Plate tectonics

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

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

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

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

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

What happens at Oceanic-Oceanic divergent plate boundaries

• Oceanic-Oceanic divergent plate boundaries

  • Two oceanic plates move apart

  • The decreasing in the overlying pressure causes parts of the underlying mantle to melt , forming magma

  • Magma rises through weak areas in the crust to the Earth's surface. And fill gaps caused by the spreading plates

  • Lava cools to form basaltic rocks

  • The rocks make up a new oceanic crust

  • An extensive underwater mountain chain - the mid ocean ridge forms

  • At the centre of the ridge is a deep rift valley with steep sides

  • Magma rises through through weak areas in the crust to the Earth’s surface forming submarine volcanoes

  • After many eruptions, the volcanoes may break the surface of the ocean to form volcanic islands such as Surtsey , Iceland 

  • Earthquakes also occur here due to the stress and tension released when plates move

  • Examples

    • The oceanic North American Plate and oceanic Euraisian plate moves apart

    • This forms the Mid-Atlantic ridge

What happens Continental-Continental divergent plate boundaries

• Two continental plates move apart

• Rocks eventually fracture to form parallel faults

• The rocks between these faults collapses to form a deep rift valley with steep sides

• As the plates move apart , the decrease in overlying pressure causes parts of the underlying mantle to melt, forming magma. Magma rises through weak areas in the crust to the Earth’s surface, forming volcanoes

• Earthquakes occur as stress and tension are released when plates move

• Example

  • The Nubian plates pulls apart from the Somalian plate

  • This forms the Great Rift Valley

  • Example of volcanoes formed : Mount Kenya , Mount Kilimanjaro 

What happens at Oceanic-oceanic convergent plate boundaries

• Two oceanic plates collide

• The denser plate subducts beneath the other plate

• This forms a depression known as an oceanic trench

• The subducting plate sinks into the mantle, the high pressure forces water out of its oceanic crust. Water lowers the melting point of the overlying mantle, causing it to melt, forming magma

• Magma rises through weak areas in the crust to the Earth’s surface

• This forms a chain of volcanoes known as a volcanic island

• Friction along the subducting oceanic plate also causes earthquakes to occur

• Examples:

  • The oceanic Pacific plate subducts beneath the oceanic Philippine Plate

  • This form the Mariana Trench , as well as volcanic islands known as the Mariana Islands

  • Guam, an island near the Mariana Trench commonly experiences earthquakes

What happens at Continental-continental convergent plate boundaries

• Two continental plates collide

• Subduction does not take place

• Enormous pressure causes the rocks to be uplifted and buckled to form fold mountains

• Friction along the convergent plate boundary also causes earthquakes to occur

• Magma does not the rise to the surface

• Examples

  • Continental Indo-Australian plate converges with the continental Eurasian plate

  • This forms the Himalayan Mountain range

  • This area experiences many earthquakes, such as the devastating 2015 Nepal earthquake.

What happens at Oceanic-continental convergent plate boundaries

• An oceanic plate collides with a continental plate

• The denser oceanic plate subducts beneath the continental plate

• This forms an oceanic trench in the subduction zone

• As the subducting plate sinks into the mantle,  the high pressure forces water out of its oceanic crust. Water lowers the melting point of the overlying mantle , causing it to melt forming magma

• Magma rises through the weak areas in the crust to the Earth’s surface forming volcanoes on the continental plate

  • Friction along the subducting oceanic plate causes earthquakes to occur 

• Enormous pressure at the plate boundary causes rocks on the continental plate to be uplifted and buckled, forming fold mountains

• Examples

  • Oceanic Nazca plate converges with the continental South American Plate

  • Denser Nazca Plate subducts beneath the South American plate forming

    • The Peru-Chile trench

    • The Andes fold mountain range

    • The Nevado del Ruiz volcano

  • Oceanic are also common here, such as the 2010 chile earthquake

What happens at Transform Plate boundaries

• Tectonic plates slide past each other

• No crust is created or destroyed

• Stress caused by the plate movement produces a fault which is a zone of fractures

• Earthquakes occur here as one plate suddenly slips past another

• Magma does not rise to Earth’s surface, hence there are no volcanoes

• Example

  • Pacific Plate sliding past the North American plate

  • This forms the San Andreas Fault in California, USA

  • Earthquakes are common here , such as the 1989 Loma Prieta earthquake in California