tectonics 1

What is the plate tectonic theory?

lithosphere 

• consists of the uppermost part of the mantle + crust

  • solid parts of the Earth 

• divided into huge parts called “tectonic plates”

  • continental (land masses)

  • oceanic (seafloor; denser than continental)

asthenosphere 

• semi-solid 

• lies below the lithosphere

• heat from the core causes rocks in this layer to melt

Plate Boundaries

• divergent plate boundaries 

• convergent plate boundaries 

• transform plate boundaries

2 Forces Responsible for Plate Movement

• slab-pull force

• convection currents 

What are the characteristics of the layers of the Earth?

Core

• innermost layer (position)

• highest temperature

  • 4400°C - 6000°C

• thickest layer

  • 3300km

Mantle

• in between core and crust

• temperature of

  • 1000°C - 3700°C

• thickness of 

  • 2900km

Crust

• outermost layer (position)

• lowest temperature 

• thinnest layer

  • 6-70km

How do convection currents lead to tectonic plate movement?

1) Heat from the Earth’s core causes

• mantle magma to become less dense

2) The mantle magma rises towards the surface

3) Rising mantle magma spreads beneath the plate

• dragging them apart

  → causing divergent plate movement

4) Mantle magma loses heat

• sinking towards the core

5) Convergent plate movement occurs where the

• convection currents collide

6) Mantle magma is heated up again

• the process repeats 

• rising and sinking of the magma forms convection currents

How does slab-pull force lead to tectonic plate movement?

1) when 2 plates converge 

• the denser oceanic plate is pulled down by gravity 

  • subducting beneath the less dense crust

2) the denser ocean crust sinks deeper into the mantle 

• under its own weight 

• then pulling the rest of the plate with it 

  → contributing to further convergence 

What is the evidence of seafloor spreading?

1) seafloor spreading occurs at

• divergent plate boundaries

  • where 2 plates move away from each other

2) magma from deep within the earth rises through the

• mid-ocean ridge

  • undersea volcanic mountains formed @ divergent plate boundaries, where seafloor is created (fyi)

3) new oceanic crust is formed

How does seafloor spreading support the plate tectonics theory? (i.e., how does this process show that plates move?) (1/2)

1) Ages of Rock (at the seabed)

• shows a pattern

  • rocks nearer to the center of the mid-ocean ridge

    → the youngest

  • rocks further away from the ridge are

    → progressively older

∴ shows that at divergent plate boundaries

• new oceanic crust is continuously formed

  • crust moves laterally on both sides of the mid-ocean ridge

    • as seafloor spreading continues

How does seafloor spreading support the plate tectonics theory? (i.e., how does this process show that plates move?) (2/2)

2) Limited Sediment Accumulation

• limited sediment accumulation is found at oceanic trenches

  • as older oceanic crust is being destroyed at oceanic trenches 

  ∴ oceanic crusts are usually younger than continental crusts 

this proves that 

• new crust is continually formed at divergent boundaries

  • at the mid-ocean ridges 

• as plates move, older crust is destroyed further away 

  • at oceanic trenches

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What is normal and reverse polarity?

Normal Polarity

• magnetic North points roughly towards the

  • geographical North 

• magnetic South points roughly towards the

  • geographical South

Reverse Polarity

• magnetic North points roughly towards the

  • geographical South

• magnetic South points roughly towards the 

  • geographical North 

Overtime, Earth’s polarity has flipped multiple times

• alternating between normal and reverse polarity

What is magnetic striping?

• the zebra-like pattern where

  • there’s strips of normal polarity rocks 

    • alternating alongside strips of reversed polarity rocks on the seafloor

  • it’s symmetrical on both sides of the

    • mid-ocean ridge

∴ stripes of rock on the seafloor with alternating magnetic properties

occurs on

How does evidence of magnetic striping support the plate tectonic theory? (1/2)

1) Presence of Basaltic Rocks

• basaltic rocks from the oceanic crust (FYI)

  • are volcanic rocks formed from iron-rich lava

• contains iron-rich magnetic materials

• when the iron-rich lava erupts from the centre of the mid-ocean ridge

  • it cools and solidifies

    → its magnetic materials point towards Earth’s magnetic North

    • recording evidence of Earth’s polarity at that time

How does evidence of magnetic striping support the plate tectonic theory? (2/2)

2) Symmetrical Zebra-Like Magnetic Pattern on both sides of the Mid-Ocean Ridge

• zebra-like pattern is symmetrical on either side of the mid-ocean ridges

  • shows that this is not a random occurrence

∴ supports the plate tectonic theory as it provides evidence that the plates move

• as oceanic plates diverge away from each other

  1) iron-rich lava erupts from the centre of the ridge

  • the lava cools and solidifies

    → forming new oceanic crust

    2) the crust is then pushed in both directions away from the centre of the ridge

    • as the oceanic plates move apart

    3) when Earth’s polarity reverses

    • the rocks record the reversals

    4) over time, a symmetrical zebra-like magnetic pattern forms

    • supporting the fact that plates move during seafloor spreading

What happens at divergent plate boundaries?

tectonic plates move away from each other

• mid-ocean ridges

• volcanoes, inclusive of

  • submarine volcanoes

  • volcanic islands

• rift systems

• earthquakes 

What happens at convergent plate boundaries?

tectonic plates move towards each other

• fold mountains

• volcanoes, inclusive of

  • submarine volcanoes

  • volcanic islands

• oceanic trenches

• earthquakes

What happens at transform plate boundaries?

tectonic plates slide past each other

• no crust created or destroyed

• faults

• earthquakes

Divergent Plate Boundaries: Oceanic-Oceanic

When 2 oceanic plates move apart

• the decrease in overlying pressure causes parts of the underlying mantle to melt

  • forms magma

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

  • fills gaps caused by the diverging plates

• the lava cools and solidifies

  • forms basaltic rocks

    • which make up new oceanic crust

    • the mid-oceanic ridge forms

      • at the centre

        → there’s a deep rift valley with steep sides

• magma rises through weak areas in the crust to Earth’s surface as lava

  • the lava cools, solidifies + accumulates over time

    • forms submarine volcanoes

      → after many eruptions, the volcanoes may break the surface of the ocean

      • forms volcanic islands 

• earthquakes occur when

  • the plates move

    • stress and tension is released

e.g.,  Oceanic North American Plate + Oceanic Eurasian Plate

• move apart to form the mid-atlantic ridge

Iceland, Surtsey

• volcanic island

Divergent Plate Boundaries: Continental-Continental

When 2 Continental plates move apart

• rocks eventually fracture

  • forms parallel faults

• rock between these faults subsides

  • forms deep rift valley with steep sides

• as plates move apart, the decrease in overlying pressure causes parts of the underlying mantle to melt 

  • forms magma 

  • magma rises through weak areas in the crust to Earth’s surface as lava

  • lava cools, solidifies, and accumulates over time
    → forms volcanoes 

• earthquakes occur when 

  • stress and tension are released 

    → when the plates move  

e.g., Continental Nubian Plate + Continental Somalian Plate

• forms the Great Rift Valley

Volcanoes formed

• Mount Kenya 

• Mount Kilimanjaro 

Convergent Plate Boundaries: Oceanic-Oceanic

When 2 Oceanic plates collide

• the denser plate subducts

  • beneath the other plate

  • forms an oceanic trench in the subduction zone

• as the subducting plate sinks into the mantle

  • high pressure forces water out of its oceanic crust

  • water lowers the melting point of the overlying mantle, causing it to melt

    → magma forms 

  • magma rises through weak areas in the crust to Earth’s surface as lava

    • the lava cools, solidifies + accumulates over time

      → forms a chain of submarine volcanoes

      • when they rise above sea level

        → become volcanic islands

• earthquakes occur cuz of

  • friction along the subducting oceanic plate 

e.g., Oceanic Pacific Plate (subducts) + Oceanic Phillipine Plate

• forms the Mariana Trench 

  • Guam, an island near the trench

    → commonly experiences earthquakes

• volcanic islands, the Mariana Island

Convergent Plate Boundaries: Continental-Continental

When 2 low-density Continental plates collide

• subduction doesn’t occur

  • continental plates are too buoyant to subduct

∴ magma doesn’t rise to the surface

• no volcanoes formed 

• enormous pressure causes the rocks on the plates to be

  • uplifted and

  • buckled 

    → forms fold mountains

• earthquakes occur cuz of

  • friction along the convergent plate boundary

e.g., Continental Indian Plate + Continental Eurasian Plate

• forms the Himalayan Mountain Range 

• this area experiences many earthquakes

  → Nepal 2015 devastating earthquake 

Convergent Plate Boundaries: Oceanic-Continental

When an oceanic plate collides with a continental plate

• the denser oceanic plate subducts

  • beneath the continental plate 

  • 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

  • magma forms

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

    • the lava cools, solidifies, and accumulates over time

      → forms volcanoes on the continental plate

• enormous pressure at the plate boundary causes the rocks on the continental plate to be

  • uplifted and

  • buckled 

    → forms fold mountains

• earthquakes occur cuz of 

  • friction along the subducting oceanic plate 

e.g., Oceanic Nazca Plate + Continental South America Plate

• forms the following;

  • Peru-Chile trench

  • Andes Fold mountain range

  • Nevado del Ruiz volcano

• earthquakes are also common here

  • 2010 Chile earthquake

Transform Plate Boundary

When 2 tectonic plates try to slide past each other

• friction causes the 2 plates to get locked

  • stress builds up

• stress caused by the plate movement produces a 

  • fault 

    → a zone of fractures between 2 plates

• no crust is created nor destroyed

  • magma does not rise to earth’s surface

    → no volcanoes

• earthquakes occur as

  • one plate suddenly slips past another

e.g., the Pacific Plate slides past the North American Plate

• forms the San Andreas Fault

  • in California, USA

• earthquakes are common here