Chapter 1

• Plate tectonic theory:

  • Earth has a three-layered structure
  • 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 (e.g. mountain ranges, oceanic trenches) and phenomena (e.g. earthquakes, tsunamis).
• Parts of earth:

  • The lithosphere is divided into huge pieces called tectonic plates.
  • There are two types of plates, continental and oceanic
  • The semi-solid asthenosphere lies below the lithosphere. Heat from the core causes the rocks in the asthenosphere to melt.
  • Plate movements give rise to three types of plate boundaries: Convergent, Divergent and Transform
  • There are two forces that are responsible for plate movements: Convection currents and Slab-pull force.
• Convectional currents:

  1. Heat from the earth's core causes the mantle material to become less dense.
  2. Mantle material rises towards the surface.
  3. The rising convection currents then spread beneath the plates and drag them apart, causing divergent plate movement.
  4. The mantle material then loses heat and sinks towards the core.
  5. Convergent plate movement occurs where the convection currents collide.
  6. The materials then get heated up again, and the process repeats.This rising and sinking of the mantle material forms convection currents.
• Slab-pull force:

  1. When two plates converge, the denser oceanic crust is pulled down by gravity as it subducts beneath the less dense crust.
  2. 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.
• Seafloor spreading:

  1. Where two plates move away from each other at divergent plate boundaries, seafloor spreading occurs.
  2. Magma from deep within the earth rises through the mid-ocean ridge.
  3. New oceanic crust is formed.
• How does seafloor spreading support plate tectonic theory:

  • Rocks nearer to the crest of the mid-ocean ridge are the youngest.
  • Rocks further away from the ridge are progressively older.
  • Shows how new oceanic crust is created at divergent boundaries and then moves laterally on both sides of the mid-ocean ridge as seafloor spreading continues.
  • Little sediment accumulation is found at oceanic trenches as plates are being destroyed
  • Proves that new crust is continually formed at divergent boundaries and as plates move, older crust is destroyed at oceanic trench
• Magnetic stripping:

  • Earth’s magnetic poles will flip overtime, alternating between normal and reverse polarity
  • Magnetic stripping is a zebra like pattern of normal polarity rocks and reversed polarity rocks
  • Occurs as magnetic material in the rock points to Earth magnetic North, recording Earth’s polarity at that time
• Magnetic stripping provides evidence that plates move, supporting the plate tectonics theory as:

  • Oceanic plates move away from each other Iron-rich lava erupts from the centre of the ridge.
  • Lava cools, solidifies, and forms new oceanic crust.
  • The crust is then pushed in both directions away from the ridge when new lava erupts and solidifies as plates move apart.
  • When Earth's polarity reverses, the rocks record the reversals.
  • Over time, as more lava moves away from the ridge due to plate divergence, a symmetrical zebra-like pattern forms.
• Oceanic-Oceanic divergent boundary:

  1. Two oceanic plates move apart.
  2. The decrease in overlying pressure causes parts of the underlying mantle to melt, forming magma.
  3. Magma rises through weak areas in the crust to the Earth's surface, and fills gaps caused by the spreading plates.
  4. Lava cools and solidifies to form basaltic rocks.
  5. The rocks make up a new oceanic crust
  6. A mid-ocean ridge forms.

  Example:

  • North American plate and European Plate diverge
  • Creating the Mid-Atlantic ridge
• Continental-Continental divergent:

  1. Two continental plates move apart.
  2. Rocks eventually fracture to form parallel faults.
  3. The rock between these faults collapses to form a deep rift valley with steep sides.
  4. As the plates move apart, the decrease in pressure causes parts of the 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

  Example:

  • Nubian Plate diverges from Somalian plate
  • This formed the Great Rift valley and volcanoes like Mount Kenya
• Oceanic-Oceanic convergent plate boundary:

  1. Two oceanic plates collide.
  2. The denser plate subducts beneath the other plate.
  3. This forms a deep depression known as an oceanic trench.
  4. 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.
  5. Magma rises through weak areas in the crust to the Earth's surface.
  6. This forms a chain of volcanoes, known as a volcanic island.
• Example of oceanic oceanic convergence:

  • Oceanic Pacific plate subducts beneath Oceanic Philippines plate
  • Forms Mariana Trench and Mariana Islands
  • Guam, which is near the Mariana Islands, commonly experiences earthquakes
• Continental-continental convergence:

  1. Two continental plates collide.
  2. Subduction does not take place because continental plates are too buoyant to subduct.
  3. Enormous pressure causes the rocks to be uplifted and buckled to form fold mountains.

  Example:

  • 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.
• Oceanic-continental convergence:

  1. An oceanic plate collides with a continental plate.
  2. The denser oceanic plate subducts forming an oceanic trench
  3. As the subducting plate sinks into the mantle, the high pressure forces water out of oceanic crust, lowering the melting point of the overlying mantle, causing it to melt, forming magma.
  4. Magma rises through weak areas in the crust to the Earth's surface, forming volcanoes on the continental plate.
  5. Enormous pressure at plate boundary cause rocks on the continental plate to be uplifted and buckled, forming fold mountains.
• Example of oceanic-continental convergence:

  • Oceanic Naza Plate converges with the continental South American Plate.
  • Denser Naza Plate subducts beneath the South American Plate, forming: Peru-Chile trench, Andes fold mountain range and Nevado  del Ruiz volcano.
  • Earthquakes are also common here, such as the 2010 Chile earthquake.
• 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