Earth’s Structure and Plate Tectonics: A Comprehensive View

Posted on Dec 10, 2024 in Geology

Earth’s Internal Structure: Geochemical and Geodynamic Models

Geochemical Model

This model categorizes Earth’s layers based on their chemical composition:

  • Crust:
    • Continental: Heterogeneous, with granite being abundant.
    • Oceanic: Homogeneous, mainly composed of basalt.
  • Mantle: Composed of igneous rocks rich in iron and magnesium silicates, primarily peridotite. The mantle is traditionally divided into:
    • Upper Mantle
    • Transition Zone (denser materials due to higher temperature and pressure)
    • Lower Mantle

    Note: These layers do not have significant differences in chemical composition; the differences are in their state and behavior.

  • Core: Almost pure iron mixed with iron sulfides and nickel. The core is divided into:
    • Outer Core
    • Inner Core

    Note: Similar to the mantle, the core layers do not have significant differences in chemical composition but differ in state and behavior.

Geodynamic Model

This model classifies Earth’s layers based on their physical states (plasticity-rigidity/density) and mechanical properties (response to variations in temperature or pressure):

  • Lithosphere: Includes the crust and the upper part of the upper mantle. It is rigid (solid) and fractured into large blocks called lithospheric plates. These plates exhibit:
    • Horizontal movements, dragged by the mantle underneath.
    • Vertical movements due to isostatic adjustment.
  • Asthenosphere (or Sublithospheric Upper Mantle): Includes the rest of the upper mantle and the transition zone (between the lithosphere and mesosphere). It is malleable, plastic, and has a tendency to flow as the lithosphere moves.
  • Mesosphere: The lower mantle, extending from approximately 670 km deep to the D” layer. It is semi-solid and flows slowly (cm/year). It is not static, as cold lithospheric plates from subduction zones descend into it, and mantle plumes can enter it from the D” layer underneath.
  • D” Layer: One of the most dynamic layers of the Earth. It is liquid due to the heat that accumulates from the outer core. Hot magma escapes from this layer, creating mantle plumes.
  • Mantle Plumes: Break through the mantle and the lithosphere, creating hotspots at the Earth’s surface.
  • Hotspots: Areas of intense volcanic activity (e.g., the Hawaiian Islands).
  • Outer Core: Liquid due to the heat of the inner core. It flows in violent convection currents that generate the Earth’s magnetic field.
  • Magnetic Field: Invisible dynamic lines of force that cross the Earth between the two magnetic poles.
  • Inner Core: Solid due to the pressure of the overlying materials. The center is 6378 km deep, with a temperature higher than 6000ºC. It produces geothermal energy.

Internal Dynamics

In the mantle, hot magma plumes from the D” layer generate ascending currents. Gravity acting on the lithospheric plates generates descending currents. Layers with different densities, like the magnetic core and the rocky mantle, cannot mix. As a result, independent convection currents are generated.

Geothermal Gradient

The internal temperature of the Earth increases with depth.

Heat Flow

The amount of heat energy that reaches the surface. Heat travels from the hot interior of the Earth to the surface, but the transmission is very slow.

Convection Current

Hot materials, which are less dense, ascend to the surface. As they cool, these materials become denser and sink again. This continuous flow is generated by high-temperature variations between the lithosphere and the D” layer.

Plate Tectonics

Plate tectonics is a scientific theory that describes the large-scale motions of Earth’s lithosphere. The model builds on the concepts of continental drift, developed during the first few decades of the 20th century by Alfred Wegener. The geoscientific community accepted the theory after the concepts of seafloor spreading were developed in the late 1950s and early 1960s.

Evidence of Plate Tectonics

  • Maps of the Seafloor: Show mid-ocean ridges, oceanic trenches, and large underwater faults.
  • Direct Measurements: Demonstrate that the plates are moving and indicate the direction of movement.
  • Sediment Deposits: Thicker at continental margins, almost non-existent at mid-ocean ridges. The newest crust is found at the center of mid-ocean ridges (age increases with distance from the ridge towards continents).

Principles of Plate Tectonics

  • The lithosphere is broken up into tectonic plates (seven or eight major plates and many minor plates).
  • Where plates meet at plate tectonic boundaries, there is intensive seismic (earthquake) and volcanic activity.
  • At these boundaries, relief is created (mountain-building, emerged or submerged, and oceanic trench formation).

Mantle Convection Currents

These are slow motions of Earth’s semi-solid mantle caused by convection currents carrying heat from the interior of the Earth to the surface.

Plate Tectonics and Mantle Convection

The lithosphere (made up of the crust and the solid upper part of the upper mantle) moves over the rest of the upper mantle. The lithosphere is divided into a number of lithospheric plates that are continuously being created and destroyed at opposite plate boundaries (ridges and trenches). Accretion occurs as mantle is added to the growing edges of a plate (seafloor spreading). This hot added material cools down by conduction and convection of heat.

Mid-Ocean Ridges

At the destruction edges of the plate, the material has thermally contracted to become denser, and it sinks under its own weight in the process of subduction, usually at an ocean trench.

Subduction Zones or Ocean Trenches

The relative motion of tectonic plates determines the type of boundary: convergent, divergent, or transform.

  • Divergent Boundaries: Where two plates are moving apart. The space created can fill with new material sourced from molten magma that forms below; the lithosphere is created. Divergent boundaries can form within continents but will eventually open up and become ocean basins.
    • On land: Divergent boundaries within continents initially produce rifts, which produce rift valleys.
    • Under the sea: The most active divergent plate boundaries are between oceanic plates and are often called mid-oceanic ridges.
  • Transform Boundaries: Where plates slide past each other. The relative motion of the plates is horizontal. They can occur underwater or on land, and the crust is neither destroyed nor created. Because of friction, stress builds up in both plates, and when it exceeds the threshold of the rocks, the energy is released, causing earthquakes.
  • Convergent Boundaries: Where two plates are colliding.
    • Subduction zones occur when one or both of the tectonic plates are composed of oceanic crust. The denser plate is subducted underneath the less dense plate. The plate being forced under is melted and destroyed.
    • Where oceanic crust meets ocean crust, island arcs and oceanic trenches occur when both of the plates are made of oceanic crust. These are often associated with submarine volcanoes.
    • Where oceanic crust meets continental crust, the denser oceanic plate is subducted, often forming a mountain range on the continent. The Andes is an example of this type of collision.
    • Obduction zones occur where continental crust meets continental crust. Both continental crusts are too light to subduct, so a continent-continent collision occurs, creating especially large mountain ranges. The most spectacular example of this is the Himalayas.