Plate Tectonics & Plate Movements

• What is Distribution?

  • Distribution refers to the spatial coverage of the hazard
  • Distribution of a hazard can also refer to the areas where the particular hazard is likely to occur
• What is Frequency + Magnitude?

  • Frequency refers to the distribution of the hazard through time whereas Magnitude assesses the size of the impact
  • The frequency-magnitude principle leads us to expect many small, insignificant events and, in the long term, increasingly fewer events as the magnitude rises
• Structure of the Earth (Core)

  • The core is made up of dense rocks containing iron & nickel alloys and is divided into a solid inner and a molten outer core
  • Temperature of over 5000C
  • Heat is produced by 2 processes

  1. Primordial Heat (leftover from the Earth's formation)
  2. Radiogenic Heat (produced by radioactive decay of isotopes)

  
• Structure of the Earth (Mantle)

  • The mantle is made up of molten and semi-molten rocks containing lighter elements i.e. silicon & oxygen
  • The lithosphere consists of the crust & rigid upper section of the mantle and is approximately 80-90km, this is where the oceanic & continental plates are found
  • The asthenosphere lies beneath the lithosphere and is the semi-molten layer the plates float & move on

  
• Structure of the Earth (Crust)

  • The crust is light because of the light elements (silicon, oxygen, aluminium, potassium & sodium)
  • The crust varies in thickness:

  1. beneath the oceans it is 6-10km thick
  2. below continents it is 30-40km thick
  3. under mountain ranges it is 70km thick

  
• Differences between Oceanic Crust (OC) & Continental Crust (CC):

  • THICKNESS: CC = 30-70km, OC = 6-10km
  • AGE: CC = >1500 million years, OC = <200 million years
  • DENSITY: CC = 2.6 (lighter), OC = 3.0 (heavier)
  • COMPOSITION: CC = "silicate and magnesium" (SIAL), OC = "silicate and aluminium" (SIMA)

  
• What is Magma?

  Molten rock, gases and liquids from the mantle accumulating in vast chambers at great pressures within the lithosphere. Upon reaching the ground surface, magma is known as lava
• What is Igneous Rocks?

  Rocks formed by the cooling of molten magma, either underground (intrusive) or on the ground surface (extrusive)
• What is Intrusive Magma?

  • Magma that cools & solidifies slowly below the surface
  • It forces coarse-grained igneous rocks
  • Vertical dykes & horizontal sills can become parts of the landscape once erosion removes the overlying rocks

  
• What is Extrusive Magma?

  • Lava that is in contact with the air or sea
  • It cools & solidifies far quicker than intrusive magma
  • The resulting igneous rocks, ted to be fine-grained with small crystals

  
• Plate Tectonic Theory (Background)

  • Alfred Wegener was a German climatologist who developed a supercontinent theory of Pangea 300 million years ago. This birthed the idea of Continental Drift
  • Previously, volcanic/seismic hazards were explained by the release of internal heat, or the movement of magma chambers. However, these theories didn't account for volcano distribution and patterns around the world.
  • The theory was rejected as Wegener was unable to provide a convincing explanation for how continental drift occurred

  
• Geological Evidence for Continental Drift

  • Puzzle Theory - East coast of South America & West coast of Africa
  • Rock Sequences in Scotland closely agreed with those in Eastern Canada
  • Evidence of Carboniferous glaciation (glacial activity & striations in rocks) across South America, Africa, and India

  
• Biological Evidence for Continental Drift

  • Fossils of ancient plant found in India & Antarctica
  • Fossil of brachiopods found in Indian limestone
  • Fossil of Mesosaurus reptile found in South America & South Africa

  
• Climatological Evidence for Continental Drift

  • Coral reefs are found in tropical environments but ancient coral reefs are found in locations where it is much too cold today.
  • Wegener suggested that these creatures were alive in warm climate zones and that the fossils had drifted to new locations on the continents.

  
• Plate Tectonic Theory Development

  • Originally Wegener's theory couldn't explain how continental movement took place
  • The discovery of the Mid-Atlantic Ridge & examination of the ocean crust on each side of the ridge suggested the sea-floor was spreading

  
• Paleomagnetism

  • Paleomagnetism provided evidence for the sea floor spreading
  • Iron particles in lava erupted on the ocean floor are aligned with Earth's magnetic field
  • Lava solidifies, providing a permanent record of the Earth's polarity

  
• Paleomagnetism (2)

  • The Earth's polarity reverse at regular intervals [approx every 400,000 years], resulting in 'stripes' on rocks aligned towards either the north or south pole
  • Sea-floor spreading implies Earth is getting bigger, therefore sea floor is destroyed elsewhere
  • Evidence of this is seen in huge oceanic trenches where large areas of ocean floor are pulled downward by subduction

  
• Mantle Convection Cells

  1. Hot spots in the asthenosphere generate thermal convection currents,
  2. Causing magma to heat & rise towards the crust, then spread at the base of the lithosphere where it cools & sinks back towards the mantle
  3. As cool material is replaced with hot material, it forms a large convection cell
  4. This is a slow, constant movement and tectonic plates move around the Earth's surface

  
• What is Subduction?

  • A process where oceanic lithosphere is recycled into the Earth's mantle at convergent (collision) boundaries
  • The heavier oceanic crust dives beneath a lighter second crust, causing it to sink into the mantle
  • "Chain on edge of table"

  
• What is Ridge Push?

  • A process where 2 divergent (splitting) boundaries move apart
  • The new exposed crust is less dense than surrounding crust so it rises forming oceanic ridges
  • Magma solidifies on top of the plates, adding weight and pushing it down

  
• What is Slab Pull?

  • A process where divergent (splitting) boundaries move apart
  • Dense oceanic plate is subducted beneath lighter continental plate, the density of the oceanic plate pulling itself into the mantle

  
• What is Gravitational Sliding?

  • New oceanic crust is formed at ocean ridges
  • As the crust moves away from the ocean ridge, it cools and becomes denser & thicker
  • This causes the lithosphere to slope away from the ridge & gravity pulls the lithosphere down this slope, pushing it forwards

  
• Why is Plate Tectonic Theory Important?

  • Explains many natural landforms and phenomena
  • Better understanding of Earth's natural processes and hazards can influence our human behaviour & save more lives
  • ^^ Domination Perception of Hazards
• Divergent Plate Boundaries:

  • Two tectonic plates moving away from each other
  • As the plates pull apart, molten material rises to the surface - causing volcanic activity
  • If a divergent boundary forms on a continent, its called RIFT or CONTINENTAL RIFT
  • When a divergent boundary forms under the ocean, its called an OCEAN RIDGE

  
• Divergent Plate Boundaries Example (Iceland)

  • Iceland is located on a divergent boundary between the North American & Eurasian plates
  • Volcanic activity occurred along the cracks, & over time the island formed
  • Scientists believe there is a mantle plume (upwelling of hot mantle material) located underneath Iceland, meaning more material is erupted

  
• Convergent Boundaries (Continent-Continent):

  • When 2 continental crusts collide, both sides have the same properties
  • Neither side of the boundary wants to sink, so they push against each other, & the crust cracks, pushing up (& down into the mantle) high mountain ranges
  • Examples of Mountain Ranges = Himalayas & European Alps

  
• Convergent Boundaries (Continent-Ocean):

  • The oceanic crust is thinner & more dense, so it is subducted below the continental crust into the mantle
  • When the subducting slab is around 100km deep, it dehydrates & releases water into the mantle wedge, which changes the melting point of the molten material
  • This forms new melt which rises & forms volcanoes
  • Subduction is a way of recycling the oceanic crust.

  
• Convergent Plate Boundaries (C-O) Example (Andes)

  • Oceanic lithosphere subducts underneath the continental lithosphere. Oceanic Lithosphere heats & dehydrates.
  • The melt rises forming volcanism
  • Examples = Andes Mountains under South American & Nazca Plate

  
• Convergent Plate Boundaries (Oceanic-Oceanic)

  • Heavier plate subducts leaving an ocean trench. Fold mountains will also occur
  • Built up pressure causes underwater volcanoes bursting through oceanic plate.
  • Lava cools and creates new land called island arcs.

  
• Transform Plate Boundaries:

  • Parallel plates move in different directions or at different speeds
  • No landforms are created
  • The movement builds a lot of pressure. In oceanic crust, this can displace a lot of water. On continental crust, fault lines can occur where the ground is cracked

  
• How are Fold Mountains formed?

  • Formed at Convergent boundaries
  • Created by a process called 'orogeny', where the extreme pressure of the plates pushing against one another forces the edges of the plates upwards into a series of folds
  • EXAMPLE - The Himalayas, formed by the Indian & Eurasian plates

  
• How are Rift Valleys formed?

  • Formed at Divergent boundaries
  • The plates move apart, causing the lithosphere to crack. Rock & debris collapse into the crack, forming a rift valley
  • As the lithosphere weakens, sometimes magma escapes, forming volcanoes
  • EXAMPLE - East African Rift System, has volcanoes, between Jordan & Mozambique

  
• How are Ocean Ridges formed?

  • Formed at Divergent boundaries
  • Convection currents cause the plates to be pulled apart. Magma rises between the plates, erupting basalt onto the seafloor which cools forming a ridge
  • EXAMPLE - Mid-Atlantic Ridge

  
• How are Deep Sea Trenches formed?

  • Found on Convergent boundaries
  • Formed by Oceanic crust being subducted, which causes the seafloor to bend & form a depression
  • EXAMPLE - Marianas Trench

  
• How are Island Arcs formed?

  • Formed by Convergent Boundaries
  • Formed by subduction, which results in the creation of undersea volcanoes. These volcanoes erupt andesite & basalt which solidify & form a string of islands
  • EXAMPLE - Aleutian Islands

  
• How are Volcanoes formed?

  • Formed on Convergent & Divergent boundaries
  • Convergent - The subducting slab reaches 100km, it releases water into the mantle wedge. The new melt forms volcanoes
  • Divergent - Magma rises in cracks between plates
  • EXAMPLE - Eyjafjallajökull

  
• Hot Spots in Hawaii:

  There are parallel chains of volcanic islands that run directly across the Pacific plate, not near plate boundaries
• How are Hot Spots formed?

  • Formed at magma plumes - vertical columns of super hot magma that rise from the mantle via convection
  • These are stationary
  • Magma is lighter than solid lithosphere so magma rises, erupting at the surface
  • Over time lava builds & hardens, creating an island

  
• How are Chain Volcanoes Formed?

  1. Magma plume remains stationary but crust above moves
  2. Volcanic activity decreases in the part of the crust that moves away
  3. New volcanoes form in part of the crust above the magma plume, creating a 'Chain of Volcanoes' (e.g. Hawaii Chain)

  
• What are the problems with Plate Tectonic theory in regards to Hot Spots?

  • Doesn't explain why or how hot spots form
  • Some hot spots (e.g. Hawaiian-Emperor seamount chain) show abrupt directional changes