Deformation of rock and geologic structure - review questions for chapter 14

(also look at questions on p.429 in the textbook)

We turn next to the study of how rock is deformed and how we can read and interpret the geologic structures formed by bending, folding, and fracturing the rocks of the earth's crust. Although it is possible for rocks to be folded and faulted without the kind of recrystallization or development of foliation that would turn them into metamorphic rock, the forces responsible for folding and faulting are generally the same as those responsible for metamorphism. Furthermore these forces are best understood within the framework of plate tectonics. Therefore we will focus on the connection between particular kinds of structures and the types of forces and related plate-tectonic environments in which they form.

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.

Two fundamental concepts that are critical in understanding how rock deforms are stress and strain. Define each of these two terms and explain the difference between tensional stress, compressional stress, and shear stress.

When exposed to high stress, rock deforms in three distinct stages: elastic deformation, ductile deformation, and fracture. Explain the differences among these three stages. How does the behavior of rock under stress change from one stage to the next?

How does the style or pattern of rock deformation change with increasing temperature and pressure?

How does rock strength vary with depth in the earth's crust?

What is a fault? How are faults similar to and different from joints? What types of movement occur along faults and what are some typical rates of motion?

What do strike and dip tell us about the orientation of an inclined plane or layer of rock?

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?

What are slickensides and fault breccias?

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.

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. (We will illustrate in class with reference to some examples from the Appalachian Mountains.)