Welcome to Plate Tectonics!
This chapter is fundamental to understanding the marine environment. The massive, slow movement of the Earth's surface directly controls the shape of the seabed, the depth of the ocean, and the location of major natural hazards like volcanoes and tsunamis.
Don't worry if this seems like geology at first. We will focus purely on how these movements affect the oceans and coastal areas!
1. The Earth's Crust and Tectonic Plates (Syllabus 1.2.1)
The outer layer of the Earth, the crust, is not a single solid shell. Instead, it is broken up into several massive pieces called tectonic plates.
Analogy: Think of the Earth's crust as the thin, hard shell of a boiled egg that has been cracked into many segments. These segments are the plates.
Key Structures Involved
- Crust: The solid, outermost layer of the Earth. Tectonic plates are sections of the crust.
- Mantle: The layer immediately beneath the crust. Although made of solid rock, the high temperature means this rock is soft and can flow very slowly—like an extremely thick, viscous liquid (sometimes called magma when it melts).
2. The Engine of Movement: Convection Currents (Syllabus 1.2.2)
Why do these massive plates move? They are floating on the mantle, and the heat deep inside the Earth causes the mantle material to circulate.
Understanding Convection Currents
- Heat from the Earth's core warms the rock deep within the mantle.
- The heated, less dense rock rises towards the crust.
- It spreads out just below the crust, dragging the tectonic plates along with it.
- As the rock cools near the surface, it becomes denser and sinks back down towards the core.
This circular motion is called a convection current, and it is the force that slowly moves the tectonic plates across the Earth's surface.
Key Takeaway: Tectonic plates float on the mantle and move because they are dragged by convection currents (cycles of rising hot rock and sinking cool rock).
3. Continental Drift: From Supercontinent to Today (Syllabus 1.2.3)
The oceans and continents we see now are the result of plate movement happening over hundreds of millions of years.
- Scientists believe that long ago, all the continents were joined together in one huge landmass called a supercontinent (Pangaea is the most famous example).
- The continuous movement of the tectonic plates caused this supercontinent to break up.
- The pieces (our current continents) slowly drifted to their present positions, forming the current arrangement of the world's oceans and seas.
Did you know? The speed at which plates move is similar to the speed your fingernails grow—only a few centimetres per year!
4. Types of Plate Boundaries (Syllabus 1.2.4)
When plates interact, they form three main types of boundaries, based on how they are moving relative to each other. The type of boundary determines the geological features present.
4a. Divergent Boundaries (Dividing)
At a divergent boundary, two plates are moving apart or pulling away from each other.
- Movement: Moving apart.
- Result: Magma rises to fill the space, cools, and forms new ocean crust, often creating underwater mountains called mid-ocean ridges.
Memory Aid: Divergent means Dividing.
4b. Convergent Boundaries (Colliding)
At a convergent boundary, two plates are moving towards each other and colliding.
- Movement: Pushing together.
- Result: Often, one plate (usually the denser oceanic plate) sinks beneath the other plate in a process called subduction. This creates deep underwater valleys known as ocean trenches.
Memory Aid: Convergent means Colliding or Coming together.
4c. Transform Boundaries (Sliding)
At a transform boundary, two plates are sliding or grinding past each other horizontally.
- Movement: Sliding alongside each other.
- Result: The plates move unevenly, building up huge stress that is released as strong earthquakes.
Quick Review: Boundary Movements
| Boundary Type | Action | Geological Feature |
|---|---|---|
| Divergent | Plates move apart | Mid-ocean ridge (new crust formed) |
| Convergent | Plates collide | Ocean trench (crust destroyed) |
| Transform | Plates slide horizontally | Earthquakes / Fault lines |
5. Earthquakes and Volcanoes Caused by Plate Movement (Syllabus 1.2.5)
Earthquakes
Earthquakes occur when the stress built up at plate boundaries (especially convergent and transform) overcomes the friction holding the plates together, causing a sudden slip.
Volcanoes
Volcanic activity is usually found at divergent and convergent boundaries:
- Convergent zones: As the subducting plate sinks into the mantle, it melts, forming magma. This magma rises and forces its way through the overlying plate, creating volcanoes.
- Divergent zones: Magma rises directly from the mantle to fill the gap between the separating plates, creating new oceanic crust and volcanic ridges.
6. Tsunami Formation and Effects (Syllabus 1.2.6 & 1.2.7)
A tsunami is a marine hazard that is directly caused by plate tectonics, specifically by rapid vertical movement of the seafloor.
How a Tsunami is Formed (Step-by-Step)
Tsunamis are most often formed by powerful underwater earthquakes that occur at convergent boundaries.
- The two tectonic plates at a convergent boundary lock together, building up massive tension.
- When they suddenly snap, the seafloor is rapidly forced up (or drops down).
- This sudden vertical movement displaces (pushes up) the entire column of water above it.
- This displacement generates a series of huge, fast-moving waves—the tsunami.
Remember: The earthquake must involve vertical displacement of the seafloor to cause a tsunami. Sliding plates (transform boundaries) usually don't cause tsunamis because the movement is horizontal.
The Effects of Tsunamis
When a tsunami reaches the coast, the wave slows down and increases dramatically in height, causing immense damage.
Effects on Marine Ecosystems
- Habitat Destruction: The powerful currents and impact can flatten structured habitats like coral reefs, smashing delicate corals and exposing the reef base.
- Mangrove Damage: Mangrove forests, which provide vital protection, can be ripped up or stripped bare.
- Sedimentation: Fine sediments are churned up and redeposited, smothering organisms that require clear water (like filter feeders) and blocking light necessary for primary producers.
Effects on Human Coastal Communities
- Loss of Life: The immediate and most severe impact is the massive loss of human life.
- Infrastructure Loss: Complete destruction of ports, harbours, fishing boats, and coastal towns.
- Pollution and Salinisation: Flooding causes severe water contamination (sewage, chemicals), and the salt water ruins coastal farmland and underground freshwater supplies (aquifers).
Final Thought
The ongoing process of plate tectonics is what determines where we find the deepest ocean trenches (like the Mariana Trench), the youngest crust (at mid-ocean ridges), and where the highest risks for major seismic events exist. This geological structure is the foundation of the entire marine environment.