GEOL 105 Lab #3

Anticline

An arch-shaped fold in rock layers where the limbs dip away from the central axial plane, with the oldest rock beds located at the core/center.

Syncline

A trough-shaped or U-shaped fold in rock layers where the limbs dip toward the central axial plane, with the youngest rock beds located at the core/center.

Axial Plane

An imaginary planar surface that divides a fold symmetrically into two limbs.

Convergent Boundary

A tectonic plate boundary where plates move toward each other, producing compressive forces that commonly form folds and reverse/thrust faults.

Divergent Boundary

A tectonic plate boundary where plates pull apart from each other, producing tensional forces that thin the crust and create normal faults.

Transform Boundary

A tectonic plate boundary where plates slide past each other horizontally, producing shear forces that result in strike-slip faults.

Law of Cross-Cutting Relationships

A fundamental geological principle stating that a feature (such as a fault or igneous intrusion) that cuts across rock layers must be younger than the layers it cuts through.

Hanging Wall

The block of rock that lies directly above an inclined fault plane.

Footwall

The block of rock that lies directly below an inclined fault plane.

Normal Fault

A fault caused by tensional stress where the hanging wall moves down relative to the footwall.

Reverse Fault

A fault caused by compressive stress where the hanging wall moves up relative to the footwall.

Strike-Slip Fault

A fault caused by shear stress where rock blocks slide past each other horizontally with little to no vertical movement.

Right-Lateral Strike-Slip Fault

A strike-slip fault where the opposite side of the fault appears to have shifted to the right relative to an observer standing on either side.

Left-Lateral Strike-Slip Fault

A strike-slip fault where the opposite side of the fault appears to have shifted to the left relative to an observer standing on either side.

Unconformity

A substantial gap or break in the geologic rock record caused by a period of erosion or non-deposition.

Angular Unconformity

An unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted or folded and eroded rock layers.

Nonconformity

An unconformity where sedimentary rock layers rest on top of older, eroded igneous or metamorphic rock.

Disconformity

An unconformity between parallel layers of sedimentary rock where the boundary represents a period of erosion or non-deposition rather than tilting.

Law of Lateral Continuity

A principle stating that layers of sediment initially extend laterally in all directions until they thin out or terminate against a barrier.

Compression

A stress force that squeezes rock layers together, commonly associated with convergent boundaries, folds, and reverse faults.

Tension

A stress force that pulls rock layers apart, commonly associated with divergent boundaries and normal faults.

Shear

A stress force that causes rock masses to slide past one another horizontally along a fault line.

Stress

The force applied to a rock per unit area (compressional, tensional, or shear).

Strain

The change in shape, volume, or orientation of a rock resulting from stress.

Brittle Deformation

The fracturing or breaking of rock when stress exceeds its yield strength (common at shallow, cool crustal depths; forms faults and joints).

Ductile Deformation

The bending, stretching, or flowing of rock without breaking (common at high temperatures and pressures deep in the crust; forms folds).

Elastic Deformation

Temporary strain where rock deforms under stress but returns to its original shape once the stress is removed.

Hinge Line (Fold Axis)

The imaginary line along a rock layer connecting points of maximum curvature in a fold.

Axial Plane

An imaginary plane connecting the hinge lines of consecutive layers, dividing a fold into two sides.

Limbs

The dipping sides of a fold located on either side of the axial plane.

Monocline

A single step-like bend in otherwise horizontal or gently dipping rock layers.

Dome

A circular or elliptical upwarp where layers dip away from the center; oldest rocks are exposed at the center.

Basin

A circular downwarp where layers dip inward toward the center; youngest rocks are exposed at the center.

Hanging Wall

The block of rock situated directly above an inclined fault plane.

Footwall

The block of rock situated directly below an inclined fault plane.

Horst

An uplifted block of crust bounded by normal faults in an extensional tectonic setting.

Graben

A down-faulted block of crust bounded by normal faults, creating a valley structure.

Joints

Fractures in rock along which no significant movement or displacement has occurred.

Dip Angle

The acute angle measuring the steepest downward slope of a rock layer relative to a horizontal plane (0°–90°).

Rule of V’s

The mapping principle where an outcrop pattern forms a "V" shape pointing in the direction of dip as it crosses a stream valley.