Plate Tectonics and the Forces Shaping Earth's Crust

Verticalist Theories and Fixism

Verticalist theories propose that movement in terrestrial materials is primarily vertical. These are considered fixist theories.

Key Aspects of Verticalist Theories

• Movement in terrestrial materials is vertical.
• They are fixist theories.
• Some related phenomena occur in the Earth's crust, such as Eustatic changes and Isostasy.

Understanding Eustatic Changes

Eustatic changes refer to global changes in sea level of greater amplitude than normal tides.

Causes of Sea Level Variation

These variations are driven by two main factors:

1. Variation in the volume of water in the oceans:
   • Glaciations: When ice sheets form, water is locked up, causing sea level to fall.
   • Thermal Contraction or Dilation: Changes in ocean temperature cause water volume to vary (thermal expansion/contraction).
2. Variation in the shape of ocean basins.

Horizontalist Theories and Mobilism

Horizontalist theories propose that movement in terrestrial materials is primarily horizontal. These are known as mobilistic theories.

Main Horizontalist Concepts

• Movement in terrestrial materials is horizontal.
• They are mobilistic theories.
• Two primary theories: Continental Drift and Plate Tectonics.

Evidence Supporting Continental Drift

The Theory of Continental Drift is supported by several key lines of evidence:

1. Geographical Arguments: The shapes of the continents fit together in certain areas, especially when considering the boundaries of the continental shelf.
2. Paleoclimatic Arguments: The distribution of ancient climates does not correspond to the current configuration, suggesting that either climatic zones or the continents themselves have moved.
3. Geological Arguments: Geological formations on opposing sides of the Atlantic Ocean show continuity.
4. Paleontological Arguments: Identical fossils of the same organisms are found on continents that are now widely separated.

Dynamics of the Ocean Floor

Creation and Destruction of Oceanic Crust

• Creation of Oceanic Lithosphere:

  The oceanic lithosphere is created at mid-ocean ridges and spreads outwards to both sides. These ridges represent the birth sites of new oceanic crust.

• Destruction of Oceanic Lithosphere:

  The oceanic lithosphere is destroyed in subduction zones, where it sinks back into the mantle. Subduction zones effectively compensate for the creation of new lithosphere at the ridges, maintaining Earth's surface area.

Principles of Lithospheric Dynamics

Lithospheric plates move across the ductile materials of the upper mantle. This movement results in constant geological change over Earth's history.

Key Characteristics of Lithospheric Plates

• The lithospheric plates move on the ductile materials of the upper mantle.
• The oceanic lithosphere is constantly renewed.
• The continental lithosphere is more permanent.
• Throughout Earth's history, the position, shape, size, and number of lithospheric plates have continuously changed.

Engines Driving Plate Movement

The primary forces driving plate displacement are thermal energy and gravity:

• Thermal Energy: Generates convection movements within the mantle, providing the underlying force for plate motion.
• Gravitational Energy: Activates plate movement in specific zones:
  • At ridges (ridge push/exit and retreat).
  • In subduction zones (slab pull/pull and sink).