Rock Deformation and Geological Processes: A Comprehensive Guide
Rock Deformation
Deformation is the change in shape or volume experienced by rocks undergoing stress.
Types of Deformation:
- Elastic: The material deforms when subjected to stress but recovers its original shape when the stress stops.
- Plastic: The material deforms when subjected to stress and does not regain its original shape when the stress ceases to act.
- Brittle: The stress causes the material to lose internal cohesion and fracture.
Changes in Rock Behavior
At high temperatures and in the presence of water, rocks may exhibit plastic behavior.
Stress on Rocks:
- Strike: The angle contained in a horizontal layer with respect to north. Usually measured clockwise.
- Dip: The angle that the surface layer makes with a horizontal plane. The direction of the dip needs to be indicated.
Folds
Folds are bends that originate in rocks when exposed to compressive stress and exhibit plastic behavior.
Elements of Folds:
- Axial Plane: The plane that divides the fold into two halves as symmetrically as possible.
- Hinge: The area of the fold with the highest curvature.
- Fold Axis: The intersection of the axial plane with the hinge.
- Limbs: The fold materials located on both sides of the hinge.
- Core: The innermost zone of a fold.
Types of Folds:
- Anticline: A fold that has the oldest material in its core.
- Syncline: A fold that has the youngest material in its core.
Based on the Position of the Axial Plane:
- Upright: Axial plane dips approximately 90 degrees.
- Inclined: Axial plane dips between 85º and 10º.
- Recumbent: Axial plane dips less than 10º.
- Overturned: Axial plane has turned over 90° from vertical.
Based on Symmetry:
- Symmetrical: The axial plane divides the fold into two approximately symmetrical halves.
- Asymmetrical: The axial plane divides the fold into two clearly non-symmetrical halves.
Fold Associations:
- Anticlinorium: A set of folded strata in the form of a large anticline.
- Synclinorium: A set of folded strata in the form of a large syncline.
Faults
Faults are fractures in which there has been displacement of one block relative to the other.
Elements of Faults:
- Fault Plane: The fracture surface.
- Fault Blocks: The two blocks into which the land has been divided by the fracture. When the blocks are at different heights, we can speak of a hanging wall (raised block) and a footwall (sunken block).
- Fault Displacement: A measure of the relative movement that has occurred.
- Fault Orientation: The direction and dip of the fault plane.
Types of Faults:
- Normal Fault: The fault plane dips toward the footwall. Originates as a result of tensional stress and an increase in surface area.
- Reverse Fault: The fault plane dips toward the hanging wall. Caused as a result of compressional stress and a decrease in surface area.
- Strike-Slip Fault: The relative displacement of the blocks occurred horizontally. This type of fault does not have a hanging wall or footwall.
Fault-Related Structures:
- Thrust Fault: A type of reverse fault with a small dip, where the hanging wall overlaps the footwall, traveling tens of kilometers over it.
- Graben: A down-dropped block bounded on both sides by parallel faults.
- Horst: An uplifted block bounded on each side by parallel faults.
Mountain Ranges (Orogenesis)
One of the most notable consequences of lithospheric plate movement is orogenesis. Mountain ranges like the Pyrenees, Himalayas, or the Andes are called fold mountains or orogenic belts.
Formation of Mountain Ranges:
- Formation of Accretionary Prism: The majority of the sediment carried by the oceanic lithosphere does not subduct at the continental margin. The subduction zone traps sediments, which will be stacked, folded, and fractured.
- Magmatism and Metamorphism: The presence of water in the subducted lithosphere and heat favor the partial melting of rocks. The magma ascends, and some of it reaches the surface, causing volcanic activity, while others remain inside, contributing to the thickening of the continental crust.
- Orogenic Uplift: Two processes contribute to this: thickening of the continental crust (produced by the accumulation of sedimentary material) and isostatic uplift (due to the thickened crust, which is less dense than the mantle).
Types of Mountain Ranges:
- Interior: Examples include the Himalayas and the Alps.
- Continental Margin: Examples include the Rocky Mountains and the Andes.
Continental Collision Process:
The process involves oceanic subduction, ocean basin closure, and continental collision.
Geological Processes
Internal Processes:
Originate from the thermal energy inside the Earth, aided by gravity.
External Processes:
Generated by solar energy, aided by gravity.
Mutual Influences:
Internal and external processes interact and influence each other. Examples:
- The uplift of a mountain range activates erosion processes, as erosion is more intense in upland areas than in lowlands.
- The displacement of a continent that approaches or moves away from the equator causes climate change.
- Intense volcanic activity changes the composition and dynamics of the atmosphere.
The interaction between internal and external geological processes shapes the relief. The landscape in each zone is the result of these interactions.