Metamorphic Rocks and Crustal Deformation: An Overview
Agents of Metamorphism
Heat, pressure, and chemically active fluids are the main agents of metamorphism.
Cataclastic or Dynamic Metamorphism
This type of metamorphism occurs in fault zones where rocks are subjected to tectonic stress and pulverization, producing finely cataclastic rocks.
Contact Metamorphism
Contact metamorphism occurs as a result of increased temperature when magma intrudes a rock body. In this case, an alteration zone called an aureole forms in the rock surrounding the magma body.
Regional Metamorphism
During the formation of mountains, large volumes of rocks are subjected to high pressure and temperatures associated with large-scale deformation, resulting in regional metamorphism.
Metamorphic Rocks of Igneous Origin
Ultramafic rocks and mafic rocks contain low silica. Examples include pelitic, greywacke, and calc-silicate rocks.
Features of Significant Metamorphic Rocks
- Slate: A very fine-grained, foliated rock composed of tiny mica crystals that are too small to be visible.
- Phyllite: Represents a gradation in metamorphic grade between slate and schist.
- Amphibolite: This rock consists mainly of dark plagioclase. The alignment of hornblende crystals produces a slightly foliated texture.
Importance and Usefulness of Metamorphic Rocks
Metamorphic rocks allow us to reconstruct the history of our planet. They help determine the age of the land, understand the distribution of continents in the past, and find evidence of past life forms.
Elastic Rebound Theory
This theory emerged after the San Francisco earthquake of 1906, proposed by H.F. Reid in 1910.
Snell’s Law
This law describes the phenomenon of reflection and refraction.
Flanks or Wings of a Fold
These are the portions of a fold found on either side of the axial plane, between the axis of an anticline and the axis of a syncline.
Fold Axis or Axial Line
This is the line of intersection between the axial plane and the plane of any of the folded layers.
Hinge of a Fold
This is where the dip of the layers changes direction.
Direction of a Layer
This is the angle formed by the intersection of the line of strike of the layer plane and the horizontal plane.
Types of Folds
- Homoclinal Fold: A fold in which the layers have a uniform and constant dip. It occurs when layers are in a more or less horizontal position. This type of fold is characteristic of areas that have experienced an uplift and then returned to their previous position.
- Dome or Plateau: A structure in which the strata dip away from a central point. Vertical forces in the central part cause the layers to remain horizontal.
- Structural Terrace: A fold where the layers have a gentle dip in a central section and are nearly horizontal elsewhere.
- Fan Fold: So named because it has a compressed core, and the core has the greatest curvature.
- Anticline: A fold that has an arch-like shape when viewed from above. The limbs diverge, and the oldest layers are at the center of the structure.
- Syncline: A fold that has a trough-like shape when viewed from above. The limbs converge, and the youngest rocks are at the center of the structure.
- Isoclinal Fold: A fold formed by parallel anticlines and synclines. Its characteristic feature is that its limbs are parallel.
- Chest or Box Fold: This is the narrowest part of an anticline.
- Chevron Fold: A fold characterized by sharp, angular bends, alternating between anticlines and synclines.
Matching of Folds
- Symmetrical
- Asymmetrical
- Overturned
- Recumbent
- Plunging
Crustal Deformation
Breaking of Rocks
- Fracture
- Fault
- Joint
- Fissure
Fault Plane
The surface along which a fault occurs, separating two blocks of rock.
Fault Scarp
The exposed surface of the fault plane.
Fault Mirror
Polished areas on the fault plane caused by the movement of the rocks.
Strike of a Fault
The direction of the intersection of the fault plane with a horizontal plane, measured relative to north.
Dip of a Fault
The angle of inclination of the fault plane relative to the horizontal.
Normal Fault
A fault caused by tensional stress, where the hanging wall moves down relative to the footwall.
Reverse Fault
A fault caused by compressional stress, where the hanging wall moves up relative to the footwall.
Thrust Fault
A low-angle reverse fault.
Horst Fault
A fault caused by tensional stress, resulting in the formation of two normal faults with opposing dips.
Graben or Rift Valley
A down-dropped block of rock bounded by two normal faults.
Shear Fault
A fault caused by horizontal shear stress.
Rotational Fault
A fault that involves rotation of the rock blocks.
Transverse Fault
A fault that cuts across the layers of rock.
Strike-Slip Fault
A fault where the movement is parallel to the strike of the fault.
Hooke’s Law
Within the elastic limit of a body, the deformation it undergoes is directly proportional to the stress applied.
Young’s Modulus
A characteristic property of solid substances that measures the stiffness of a material.