Plate Tectonics

• There are two types of crust:

  • Oceanic – an occasionally broken layer of basaltic rocks known as sima (because they are made up of silica and magnesium).
  • Continental – bodies of mainly granitic rocks known as sial (because they are made up of silica and aluminium).
• Together, the crust and the upper mantle are known as the lithosphere
• The rocks in the upper mantle are solid and sit on top of the asthenosphere, a layer of softer, almost plastic-like rock. The asthenosphere can move very slowly, carrying the lithosphere on top.
• The mantle is the widest section of the Earth – it is 2900 km thick.
• Primordial Heat - Heat remaining from the initial collisions that formed the universe

  Radiogenic Heat - Heat released from the breaking down of radioactive material within the mantle.
• The Earth’s surface is made up of seven major and several minor tectonic plates. Each plate is an irregularly shaped ‘raft’ of lithosphere effectively floating on the ‘plastic’ asthenosphere beneath.
• Continental plates are permanent and may extend far beyond the margins of current land masses. They will not sink into the asthenosphere because of their relatively low density.
• Oceanic plates are continually being formed at mid-ocean ridges and destroyed at deep ocean trenches – hence their relatively young age
• The plates move relative to each other at varying rates from 2 cm to 16 cm a year.
• Ridge Push - Gravity acting on the weight of the lithosphere near the ridge ‘pushes’ the older part of the plate in front, causing the plates to push apart
• Slab Pull - Following subduction, the lithosphere sinks into the mantle under its own weight, downwarping and gravity pulls the rest of the plate with it.
• When two plates separate (diverge) they form a constructive margin
• in oceanic areas, sea-floor spreading occurs on either side of mid-ocean ridges (e.g. the Mid-Atlantic Ridge)
• in continental areas, stretching and collapsing of the crust creates rift valleys (e.g. the Great African Rift Valley).
• Mid-Ocean Ridge

  • Form when oceans diverge
  • Breaks called transform faults cut across the ridges
  • These faults occur at right angles to the boundary, seperating sections of the ridge
  • These widen at different rates, causing earthquakes
  • Middle of the ridge marked by a deep rift valley which are widened by magma rising from the asthenosphere
• Rift Valley

  • Formed due to continental divergence
  • Formed when the lithosphere stretches, causing it to fracture into sets of parallel faults
  • The land between the faults collapses into deep, wide valleys separated by upright land known as horsts
• When two plates collide (converge) they form a destructive plate margin.
• Three types of convergence are possible

  • oceanic plate meeting continental plate, such as along the Pacific coast of South America
  • oceanic plate meeting oceanic plate, such as along the Mariana Trench in the western Pacific
  • continental plate meeting continental plate, such as the Himalayas.
• Alfred Wengener (1915) - Came up with the theory of continental drift and that tectonic plates have moved. Demonstrated by South America and Africa looking like they fit together.
• Arthur Holmes (1931) - Came up with concept of convection currents within the mantle and that sea floor spreading occurs.
• Alexander du Toit (1937) - Illustrated and mapped the two super continets Laurasia and Godwana.
• J. Tuzo Wilson (1965) - The first person to sue the term 'plates', allowed people to further understand how continents can move
• Marie Tharp (1940s-1950s) - Stayed in a lab (wasn't allowed on ships) and plotted the ocean floor on a map. She plotted ocean ridges and discovered that a large amount of earthquake epicenters were located here also.
• Oceanic - Continental convergent boundary

  • Oceanic plate is lighter so subducts beneath continental plate, forming a deep ocean trench e.g. Peru-Chile trench.
  • As plates converge, continental land mass is uplifted, and forms an accretionary wedge which forms fold mountains.
  • Folds become asymmetric and make a recumbent fold, which, if compression increases, breaks and forms a nappe.
  • Subducting plate reaches a zone of melting called the benioff zone, where friction build and is released as earthquakes.
• Oceanic - Oceanic convergent boundary

  • One plate (faster or denser) subducts beneath the other, leading to the formation of a deep ocean trench.
  • Magma rising from the benioff zone forms crescents of submarine volcanoes which grow to form island arcs
  • e.g. Mariana Trench where the Pacific plate is subducted beneath the Philippine plate
• Continental - Continental convergent boundary

  • Subduction does not occur as they are lower density than the asthenosphere.
  • Sediment on the plates buckle and form high fold mountains such as the Himalayas
  • Shallow earthquakes can occur
  • Fold mountains are growing due to continual compression
• Two plates sliding past each other forms a conservative plate margin
• On conservative plate margins, plates stick to each other and friction builds up in extremely powerful shallow-focus earthquakes such as the one in Los Angeles in 1994
• 95% of the world's earthquakes are located along plate margins
• Radioactive decay within the Earth's core generates hot temperatures. If this decay is concentrated, hot spots form at the core.
• Hot spots heat the lower mantle creating localised thermal currents where magma plumes rise vertically. These plumes occasionally rise within the centre of plates and then ‘burn’ through the lithosphere to create volcanic activity on the surface.
• Hot spots remain stationary, so the movement of the overlying plate results in the formation of a chain of active and extinct volcanoes
• There is a clear relationship between volcanoes and tectonic plate margins e.g. Pacific Ring of Fire has hih densities of volcanoes along plate boundaries
• Volcanic activity is common at divergent and convergent boundaries and absent at transform boundaries. Some volcanoes also occur within plates.
• Stratovolcano formation

  • Convergent boundaries
  • Sediment on the subducting plate melts in the Benioff Zone, this leads to flux melting where the addition of water and CO2 lowers the melting point of the asthenosphere, forming molten magma.
  • These plutons of magma rise because they are less dense than the asthenosphere.
  • These volcanoes have andesitic magma which escapes through fault lines and doesn't travel far.
• Andesitic Magma - more viscous
• Shield Volcano

  • Divergent boundary
  • Plats part and a rift form, reducing pressure in the asthenosphere, so decompression magma forms as the melting point lowers, this is molten
  • As it is molten, it is less dense than the asthenosphere, so it rises and erupts
• Basaltic magma - less viscous, darker in colour
• Rhyolitic magma has a higher silica content, and then andesitic magma, then basaltic magma
• Geological evidence for continental drift = The 'jigsaw fit' of South America and West Africa, evidence of similar glacial deposits in South America, Antarctica and India