Plate Boundaries

Thermal Convection and Plate Tectonics

In the mantle, hot spots of semi-molten magma rise towards the lithosphere.

When this magma reaches the lithosphere, it spreads out horizontally, causing plates to diverge.

As the magma cools near the lithosphere, it becomes denser and sinks back into the mantle, forming a convection current.

These currents drive the movement of tectonic plates, leading to sea-floor spreading, continental drift, and geological activities such as earthquakes and volcanic eruptions.

This cyclical process, where heated magma rises, cools, spreads, and sinks, underpins the theory of plate tectonics.

Plate Boundary: Zone where plates collide, separate, or slide past one another.

• Converge: plates collide.

• Diverge: plates separate.

• Transform: plates slide past one another. There are three types of plate boundaries:

• Divergent: boundaries of construction. The plates are separating.

• Convergent: boundaries of destruction. The plates are colliding. There are three types:

  • Oceanic-Oceanic boundaries
  • Continental-Continental boundaries
  • Oceanic-Continental boundaries.

• Conservative: passive boundaries. The plates slide past each other.

  

Convergent Plate Boundaries

Three types:

• Oceanic-Oceanic
• Oceanic-Continental
• Continental-Continental A destructive plate boundary occurs where two plates converge (collide). Part of the Earth's crust is destroyed as a result.

Oceanic-Oceanic Boundaries

Two Oceanic Plates Collide

Example: Pacific Plate collides with Eurasian Plate

The process that occurs at these boundaries is called subduction. Landforms formed include Ocean Trenches and Island Arcs. An example of where this occurs is off the coast of Japan.

• The heavier, denser oceanic plate subducts into the mantle. (subducted beneath the lighter plate)
• Convection currents drag the plate downwards. An arc-shaped ocean trench may form at the subduction zone (e.g., Japan Trench).
• As the crust sinks, it releases fluids (water) and gases (carbon dioxide and sulfur dioxide).
• Temperatures increase rapidly and the sinking plate melts.
• Silica and gas-rich magma forms from the destroyed crust.
• The new magma rises and burns a path through the overlying, lighter oceanic crust.
• An explosive volcanic eruption occurs on the sea floor when the magma reaches the Earth's surface.
• Continued volcanic activity forms a chain of volcanic islands, known as an island arc, parallel to the oceanic trench (e.g., the Japanese islands).

Oceanic-Continental Boundaries

An Oceanic Plate Collides with a Continental Plate

Example: The Nazca Plate collides with the South American Plate

Subduction occurs at these boundaries as well. Landforms formed include Fold Mountains, Volcanoes and Ocean Trenches. The Andes Mountains were formed at an Oceanic-Continental plate boundary.

• The heavier, denser oceanic plate subducts under the lighter continental plate.

• Convection currents drag the oceanic plate downwards. A deep arc-shaped ocean trench forms at the subduction zone.

• The subducting crust melts, creating hot silica and gas-rich magma.

• This less dense magma rises and burns a path to the surface of the continental plate.

• A volcanic eruption occurs when the magma reaches the surface.

• Over time, a chain of volcanic mountains forms parallel to the subduction zone (e.g., Andes volcanic arc). Also:

• Stress builds at the continental plate boundary as the plates collide.

• This stress causes rocks at the edge of the continental plate to buckle upwards.

• A range of fold mountains forms, such as the Andes in South America.

Continental-Continental Boundaries

Two Continental Plates Collide

Example: The Eurasian Plate collides with the African Plate

Fold Mountains, such as The Alps, are the main feature formed at these boundaries.

• Little subduction occurs due to the similar age, weight, and density of both plates.
• The plates buckle upwards, creating fold mountain ranges like the Alps.
• Magma intrudes into gaps between the folded rock layers.
• This magma cools slowly, forming new rock beneath the Earth's surface.

Divergent Plate Boundaries

A divergent plate boundary, also known as a constructive plate boundary, occurs where two tectonic plates move apart from each other. This movement creates new Earth's crust as a result of the divergence. Sea-floor spreading is the main process at these boundaries.

The Mid-Atlantic Ridge has formed at a boundary where the oceanic plate margins of the North and South American plates are slowly separating from the Eurasian and African plates respectively. This is what happens:

• Convection Currents, Tension and Cracks Convection currents in the Earth's mantle pull the plates in opposite directions, creating tension (stretching) on the Earth's crust. This tension is so great that the crust tears apart, forming numerous cracks called fault lines and fissures. Earthquakes are common as these faults and fissures develop.

• Magma Emergence and New Crust Formation Magma from the Earth's mantle rises through these fissures. As it reaches the surface, it helps to push the plates further apart. The magma cools and hardens quickly, forming new oceanic crust. This continuous process of magma emergence and hardening is known as sea-floor spreading.

• Mid-Ocean Ridge Formation The continuous rise of magma builds up a mid-ocean ridge called the Mid-Atlantic Ridge. While most of this ridge remains below sea-level, some parts rise high enough to form islands, such as Iceland.

• Volcanic Mountains: In some areas, magma emerges more violently through individual pipes, forming volcanic mountains. Most of these mountains are submerged beneath the Atlantic Ocean.

Transform Plate Boundaries

Conservative plate boundaries occur where two plates slide past each other. Crust is neither created nor destroyed. Seismic activity is the main process present at these boundaries.

• Fault Lines: The movement of plates past each other creates large fissures or fault lines, known as transverse or tear faults.
  • San Andreas Fault in California, USA.
  • 1,300-km-long fault.
  • Marks the boundary between the Pacific Plate and the North American Plate.
• Earthquakes:
  • The plates at the San Andreas Fault are moving in the same direction (northwesterly).
  • However, they move at different speeds. (Pacific Plate moves about six times faster than the North American Plate.)
  • Slippage is not smooth. Friction between the moving plates causes massive pressure to build up.
  • When the pressure is released, the plates lurch past each other, releasing shock waves that cause tremors or earthquakes.