GEOLOGY GCSE Plate Tectonics and Hazards Name: ______________________________ Whitmore High School 1 EARTH’S INTERNAL STRUCTURE From gathering all of the available evidence from: * Earthquake waves * Direct evidence * Density readings * Meteorites It is possible to build up an idea of what the interior of the Earth is like: Main four compositional layers of the Earth The structural layers of the Earth Solid Solid Semi solid 2 The Layered Structure of the Earth (not drawn to scale) o o o o The various layers are separated by their composition and behaviour: Mechanical Depth (km) Layer Composition Layer behaviour 0 Rich is Silicon (Si), Oxygen (O) & Crust Aluminium (Al) - Granitic Cold, rigid outer shell 10-70 Lithosphere 100 Weaker layer with Asthenosphere some partially melted material Rich in Iron (Fe) & Magnesium 200 Mantle (Mg) - Peridotite Lower mantle Hot, solid layer 2900 Layer of flowing liquid Outer core metal Core Rich in Iron (Fe) & Nickel (Ni) 5100 Inner core Very hot, solid, dense layer 6370 3 PLATE TECTONIC THEORY Continental Drift A very long time ago, the continental plates were all joined together to form one super-continent called Pangaea. Over the last 200 million years, the continental plates have moved apart. Alfred Wegener developed the theory called continental drift in 1915. 4 There are many pieces of evidence that back up this theory suggested by Wegener: The Shapes Match - The continents look as if they were pieces of a giant jigsaw puzzle that could fit together to make one giant super- continent. The bulge of Africa fits the shape of the coast of North America while Brazil fits along the coast of Africa beneath the bulge. The Plants and Animals Match - Wegener noted that plant fossils of the same age found on several different continents were quite similar. This suggests that they evolved together on a single large land mass. To him, the presence of identical fossil species along the coastal parts of Africa and South America was the most compelling evidence that the two continents were once joined. The Rocks Match - Broad belts of rocks in Africa and South America are the same type. These broad belts then match when the end of the continents are joined together. Direct Evidence – Experiments carried out along rift valleys proved that the plates were moving apart in some places. These have been updated recently using laser measurements to calculate the rate of movement. A summary of some of the evidence 5 All of the World’s tectonic plates are moving in different directions. The places where they meet are called plate boundaries or plate margins. This early theory of continental drift was replaced by the more complex idea of sea floor spreading. This theory suggested by Harry Hess (1960) states that oceanic plates are being made and/or destroyed in various places, while the continental plates remain fairly constant. Evidence for this theory came from observations of the ocean floor in the Atlantic, where it was noticed that away from the central mountains the two sides were very similar. Also Iceland appears to be splitting along a central ridge. Iceland splitting along the Mid Atlantic ridge as the sea floor spreads. Sea floor spreading - as new oceanic plates are made by rising magma, the ocean floor increases in size. In 1963 Fred Vine and Drummond Matthews began to explore the magnetic changes seen in rocks on the ocean floor. From these ‘magnetic stripes’ they deduced that the oceanic crust must have been created either side of a central ridge. 6 The problem with all of these theories was that no one knew what was moving these huge continents and oceans. In 1965, John Tuzo- Wilson came up with the theory of Plate Tectonics. This for the first time suggested a method for the plate movement. Wilson noticed that in Hawaii there was a chain of volcanoes getting older and becoming extinct. The volcanoes in Hawaii get older to the north-west and the active ones are to the south-east. Wilson suggested that they were being moved over a ‘hotspot’ by convection currents below the solid crust. Exercise: 1) The island of Kauai is now 400km from the fixed ‘hot spot’. If the basalt lava flows on the island are 5 million years old, how fast has the Pacific plate moved? Remember: _____________ cm/yr-1 2) How old are the lava flows on Oahu, which is 250km away from the fixed ‘hot spot’? _____________ Million years Plate Movement Today we know that the movement of the tectonic plates is being driven by convection currents in the mantle. The crust is actually joined to the solid top layer of the mantle, together they form the lithosphere. This floats on top of the weak, partially melted layer underneath known as the asthenosphere. Heat rises from the lower mantle and moves around easily in the partially melted asthenosphere as convection currents. Convection currents driving plate movements 7 Plate Boundaries/Margins These are the tectonically active zones of the World. Here we find most volcanoes and earthquakes happening. Exercise: Shade in the active zones and mountain belts on the map below. 8 PLATE BOUNDARIES/MARGINS For each of the plate boundaries below label the key terms on the diagram, describe the processes and landforms created. A) Divergent (constructive) boundary At this boundary, plates are moving apart and new oceanic plate is being created. There is high heat flow from rising convection currents and partially melted mantle material generates basaltic magma. This is added to the edge of each plate. Label: * rift valley * abyssal plain * area of high heat flow * ridge mountains * basaltic magma * convection currents Processes: ______________________________________________ _______________________________________________________ _______________________________________________________ Landforms: _____________________________________________ _______________________________________________________ _______________________________________________________ Example: _______________________________________________ 9 Sea Floor Spreading This is the theory put forward by Harry Hess in 1960 that the ocean floor is spreading apart at mid-ocean ridges. New ocean crust is created as magma rises from the mantle. Therefore, rocks at the ridge are younger and get older further away from the central point. Sea floor spreading 1) Explain how new ocean crust is being made at a constructive (divergent) plate boundary. (3) __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ 2) Name an example of a constructive (divergent) plate boundary. ________________________________________________ (1) Every few million years the Earth’s magnetic field reverses so that a compass would no longer point to what we understand as North. A magnetic reversal 10 In 1963 Vine and Matthews noticed that as magma cools at the plate boundary the iron particles in the crystals align in the direction of the magnetic field at the time of their formation. However, if the magnetic field reverses they will be aligned in the opposite direction. These form magnetic ‘stripes’ on the sea floor of Magnetism preserved in the rocks of the sea normal and reversed polarity. floor Iron particles aligning as they cool 3) Name the type of plate boundary shown. __________________ (1) 4) Locations R and S in Figure 13 are 800km apart on the ocean floor. How fast are R and S spreading apart from each other? Show your calculation below: (2) Answer ________ cm per year 11 5) Which two of the following are associated with mid-ocean ridges such as this? Tick only two boxes. (2) Basalt pillow lavas Huge earthquakes Tsunamis High heat flow Regional metamorphism 6) Explain how the magnetic stripes in Figure 13 have formed. (4) __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ __________________________________________________________ 7) Which two features are always found at this type of plate boundary? Tick only two boxes. (2) Ocean trench Island arc Undersea volcanoes Rift valley Andesitic eruptions 8) Why is this pattern of ‘stripes’ symmetrical? (2) __________________________________________________________ __________________________________________________________ __________________________________________________________ 12 B) Convergent (destructive) boundaries 1) Continental-Oceanic (C-O) Here an oceanic plate moves towards a continental one. The denser oceanic plate is subducted and begins to partially melt into the mantle. This generates andesitic magma, which is very gassy and explosive. It is also viscous (sticky) so the gas can’t escape, building pressure creating explosive eruptions at cone volcanoes. Plutons of granite with form if the magma does not reach the surface of the continental plate. Label: * Partially melted material * Andesitic magma * Ocean trench * Cone volcanoes * Subduction zone * Zone of descending earthquakes Processes: ______________________________________________ _______________________________________________________ _______________________________________________________ Landforms: _____________________________________________ _______________________________________________________