Causes and Types of Plate Tectonics, Mountain Formation and Folding

Causes of Lithospheric Plate Movement

Lithospheric plates move due to the internal heat activity of the Earth. The large amount of thermal energy built up in the planet's interior produces convective movements.

Convection currents in the interior of the fluid mantle consist of ascending and descending flows. The upper part of the fluid, which is less dense, tends to rise, while the lower part, which is denser, tends to sink.

The manifestations of convection caused by convective movements include: terrestrial magnetism, continental drift, volcanism, seismicity, and the segregation of materials by density.

How Lithospheric Plates Move

Lithospheric plates move through three mechanisms:

1. Convection currents: The rock of the mantle is solid but very plastic. The heated rock near the core loses density as it approaches the crust, cools, becomes denser, and sinks again. These cycles of mantle rock form convection currents that move the lithospheric plates like conveyor belts.
2. Gravitational displacement: Plates slide "downhill" due to the height difference between the dorsal ridge and the trench.
3. Slab pull: The subducting plate "pulls" or drags the rest of the plate down.

Thermal columns are rocky masses of hot, less dense material that rise from the base of the mantle, where they originate intense hotspot volcanism.

Solifluction is the behavior in which solid material can flow slowly like a very viscous liquid, a characteristic of the mantle.

Formation of Mountain Ranges and Orogenies

Mountain ranges or orogenies are characterized by sedimentary rocks of marine origin, folds, and fractures in the strata. They also contain volcanoes, batholiths, and plutons.

There are two types of orogenies:

Orogenies of Continental Active Margins

These form when the oceanic lithosphere subducts beneath the continental lithosphere.

1. Initially, the continental edge is a passive margin where sediments accumulate, sometimes exceeding 11,000 m in thickness due to continental erosion.
2. Subduction begins, and convergent forces cause the movement of the plates, which fold and fracture the sediments.
3. Oceanic lithosphere is transported into the trench and is either adhered to the continent or sinks beneath it. As the subducting oceanic plate partially melts, it generates magma that tends to rise to the surface, forming volcanoes and batholiths. Sediments accumulate in the deformed margin, and the convergent forces thicken the continental crust.

The folding of materials caused by the intrusion of batholiths in the cordillera results in thickening.

Collisional Orogenies

Collisional orogenies originate from the collision of two continents. They are caused when the first plate subducts beneath the first continental plate, progressively consuming the oceanic basin until the two continents collide. The sediments of the basin are folded and fractured, forming part of the emerging cordillera.

Deformation of Materials: Folds and Fractures

According to the original horizontal principle, sedimentary strata are deposited in horizontal layers. On the other hand, materials offer resistance: rigid materials, which are brittle, break rather than deform, while plastic materials deform easily. Thus, the surface rocks of the Earth are brittle and fragile, but at a certain depth, where temperature and pressure increase, rocks behave like plastic materials that deform readily under stress.

Types of Stress

• Compression: Two forces aligned and convergent act on the material.
• Tension: Two divergent forces aligned act on the material, producing stretching.
• Shear: Two unaligned and convergent forces act on the material.

Plastic Deformation: Folds

A fold is a set of undulating strata that were originally flat and horizontal but became curved as a result of plastic behavior.

Elements of a fold:

• Limb: The lateral part of a fold.
• Hinge: The zone of maximum curvature.
• Hinge line: The line joining the points of maximum curvature.
• Axial plane: The plane that passes through each of the hinges of the strata of the fold.
• Core: The central part of the fold.

Types of Folds

According to the age of the core material:

• Anticline: The core contains the oldest strata.
• Syncline: The core contains the most recent layers.

According to the dip of the axial plane:

• Upright fold: A fold whose axial plane dip is greater than 80° and less than 100°.
• Inclined fold: A fold whose axial plane dips between 10° and 80°.