Earth’s Internal Dynamics and Plate Tectonics

Posted on Nov 26, 2024 in Geology

1. Earth’s Internal Dynamics

Earth’s internal activity stems from the vast thermal energy stored within its interior, driving convective currents.

Convection Within Earth

Convection involves updrafts and downdrafts within a fluid, occurring when the top layer becomes denser and sinks, while the less dense bottom layer rises. As hot fluid expands, its density decreases, leading to updrafts. Conversely, colder, denser fluid sinks, creating downdrafts. This process occurs in two distinct layers: the metallic core and the rocky mantle. The denser metallic core and the rocky mantle do not mix, though both experience convection.

2. Manifestations of Convection

Convection manifests in various ways: terrestrial magnetism, continental drift, volcanism, seismicity, and material density segregation.

3. Convection and Mantle Relief

Thermal Plumes and Hot Spots

Thermal plumes are columns of hot rock rising from the mantle base to the surface, creating areas of intense volcanism known as hot spots.

Solifluction describes the slow, viscous flow of seemingly solid material.

Volcanic Archipelagos

These form when a thermal plume lies beneath oceanic lithosphere, resulting in volcanic activity.

Downwelling Convective Flows

Thermal plumes affect the African lithosphere through two processes: heat conduction and material expansion.

4. Subduction

Subduction involves the descent of oceanic lithosphere, forming convective currents.

Characteristics of Subduction Zones

  1. Occur in oceans, as an oceanic plate always subducts.
  2. The overriding plate can be oceanic or continental.
  3. The subducting plate bends, forming a deep, elongated trench.
  4. Thick sediment accumulates in the oceanic trench.
  5. Compressed sediments against the overriding plate can form an accretionary prism.
  6. High seismicity zones due to friction. Earthquakes follow an inclined plane called the Benioff zone.
  7. The subducting plate undergoes partial melting, generating magma that rises to the overriding plate, causing volcanism and plutonic intrusions.
  8. The subducting plate’s thrust compresses the overriding plate, increasing thickness and creating a volcanic orogen.

Subduction zone interactions produce seismicity, volcanism, and orogen formation.

5. Orogen Formation

Orogens are mountain ranges linked to subduction zones and plate movements. Four types exist: island arcs, thermal orogens, collisional orogens, and intraplate orogens.

6. Tectonics and Rock Deformation

Stress creates three types of rock deformation:

  • Elastic deformation: Reversible deformation where rocks momentarily deform under stress but return to their original shape. This causes seismic waves.
  • Plastic deformation: Irreversible rock folding under intense compression over millions of years.
  • Brittle deformation: Rock fracturing under stress exceeding elastic or plastic limits.

Folds: Geometric Elements

Folds are plastic deformations. Key elements include:

  • Hinge: Area of maximum curvature.
  • Limbs: Sides of the hinge.
  • Core: Central part of the fold.
  • Axial plane: Imaginary plane bisecting the hinge, separating the limbs.

Fold Types

  • Anticlines: Younger layers wrap around older ones. In vertical axial planes, limbs point down, and the hinge points up. The core contains the oldest layers.
  • Synclines: Older layers wrap around younger ones. In vertical axial planes, limbs point up, and the hinge points down. The core contains the youngest layers.

Brittle Deformation: Joints

Joints are rock fractures without displacement. Causes include:

  • Shrinkage cracks: Drying clay and mud.
  • Frost wedging: Water freezing in cracks.
  • Columnar jointing: Cooling lava.
  • Exfoliation: Rocks formed under high pressure expand and fracture at the surface.

Faults: Geometric Elements and Types

Faults are fractures with block displacement. The fracture plane is the fault plane. Types include:

  • Normal faults: Caused by tensional stress.
  • Reverse faults: Caused by compressional stress.
  • Strike-slip faults: Caused by shear stress.

Overthrust Faults

These are low-angle reverse faults where the upper block (allochthon) has moved horizontally over the lower block.