fundamental concepts that are critical in understanding how rock deforms are stress and strain. Stress is defined in the textbook as the amount of force acting on a rock mass to change its shape and/or volume; strain represents a measure of how much a rock deforms or changes shape in response to an imposed stress. You should be able to explain the differences between tensional stress, compressional stress, and shear stress, all of which are defined as differential stresses, i.e. stresses that do not act equally in all directions.
• When exposed to high stress, rock deforms in three distinct stages: elastic deformation, ductile or plastic deformation, and brittle deformation or fracture.   An elastic material will spring back to its original shape after stress is removed (example: a rubber band after being stretched), but there is generally a limit beyond which elastic behavior does not occur and the material is permanently deformed (example: stretch a rubber band until it breaks). Brittle and ductile behavior are two alternative styles of deformation.
• How does the style or pattern of rock deformation change with increasing temperature and pressure? The biggest change is that you tend to get ductile behavior instead of brittle behavior, but this also depends on the properties of the rock itself; some rocks are more brittle than others even at the same pressure and temperature.
• What are strike and dip and what do they tell us about the orientation of an inclined plane or layer of rock? How do the symbols for strike and dip on a geologic map allow us to construct cross-sections of the underlying structures?
  
• What are folds and how do they form?
• Be prepared to define or identify the following, all of which are associated with folding: monoclines, anticlines, and synclines; limbs, axis, and axial plane of a fold; plunge of a fold; symmetrical, asymmetrical, overturned, and recumbent folds.
• What are domes and basins, and how are they similar to or different from anticlines and synclines.
• What is a fault? How are faults similar to and different from joints? What types of movement occur along faults?
• Be prepared to identify which side of a fault is the hanging wall and which is the footwall. What is the distinction between normal and reverse faults, and how are both of these different from strike-slip faults? What is a thrust fault and how is it similar to or different from a reverse fault? What is a transform fault and how is it related to strike-slip faults?
• What are horsts and grabens, what type of fault are they associated with, and what type of stress causes them to form?
• What type of stress and which type of plate-tectonic environment is typically associated with each of the major types of faults?
• Describe the association between folding and faulting at a convergent plate boundary or in the vicinity of a continental collision; explain the development of thrust sheets.
  
• A discussion of the San Andreas fault system at the end of the chapter may serve as a convenient lead-in to the next chapter on earthquakes.
• A discussion of the U.S. Geological Survey expedition to Turkey after the August 1999 earthquake (and the major aftershock of November 12!) provides background on earthquakes but also provides a comparison of the San Andreas fault with the North Anatolian fault in Turkey.