Deformation

Deformation

All changes in the original form and/or size of a rock body.

Stress

Force per unit area.

Strain

Change in length divided by the original length.

Strain rate

Determines whether material behaves in a brittle (breaks) or ductile (plastic) manner; equal to strain divided by time.

Compression, tension, shear

The three types of stress orientations.

Compressive forces

Forces that squeeze and shorten rocks.

Tensional forces

Forces that stretch and pull rocks apart.

Shearing forces

Forces that shear two parts of a rock formation in opposite directions.

Types of deformation

Elastic, plastic, brittle.

Elastic deformation

Temporary deformation in response to applied stress; rock returns to original shape upon removal of stress.

Plastic deformation

Permanent deformation in response to applied stress; rock does not return to original shape.

Brittle deformation

Occurs when rock is stressed to the point of failure.

Folding

Type of ductile deformation; includes anticline and syncline.

Anticline

Ductile; upfolded, or arched, rock layers (A shaped); oldest rock in center.

Syncline

Ductile; downfolded rock layers (U shaped) with youngest rock in center.

Symmetrical folds

Ductile; folds with limbs that dip symmetrically from the axial plane.

Asymmetrical folds

Ductile; folds with one limb that dips more steeply than the other.

Overturned folds

Ductile; folds with limbs that dip in the same direction, one limb tilted beyond vertical.

Plunging fold

Ductile; fold with an axis at an angle to the horizontal.

Dome

Ductile; geological structure where oldest rocks are exposed on the surface, youngest at the edge.

Basin

Ductile; geological structure with youngest rocks in the core.

Fault

Zone of localized, brittle deformation.

Dip slip fault

Movement along the inclination (dip) of a fault plane; includes normal, reverse, and thrust faults.

Strike slip fault

Fault where the dominant displacement is horizontal and parallel to the trend or strike.

Reverse faulting

Caused by compression; hanging wall block moves up with dips greater than 45 degrees.

Normal faulting

Caused by tension; hanging wall moves down.

Strike slip faulting

Caused by shearing forces.

Pseudotachylyte

Veins of glassy rocks formed from frictional melting during earthquakes, landslides, and impact events.

Slickenlines/fibers

Mineral fibers precipitated in open fault zones or on polished surfaces with lines gauged by small protrusions.

Geologic cross sections

Diagrams showing features visible if vertical slices were made through part of the crust.

Strike

The orientation of a horizontal line within a plane.

Dip

The angle a layer of rock is tilted with respect to horizontal.

Tensional tectonics

Extension of continental crust producing normal faults with high dip angles that flatten with depth, forming curved fault surfaces.