Geologic Structures, Maps, and Block Diagrams - Parts 1 and 2

Basin

In tectonics, a circular, syncline-like depression of strata.

Limb

One side of a fold. The dipping rock units between the crest of an anticline and the trough of a syncline.

Compression

Stress that reduces the volume or length of a rock, as that produced by the convergence of plate margins.

Shear

Stress that slices rocks into parallel blocks that slide in opposite directions along their adjacent sides. Sharing stress may be caused by transform motion.

Tension

Stress that stretches or extends rocks, so that they become thinner vertically and longer laterally. Tension may be caused by divergence or rifting.

Dip

The angle that a rock unit, fault or other rock structure makes with a horizontal plane. Expressed as the angular difference between the horizontal plane and the structure. The angle is measured in a plane perpendicular to the strike of the rock.

Hangingwall

The body of rock that lies above an inclined fault plane.

Footwall

The body of rock that lies below an inclined fault plane.

Strike

The geographic direction of a line created by the intersection of a plane and the horizontal. Often used to describe the geographic "trend" of a fold or fault.

Fold Axis

The line formed by the intersection of the axial plane of a fold with a bedding plane, marking where the bed shows its maximum curvature.

Structural Geology

The scientific study of the geological processes that deform the Earth's crust and create mountains.

Fold

A bend or flexure in a rock unit or series of rock units that has been caused by crustal movements.

Strain

A change in the volume or shape of a rock mass in response to stress.

Bedding

A collective term used to signify presence of beds, or layers, in sedimentary rocks and deposits.

Unconformity

A contact between two rock units of significantly different ages. An unconformity is a gap in the time record for that location.

Anticline (Antiform)

A convex fold in rock, the central part of which contains the oldest section of rock.

Dip-slip Fault

A fault in which two sections of rock have moved apart vertically, parallel to the dip of the fault plane.

Strike-Slip Fault

A fault with horizontal displacement, typically caused by shear stress.

Left Lateral Fault

A fault with horizontal movement. If you are standing on one side of the fault and look across it the block on the opposite side of the fault has moved to the left.

Right Lateral Fault

A fault with horizontal movement. If you are standing on one side of the fault and look across it, the block on the opposite side of the fault has moved to the right.

Normal Fault

A fault with vertical movement and an inclined fault plane. The block above the fault has moved down relative to the block below the fault.

Reverse Fault

A fault with vertical movement and an inclined fault plane. The block above the fault has moved upwards relative to the block below the fault.

Plunging Fold

A fold in which the axis is inclined at an angle from the horizontal.

Overturned Fold

A fold that has both limbs dipping in the same direction, resulting from one of those limbs being rotated through an angle of at least 90 degrees. Overturned folds are found in areas of intense deformation. The name overturned is given because the strata on one limb of the fold are "overturned" or upside down.

Stress

A force acting upon or within a rock, expressed in terms of unit weight per surface area such as tons per square inch.

Fault

A fracture or fracture zone in rock along which movement has occurred.

Axial Plane

A geometric plane that intersects the trough or crest of a fold in such a way that the limbs of the fold are more or less symmetrically arranged with reference to it.

Formation

A laterally continuous rock unit with a distinctive set of characteristics that make it possible to recognize and map from one outcrop or well to another. The basic rock unit of stratigraphy.

Thrust Fault

A reverse fault that has a dip of less than 45 degrees.

Syncline (Synform)

A trough-shaped fold with youngest strata in the center.

Monocline

An area of increased dip in otherwise gently dipping strata.

Angular Unconformity

An erosional surface that separates rock units of differing dips. The rocks below the surface were deposited, deformed, and eroded. The younger rocks above them accumulated upon the erosional surface.

Disconformity

An unconformity in which the beds above the unconformity are parallel to the beds below the unconformity.

Nonconformity

An unconformity that separates different rock types, such as sedimentary rocks from metamorphic rocks.

Dome

An uplift that is round or elliptical in map view with beds dipping away in all directions from a central point.