Oceanic Lithosphere & Plate Tectonics Dynamics

Characteristics of Oceanic Lithosphere

• Little Antiquity: Oceanic lithosphere is about 200 million years old.
• Ridges Are Formed by Recent Basalt Rocks and Have Transform Faults: Oceanic lithosphere is older as it moves away from the ridge.
• There is more sediment in areas far from the ridge.

Dynamics of the Oceanic Floor

Oceanic lithosphere is created in some areas and is destroyed in others, so Earth's size doesn't increase.

Creation of Oceanic Lithosphere

• The oceanic lithosphere is created in ridges extending to both sides.
• Ridges represent the birth of a new oceanic lithosphere.

Destruction of Oceanic Lithosphere

• The oceanic lithosphere is destroyed in the subduction zones, disappearing towards the mantle.
• Subduction zones compensate for the creation of oceanic lithosphere in ridges.

Plate Boundaries

Lithospheric plates are separated by plate boundaries which can be grouped into three types:

• Ocean Ridges: Constructive borders.
• Subduction Zones: Destructive edges.
• Transform Faults: Passive edges.

Lithospheric Dynamics

The lithospheric plates move on the ductile materials of the upper mantle. The engines of displacement are thermal energy and gravity:

• Thermal Energy: Produces convection movements in the mantle.
• Gravitational Energy: Activates the movement of the plates in the ridges (exit and retreat) and in the subduction zones (pull and sink).

The oceanic lithosphere is constantly renewed whereas the continental lithosphere is more permanent. Throughout the history of Earth, the position of the plates, their shape, their size, and the number of them have changed.

Movements of the Lithosphere

The movements of the lithosphere can be explained by different models.

Wilson Cycle

Set of stages of plate tectonics grouped in tension processes (formation of oceans) and compression processes (formation of orogens). The main stages are:

1. Initial Pre-drift Extension: Thinning of the continental lithosphere.
2. Rift-to-drift Phase: Opening of an oceanic basin and formation of an obduction zone.
3. Seafloor Spreading: The distance increases between the continental borders.
4. Widening of the Basin: A new ocean is formed.
5. Subduction of Oceanic Lithosphere: Closure of the basin by a subduction zone.
6. Continent-continent Collision: Disappearance of the ocean by an obduction zone.

History of Earth's Continents

Pangaea supercontinent formed 250 million years ago. It began to divide, forming new continents that continued existing until today.

Opening of an Ocean

It is divided by the opening of an ocean:

The lithosphere, the outermost and rigid layer of Earth, acts as several plates:

• Large Plates: Eurasian, African, Indo-Australian, Pacific, North American, South American, and Antarctic.
• Medium Size Plates: Nazca, Caribbean, Cocos, Arabian, Philippine.

The great African Rift Valley is experiencing right now the opening of an ocean:

• Great Dome of Kenya: Large thermal bulge.
• Ethiopian Rift: Formation of a valley and oceanic crust.
• Red Sea: Narrow sea.

Stages of Ocean Opening

1. Formation of a Thermal Dome:
   • Generation of a thermal plume (rising magma) in the upper mantle.
   • Lithosphere in contact rises and arcs.
2. Formation of a Continental Rift:
   • Stresses cause failures (fractures) and central area collapses.
   • A valley or rift forms bounded by ridges.
3. Creation of Oceanic Lithosphere:
   • Magma comes out and solidifies.
   • The valley widens and the continental blocks separate.
   • A narrow sea forms.
4. Development of an Ocean:
   • If the process of extension continues, a growing ocean is created.

Orogenesis

It is the folding ridge (orogens or mountains) formation process. Formed by:

• Accretionary wedge.
• Magmatism.
• Overlapping of two continental crusts.

Can be accretionary or collisional.

Accretionary Orogenesis

Formed in subduction zones with oceanic lithosphere under continental lithosphere. Stages:

1. Formation of an accretionary wedge: in the subduction zone.
2. Magmatism and metamorphism: formation of magma, magmatic rocks, and metamorphic rocks. Ex: Andes.

Collisional Orogenesis

Formed in subduction zones with continental lithosphere colliding with another continental lithosphere. Stages:

1. Oceanic subduction: disappearance of the oceanic basin.
2. Closure of the ocean basin and continental collision: by an obduction process. Ex: Himalayas and Alps.

Stresses and Deformations

Stresses are directed pressures that tend to deform rocks. Main types are:

• Compression: Convergence.
• Tension: Divergence.
• Shear: Parallel directions and opposite directions.

Deformations: changes in the position, shape, or volume in rocks. Types:

• Elastic: Material recovers its shape when effort ceases.
• Plastic: Material doesn’t recover when effort ceases.
• Rupture: Material fractures due to stress being too great.

Rocks can behave differently according to the environmental conditions:

• Rocks fracture under surface temperature and humidity conditions.
• At depth, temperature and water favor the plastic behavior of the rocks.

Folds and Faults

Ripples of rocks subjected to compressive stresses. There is a plastic behavior.

Types of Folds

• According to the age of the materials: anticline, syncline.
• According to the position of the axial plane: right, inclined, tipped over, lying down, inverted.
• According to symmetry: symmetrical, asymmetrical.

Fractures

Breakage of rocks subjected to compression and strain stresses. They can be joints or faults.

• Joints: Non-displaced fractures in the blocks. Formed at the same time as the rocks or later.
• Faults: Fractures with displacement of one block relative to another.

Types of Faults

• Normal, reverse, strike-slip fault.

Fault Associations

• Graben or rift valley
• Horst or tectonic pillar