Plate Tectonics: A Comprehensive Guide

Posted on Nov 6, 2024 in Geology

Plate Tectonics

The Dorsal and Oceanic Crust

The dorsal is an undersea rise, 2 to 3 km above the abyssal plain, periodically interrupted by transform faults. It features a central groove called a rift. All rocks on the ocean floor are younger than 185 million years old. The Earth is 4.5 billion years old and continents started developing around 3.8 billion years ago.

Three important data highlights about the dorsal:

  • Rocks Present: Basalts younger than a million years old are present.
  • Aging Oceanic Crust: The age of basaltic rock increases as we move away from the ridge, creating an almost symmetrical distribution.
  • Sediment Power: The thickness of sediments is related to the age of the seafloor.

Distribution of Volcanoes and Earthquakes

Volcanoes and earthquakes are internal geological processes driven by thermal energy originating inside the Earth. Mapping their distribution reveals that:

  • Uneven Distribution: Volcanoes and earthquakes are not regularly distributed.
  • Concentrated Activity: Many areas show a concentration of seismic and volcanic activity.
  • Overlapping Activity: There are numerous places where seismic and volcanic activity overlap.

Initial Conclusions

  • Geologically Active and Stable Areas: Some areas are very geologically active, while others are very stable.
  • Movement of Material: There are places where large masses of material must move with respect to others.

Lithospheric Plate Boundaries

A plate boundary is the border between one lithospheric plate and another. There are three types:

  • Oceanic Ridges: Boundaries where new oceanic lithosphere is generated.
  • Subduction Zones: Boundaries where lithosphere is destroyed.
  • Transform Faults: Boundaries where plates slide past each other laterally, neither creating nor destroying lithosphere.

The edges of the plates define seven major plates: Eurasian, African, Indo-Australian, Pacific, North American, South American, and Antarctic. There are also smaller plates like the Nazca, Caribbean, and Arabian plates.

Convection Currents

Convection currents occur when a fluid expands upon heating, loses density, and rises. In cooler areas, it cools, becomes denser, and descends.

Moving Plates

Plate movement is driven by thermal energy from the Earth’s interior and aided by gravity.

How to Divide a Continent

Columns of hot material (plumes) originate in the deep mantle. Their lower density allows them to rise to the base of the lithosphere.

Ocean Formation Process

  1. Lithosphere Rises: The ancient lithosphere rises due to pressure from the plumes.
  2. Rift Formation: Tension creates fractures in the upper part, forming a ridge with a central valley (rift).
  3. Oceanic Lithosphere Formation: Magma flows through the rift fractures and solidifies, widening the valley. Continental blocks separate, and oceanic lithosphere begins to form, creating a narrow sea.
  4. Ocean Formation: The process continues, with new lithosphere forming and widening the ocean.

Plate Tectonics Summary

Plate tectonics explains the movements of oceans and continents, as well as the origin of volcanoes, earthquakes, and mountain ranges. Its main ideas are:

  • The lithosphere is divided into rigid pieces called plates.
  • Plate edges are constantly changing and can be of three types (ridges, subduction zones, and transform faults).
  • Oceanic lithosphere is continuously renewed, while continental lithosphere is permanent.
  • Plates move over the plastic asthenosphere (upper mantle).
  • Plate movement is driven by thermal and gravitational energy.
  • The Earth’s landmasses have changed not only in position but also in shape, size, and the number of plates.

Rock Deformation

Deformation refers to changes in the position, shape, or volume of rocks subjected to stress. There are three types of deformation:

  • Elastic: The rock deforms but returns to its original shape when the stress is removed.
  • Ductile: The rock deforms permanently but does not break.
  • Brittle: The rock breaks under stress.

Elements of a Fold

  • Axial Plane: The plane that divides the fold into two halves.
  • Hinge: The zone of maximum curvature in a fold.
  • Fold Axis: The intersection of the axial plane and the hinge.
  • Flanks: The sides of the fold on either side of the hinge.
  • Core: The innermost zone of the fold.

Fold Types

  • Anticline: A fold with older rocks in the core.
  • Syncline: A fold with younger rocks in the core.
  • Upright: A fold with a vertical axial plane (90°).
  • Inclined: A fold with an axial plane between 10° and 85° from vertical.
  • Recumbent: A fold with an axial plane less than 10° from horizontal.
  • Overturned: A fold with an axial plane rotated more than 90°.
  • Symmetrical: A fold where the axial plane divides the fold into two symmetrical halves.
  • Asymmetrical: A fold where the axial plane divides the fold into two non-symmetrical halves.

Faults and Diaclases

Diaclases: Fractures in rocks where the blocks do not move significantly. They are often grouped in systems.

Faults: Fractures in rocks where there has been a displacement of one block relative to another.

  • Fault Plane: The surface of the fracture.
  • Fault Blocks: The two blocks separated by the fault.
  • Fault Displacement: The measurement of the movement along the fault.
  • Fault Orientation: The strike and dip of the fault plane.

Fault Associations

  • Graben: A down-dropped block bounded by parallel faults.
  • Horst: An uplifted block bounded by parallel faults.

Orogeny: Mountain Building

An orogen is a mountain belt consisting of folded and fractured igneous, metamorphic, and sedimentary rocks.

Formation of a Mountain Chain

  1. Accretionary Prism Formation: Sediments carried by the oceanic lithosphere are scraped off and accumulate at the subduction zone, forming a folded and fractured accretionary prism.
  2. Magmatism and Metamorphism: Water and heat generated in the subduction zone cause partial melting of rocks. Magmas rise, some reaching the surface as volcanoes, others solidifying within the crust, contributing to its thickening. High pressures and temperatures cause metamorphism of some rocks.
  3. Orogenic Uplift: The thickening of the continental crust and isostatic uplift cause the mountain belt to rise.

Continental Collision

  1. Oceanic Subduction: Oceanic lithosphere subducts, and sediments carried by it accumulate, fold, and fracture.
  2. Oceanic Basin Closure: Eventually, the entire oceanic lithosphere is consumed, and the continents on either side of the subduction zone converge. The continental crust, being less dense, resists subduction. The thicker the continental crust, the more difficult it is to subduct.
  3. Continental Collision: The two continents collide, causing compression, folding, fracturing, and uplift of the rocks. If the process continues, one continent’s crust may be thrust over the other, thickening the continental crust in that area and leading to significant isostatic uplift.