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.
