plate tectonics (W2)

• continental drift is the large scale movement of continents
• seafloor spreading is the process where new oceanic crust is formed at divergent boundaries due to convection occurring in the mantle
• how many major tectonic plates are there?

  13
• there are three type of plate boundaries: divergent, convergent boundaries, and transform faults
• divergent boundaries are where two plates move away from each other and new lithosphere is created (plate area increases)
• convergent boundaries are where two plates collide and one plate is forced under the other where it is recycled into the mantle(plate area decreases)
• transform faults occur when plates slide horizontally past each other (the plate area does not change)
• a combination of transform faults and divergent or convergent boundaries is know as an oblique boundary
• oceanic and continental crust behave differently at plate boundaries
• continental crust is lighter than oceanic so it is not as easily recycled back into the mantle
• Continental crust is weaker than oceanic so the plate boundaries tend to be more spread out and complicated
• the boundary of diverging oceanic plates is called a mid-ocean ridge (undersea mountain chain)
• continental rifting is often the start process of seafloor spreading. As the continent separates new oceanic crust begins to form along the fault between
• Continental rifting can sometimes stop before separation of the continent occurs
• subduction is the converging of oceanic plates, where one plate descends beneath the other. This sinking produces a long, narow deep-sea trench.
• the Marianas Trench a long narrow deep-sea trench, formed by the convergence of two oceanic plates, where subduction has occurred
• volcanos form along converging plates (O-O & O-C) due to water being carried downwards by the subducting ocean lithosphere, causing areas of the mantle to melt, resulting in magma rising
• Subduction results in the formation a volcanic island arc behind the deep-sea trench that also forms
• continent-continent convergence creates a double thickness in crust and the formation of immense mountain ranges
• the sea floor has narrow alternating (high and low) parallel bands called magnetic anomalies that are almost perfectly symmetrical
• magnetic anomalies occur due to thermoremanent magnetisation, where iron-rich lavas cool and become permanently magnetised in the direction of Earth's magnetic field, remaining so long after the magnetic field has changed
• earth's magnetic field has flipped frequently over geologic time. Major periods of flips occur on average every 0.5 million years.
• within major chrons (periods) there are short lived reversals of the field know as magnetic subchrons which last 3000-200,000 years
• sea-floor spreading and the magnetic anomalies of the sea-floor, can be used to determine the relative plate velocity (the velocity at which one plate moves relative to the other)
• Older sections of oceanic crust have been determined through deep sea drilling, using the age of single cell organisms. With rock age increasing the further away mid-ocean ridges occur.
• the contours of equal seafloor age are called isochrons
• the speed of plate movement is likely due to fast plates having subduction occur along large fractions of their boundaries, with gravity of the dense plate pulling the plate into the mantle
• the supercontinent of Pangaea is believed to have broken up due to the formation of bulges (from heat build up under the continent), which raised the middle of the plate causing the continental masses to begin splitting along the ridge
• plate recycling is believed to throughout the whole mantle (down to the core-mantle boundary at 2890km) this theory is called whole-mantle convection
• whole-mantle convection is supported by evidence from seismic waves which show old subducted crust (colder than surrounding mantle) descending to the core-mantle boundary.
• stratifed convection is the theory that the lithosphere is only recycled in the upper mantle (down to 700km) however there is less supporting evidence than the whole-mantle convection theory
• sea-floor spreading is believed to be a passive process where subduction at other boundaries along the plate are responsible for plate movement, and magma rises to the surface along the divergent boundary to 'fill' the gap being formed
• mantle plumes (controversial theory) are a narrow (100km or less) jet-like upwellings occurring in the centre of plates that can burn a hole through the crust and form huge volcanoes far from any spreading centre
• divergent boundaries can be called extensional boundaries while convergent boundaries can be called compressional and transform faults are referred to as shear faults
• a triple junction is the meeting of three plates
• transform faults can cause shallow earthquakes and are responsible for off-setting other plate boundaries
• Accretionary prisms are piles of ocean sediment in front of continental plates at subduction zones between ocean-continental plates.
• most continental crust forms at subduction zones, where large quantities volcanic activity occurs on the overriding plate
• plate tectonics is driven by three forces: mantle convection, ridge push and slab pull
• the principle of isostasy is the rising and falling of sections of the lithosphere due to an increase or decrease in weight. In order to maintain equilibrium between buoyancy forces that push the lithosphere upward, and gravity forces that pull the lithosphere downward