Structural Geology - the study of faults, folds, and fractures in the Earth. These fault, folds, and fractures are a consequence of large stresses caused by plate tectonic processes.

Basic Concepts - Structures are often defined by the attitude of beds which are no longer flat but tilted. If beds remain planar but have been tilted, the amount of tilt is called the dip of the beds. Any plane which has been tilted still has a horizontal line which denotes the intersection of the tilted bed (i.e., plane) with the horizontal plane. The orientation of the line of intersection is called the strike of the tilted bed. If beds are not longer planar, then the local orientation of the bed can be characterized by a strike and dip. Of course, the strike and dip vary depending on location on a folded bed.

Folds - bends in layered bed rock. Folded rock such as the Appalachians are similar to layers of rugs which have been forced into a series of arches and troughs.

Anticlines - arches or up domed folds.

Synclines - troughs or down folded rock.

Fold axis - the hinge line of an anticline or syncline where rocks change dip from one direction to the opposite direction.

Limb - the portion of a fold between an anticline and syncline which dips in the same direction. The limb is between fold axis.

Plunging fold - While some fold axes are horizontal, most plunge so the outcrop expression of a fold often gives the impression that the fold is a cone.

Monocline - a local steepening of an otherwise uniformly dipping or flat pile of strata.

Dome - a fold where beds dip away from a center point in all directions.

Structural basin - a fold where beds dip toward a center point.

Fold shapes - the geometry of fold limbs relative to each other.

Open folds - both limbs of folds dip gently away from each other.

Isoclinal folds - both limbs of any fold are parallel to each other.

Overturned folds - strata in one limb have been tilted beyond the vertical.

Recumbent folds - axial planes are horizontal or nearly so.

Fractures - when rock is subject to higher differential stress near the surface of the Earth, it often fails by brittle fracture.

Joints - cracks which form when large tensile stresses develop. The joint opens by displacements normal to the crack wall.

Shear fractures - a fractures which form when the rock is subject to large compressive stresses near the surface of the earth. These fractures form by shear displacement parallel to the walls of the fracture. Shear displacement is usually considered to be less than one centimeter.

Faults - shear fractures with displacements greater than one centimeter. Most faults start as shear fractures. Often deformation continues so that the fault slips a number of times. Large amounts of slip are associated with earthquakes.

Fault Blocks - the fault of usually a planar surface which had a dip. The fault divides rocks above and below the fault into two blocks.

Hanging wall block - The block above the fault plane.

Footwall block - The block below the fault plane.

Normal Faults - the hanging wall moves down relative to the footwall on a fault plan which dips about 60°.

Graben - a down-dropped block between two uplifted blocks (horsts). This combination of faults leads to the formation of rift zones such as is found in Africa.

Half-graben - a down-dropped block usually found at continental margins where rift zones have filled with ocean crust.

Horst -the uplifted block next to a graben.

Rift - a graben in continental crust which will eventually fill with oceanic crust as continents spread apart.

Reverse Faults - the hanging wall moves up relative to the footwall on a fault plan which dips about 60°.

Thrust Faults - the hanging wall moves over the foot wall on a fault zone horizontal or dipping at a relatively shallow angle.

Strike-slip faults - a vertical fault along which either wall moves in a horizontal direction relative to the opposite wall.

Transform faults - strike-slip faults at mid-ocean ridges where the ridges appear to be offset in one direction but the sense of slip on the fault is actually in the opposite direction.