Learning Objectives

1. Tectonic forces move and deform parts of the earth's crust. Stress is force applied to an object, while strain is a change in size and shape or both, while an object is undergoing stress. Compressional stress produces shortening strain, tensional stress produces stretching or extensional strain, and shear stress produces shear strain: parallel movement in opposite directions.

2. A body responding with elastic strain recovers its original shape. A body responding with plastic strain does not return to its original shape, while brittle strain produces fractures. Sedimentary rocks at the earth's surface are brittle.

3. A geologic map depicts rock types and structures, and a cross section is a vertical representation of a portion of the earth. Strike is the compass direction of the line formed by the intersection of an inclined bedding plane with a horizontal plane. Dip is the angle formed by the bed and a horizontal plane, and it is always measured perpendicular to strike. Strike and dip are measured in the field with a Brunton pocket transit. Horizontal and vertical beds have special symbols.

4. Folds are bends in layered rock produced by plastic strain. The axial plane (visualized as connecting the hinge lines formed by the bending of each bed in the fold) divides the fold into limbs. An anticline is an arch in which the beds dip away from the hinge line, while a syncline is a trough in which the beds dip toward the hinge line. Anticlines have their oldest beds exposed along the hinge line, while synclines have their youngest beds exposed along their hinge line.

5. Plunging folds have hinge lines that dip and produce V-shaped patterns of exposed strata.

6. Structural domes (doubly plunging anticlines) have beds that dip away from a central point, while structural basins (doubly plunging synclines) have beds that dip toward a central point.

7. Open folds have limbs with gentle dips; Isoclinal folds have limbs parallel to one another. Overturned folds have limbs that dip in the same direction. Recumbent folds have limbs that are essentially horizontal.

8. Brittle strain produces fractures in rocks called joints, if no displacement occurs. Columnar jointing and sheet jointing were mentioned in earlier chapters. Compression produces multiple joint sets.

9. Faults are fractures along which displacement occurs. The fault surface or plane separates the two sides of the fault into a hanging wall (above the fault plane) and a footwall (below the fault plane).

10. Dip-slip faults exhibit movement parallel to the dip of the fault plane. Normal faults have a hanging wall that moved down in response to tensional stress. Blocks bounded by normal faults produce grabens and rifts, if dropped down, or horsts, if raised up. Reverse faults are dip-slip faults that have a hanging wall that moved up in response to compressional stress. A thrust fault is a reverse fault with a low angle fault plane. Strike-slip faults are associated with shearing and have no vertical displacement. Left-lateral and right-lateral movement is determined by looking at displacement across the fault plane.