Transcript Document
Revision topics – Plate Tectonics • Where does it fit in the specifications? • GL1 II Energy KI 3 Key terms: • Lithosphere, Asthenosphere, LVSZ. • Types of plate boundaries, convection currents. • Earth’s magnetism, Curie Point, sea floor spreading • Polar wandering curves LITHOSPHERE • Rigid part of Earth’s CRUST • and includes the topmost part of the MANTLE • Granite based composition ASTHENOSPHERE • Layer of upper mantle, just underneath the lithosphere • 5% molten • Plastic • Ability to flow and move LVSZ • LOW VELOCITY SEISMIC ZONE • In the Asthenosphere • Because it is <5% molten, it slows the seismic waves as they travel through them • seismic waves recorded at distances of 100 km to 250 km arrive later than expected (waves slowed) • while both the P and S waves travel more slowly, the S waves are weakened. • GeoScience • page 12 and 13 (lithosphere and asthenosphere) • Page 30 (strong and weak rocks) CONVECTION CURRENTS • Molten asthenosphere can move • As minerals in the mantle break down (radioactive processes) generate heat • Heat rises upwelling plume • Eventually the warmth will be lost and the cold mantle will sink • MANTLE PLUMES – new term TYPES OF PLATE BOUNDARIES • CONSTRUCTIVE (Diverging ) • DESTRUCTIVE (Converging ) • CONSERVATIVE CONSTRUCTIVE PLATE BOUNDARIES • • • • • • Plates are diverging Moving apart Due to upwelling convection current Drags plates apart BASALTIC magma rises to fill the gap left Key terms • • • • • • • • Upwelling convection current Basaltic magma Partial melting of mantle rock Thin oceanic crust Mid Ocean Rift (MOR) Mid Atlantic Ridge (MAR) Iceland Shallow seismic activity DESTRUCTIVE PLATE BOUNDARIES • • • • Plates are converging Moving together Due to downwelling convection current • Pushes plates together • One plate is pushed under the other in a SUBDUCTION ZONE DESTRUCTIVE PLATE BOUNDARIES 2 • Different types of collisions: • Ocean crust colliding with ocean crust (eg. Marianas trench) • Ocean crust colliding with continental crust (South America, Andes mountains) • Continental crust colliding with continental crust (Himalayas) Key terms • Downwelling convection current • Partial melting of subducted ocean crust • Andesitic volcanoes • Trench • Subduction zone, Benioff Zone • Andes mountains, Himalayas • Deep seismic activity Associated metamorphism • Intense squeezing of two continents together creates REGIONAL METAMORPHISM CONSERVATIVE PLATE BOUNDARIES • Plates rub alongside each other • No crust made or destroyed • San Andreas Fault line is best example • Think of rubbing two matchboxes together (sandpaper sides together!) • Friction “sticks” then sudden jerk movement (earthquake!) San Andreas Fault scar. Hot Spot • • • • Upwelling plume of magma Bit like a lava lamp! Best example Hawaii Hot spot stays still, plate moves over the hot spot and a chain of volcanic islands is created • Basalt Earth’s magnetism • Liquid OUTER CORE generates the magnetic field for the Earth • Earth’s magnetism is preserved in rocks • Iron minerals in the rocks line up and preserve the direction of the magnetic field How do rocks preserve the magnetic field? • When rocks are molten, the iron minerals are free to move around • As the rock starts to cool, these iron crystals ALIGN to the magnetic field of the time • This is LOCKED in when the rock solidifies Curie Point • The temperature at which the iron minerals ALIGN in the molten rock • Different for different rocks! Earth’s magnetic field • Is constantly moving • North Pole moves in a predictable manner (PRECESSION) Poles also FLIP • Due to currents in the liquid outer core – the N/S can be flipped • This has happened a lot in geological time • It is recorded in rocks as normally magnetised rocks and reversely magnetised rocks Polar wandering • Rocks which have the magnetic field preserved can be used to deduce their LATITUDE of formation • and also point to where the N Pole was Polar wandering • As the plates move, the direction to the N Pole changes • Graphs can be drawn which convert direction and distance into an easy to see pattern • These are called POLAR WANDERING CURVES Jigsaw fit • JLC updated March 2007.