Rock deformation shapes Earth's crust through folds, faults, and fractures. These structures form when tectonic forces apply stress to rock, and the type of structure that develops depends on the kind of stress, the rock's properties, and the conditions deep underground. Recognizing these structures helps geologists reconstruct the history of a region and understand the forces that shaped it.
Folds, Faults, and Fractures
Stress-Strain in Rock Deformation
Before diving into specific structures, it helps to understand the forces behind them.
Stress is the force per unit area acting on a rock. There are three main types, and each one produces different structures:
- Compressional stress squeezes rock together, shortening it. This produces folds and reverse faults. You see this at convergent plate boundaries.
- Tensional stress pulls rock apart, stretching it. This produces normal faults and rift valleys, like the East African Rift.
- Shear stress pushes rock in opposite horizontal directions. This produces strike-slip faults, like those along transform plate boundaries.
Strain is how the rock actually deforms in response to stress. There are three types:
- Elastic strain is reversible. Remove the stress, and the rock snaps back to its original shape.
- Plastic strain is permanent. Once stress exceeds the rock's yield strength, it deforms and stays that way.
- Brittle deformation happens when stress exceeds the rock's breaking strength, and it fractures.
Whether a rock deforms in a ductile (bendy) or brittle (breaky) way depends on several factors:
- Composition — Quartz-rich rocks resist deformation more than clay-rich rocks.
- Temperature and pressure — Higher temperatures and confining pressures make rocks more likely to deform plastically rather than fracture. That's why deep metamorphic rocks tend to fold, while shallow rocks tend to fault.
- Strain rate — Slow, gradual stress favors ductile deformation (producing features like mylonites). Rapid stress favors brittle deformation (producing fault gouges and breccias).
At the atomic level, ductile deformation works through two main mechanisms:
- Dislocation creep — Linear defects in the crystal lattice migrate through the mineral, allowing it to change shape gradually.
- Diffusion creep — Atoms migrate from high-stress areas to low-stress areas. Pressure solution in limestones is a common example.
Types of Geological Folds
Folds form when rocks undergo ductile deformation, bending instead of breaking. You can describe folds by their geometry, orientation, symmetry, and tightness.
Fold geometry refers to the basic shape:
- Anticlines arch upward, with the oldest rock layers in the core.
- Synclines arch downward, with the youngest rock layers in the core. The Appalachian Mountains contain many well-exposed synclines.
- Monoclines are step-like bends where one limb dips steeply and the other stays nearly horizontal. The Colorado Plateau has classic examples.
Fold orientation describes the inclination of the fold axis (the imaginary line running along the crest or trough of the fold):
- Non-plunging folds have a horizontal fold axis, like those in the Zagros Mountains of Iran.
- Plunging folds have a fold axis that tilts into the ground at some angle. A fold with a 30° plunge, for instance, means the axis dips 30° from horizontal. Plunging folds create distinctive V-shaped or horseshoe-shaped outcrop patterns on geologic maps.
Fold symmetry depends on how the two limbs compare:
- Symmetrical folds have limbs dipping at equal angles on either side of the axial plane. Chevron folds are a sharp-hinged variety.
- Asymmetrical folds have limbs dipping at unequal angles. Taken to an extreme, an overturned fold has one limb rotated past vertical, so both limbs dip in the same direction.
Fold tightness is measured by the interlimb angle, the angle between the two limbs:
- Gentle folds — interlimb angle greater than 120° (Jura Mountains, Switzerland)
- Open folds — 70° to 120° (Sheep Mountain Anticline, Wyoming)
- Tight folds — 30° to 70°
- Isoclinal folds — less than 30°, where the limbs are nearly parallel. These are common in highly deformed gneiss terranes.
Classification of Fault Types
Faults form when rocks undergo brittle deformation, breaking and sliding past each other. Two key terms to know: the hanging wall is the block above the fault plane, and the footwall is the block below it.
Dip-slip faults involve mostly vertical movement:
- Normal faults — The hanging wall drops down relative to the footwall. These form under tensional stress. The Basin and Range Province in the western U.S. is full of them, creating alternating raised blocks (horsts) and down-dropped blocks (grabens). The East African Rift is another major example.
- Reverse faults — The hanging wall moves up relative to the footwall. These form under compressional stress, like in the Himalayas. A thrust fault is a special case: a reverse fault with a dip angle less than 45°, so the fault plane is relatively flat. The Glarus Thrust in Switzerland is a famous example.
Strike-slip faults involve mostly horizontal movement:
- Right-lateral (dextral) — If you stand on one side and look across the fault, the opposite block has moved to the right. The San Andreas Fault in California is the classic example.
- Left-lateral (sinistral) — The opposite block has moved to the left. The Dead Sea Transform in the Middle East is a well-known example.
Oblique-slip faults combine both vertical and horizontal movement. The Denali Fault in Alaska is one such fault.
Joints and Fractures in Rocks
Not all breaks in rock involve movement. Joints are fractures along which no significant displacement has occurred. Fractures is the broader term for any break or discontinuity in rock.
Joints form from mechanical stress, cooling, or unloading (when overlying rock is eroded away and the rock below expands). There are two main types based on how they form:
- Extensional joints open perpendicular to the direction of minimum stress. Columnar jointing in basalt is a dramatic example.
- Shear joints form in conjugate pairs (two sets at angles to each other), oriented relative to the maximum stress direction.
Geologists also describe how joints are organized:
- A joint set is a group of roughly parallel joints with similar orientation.
- A joint system consists of two or more intersecting joint sets, sometimes creating polygonal patterns on rock surfaces.
Columnar jointing deserves special mention. When lava flows or shallow intrusions cool, they contract and crack into polygonal columns. Devil's Tower in Wyoming is one of the most striking examples of this process.