Plate tectonics

1. Plate tectonics

1. Plate tectonic theory

• The plate tectonic theory explains that

  • The earth has a three layered structured

  • The earth’s lithosphere is broken up into huge pieces called tectonic plates

  • Tectonic plates are constantly moving 

  • Plate movements result in formation of landforms and phenomena

• The uppermost part of the mantle and the crust are the solid parts of the earth (Lithosphere)

• The lithosphere are divided into huge pieces called tectonic plates

  • Continental (land masses)

  • Oceanic (seafloor)

  • The semi solid asthenosphere lies below the lithosphere. Heat from the core causes the rocks in the asthenosphere to melts

• Plate movements give rise to three types of plate boundaries.

  • Divergent plate boundaries

  • Convergent plate boundaries

  • Transform Plate boundaries

2. Convection Currents

• How do convection currents lead to plate tectonic movements

  • Heat from the earth’s core cause the mantle material to become less dense

  • The mantle material rises towards the surface

  • The rising mantle material then spread beneath the plate and brings them apart causing divergent plate movement

  • The mantle material loses heat and sinks towards the core 

  • Convergent plate movement occurs where the convection currents collide

  • The materials then get heated up again and the process repeats, thus forming a convection current

3. Slab-pull force

• How does the slab-pull force contribute to plate tectonic movement

  • When two plate converge the denser oceanic crust is pulled down by gravity as it subducts under the less dense crust

  • The denser oceanic crust sinks deeper into the mantle under its own weight pulling the rest of the plate with it contributing to further convergence

4. Seafloor spreading

• What is the evidence of seafloor spreading

  • Where two plates move away from each other at divergent plate boundaries, seafloor spreading occurs

  • Magma from deep within the earth rises through the mid-ocean ridge

  • New oceanic crust is formed

• Why does evidence of seafloor spreading support the plate tectonic theory?

  • The age of the the rocks at the seabed show a pattern 

    • The rocks nearer to the crest of the mid-ocean ridge are the youngest

    • Rocks further away from the ridge are progressively older

  • Shows how the new oceanic crust are created at divergent boundaries

  • Little sediment accumulation is found at oceanic trenches as older oceanic crust are destroyed there

  • Prove that new crust are continually formed at divergent boundaries at the mid ocean ridge and destroyed further away at oceanic trenches

5. Magnetic striping

• What is magnetic striping

  • Earth has a geographic North and a geographic South as well as a magnetic North and magnetic South

  • The geographic North and South do not change

  • But the magnetic North and south changes

  • When the magnetic North = geographic North it is called normal polarity

  • But when the magnetic North is roughly pointed at the geographic North then it is called reverse polarity

  • Over time the Earth’s polarity flips

  • Magnetic striping is the zebra like pattern where there are strips of normal polarity rocks alternating alongside strips of reversed polarity rocks on the seafloor

  • It is symmetrical on both sides of the mid ocean ridge

  • Magnetic striping occurs as:

    • Basaltic rocks from the oceanic crust are volcanic rocks formed from iron-rich lava

    • They contain magnetic material

    • When the iron-rich lava erupts it cools and solidifies 

    • Its magnetic material points toward the Earth’s magnetic North recording evidence of Earth’s polarity at that time

• Why does evidence of magnetic striping support the plate tectonic theory

  • The zebra-like pattern is symmetrical on either side of mid-ocean ridges showing that this is not a random or isolated occurrence

  • Provides evidence that plates move 

    • Oceanic plates move away from each other 

    • Iron rich lava erupts from the centre of the ridge

    • Lava cools and solidifies forming new oceanic crust

    • When the Earth’s polarity reverses the rocks record the reversals