1.4 Tectonic Processes at Different Plate Boundaries

What happens at plate boundaries when tectonic plates move?

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