Lecture 4: Seismic Waves, Discontinuities, Earth's Magnetism PDF

Previous Class – This Class – 1. Earth and its 1. Seismic waves birth 2. Discontinuities 2. The 3 S 3. Earth’s Magnetism 3. Geoid 4. What lies under our feet? Is the inner core growing? Grows about 1mm/year....

Previous Class – This Class – 1. Earth and its 1. Seismic waves birth 2. Discontinuities 2. The 3 S 3. Earth’s Magnetism 3. Geoid 4. What lies under our feet? Is the inner core growing? Grows about 1mm/year. Grows as liquid outer core solidifies/cools However, this growth is not uniform  Seismic Waves - Tools for understanding the underground Seismic wave are vibration generated by an earthquake, explosion, or similar energetic source that travel within the Earth or along its surface. Seismographs record the amplitude and frequency of seismic waves and yield information about the Earth and its subsurface structure. Aritificially generated seismic waves use for hydrocarbon (oil or gas) prospecting  Two main types of seismic waves P waves ▪ Primary waves/ compressional waves/ pressure waves ▪ First ones to reach the recording station ▪ Cause ground to compress and expand ▪ Particles move parallel to the direction of propagation ▪ Through solid and liquid ▪ Slows down in liquid S waves ▪ Secondary waves/shear waves ▪ Particles move perpendicular to the direction of propagation ▪ Move the ground up and down or side and side ▪ Passes through solid only ▪ Slower than P waves  Seismic shadow zone ▪ Area of the earth that cannot be recorded by P and S waves ▪ P waves don’t appear between angular distances 103˚ and 140 ˚ from the epicentre – why? ▪ S waves don’t appear beyond an angular distance of ~103˚ from the epicentre – why? P waves refracted by the liquid outer core S waves cannot pass through liquid outer core With this liquid outer core was discovered in 1900s Earth’s interior is inhomogeneous – differences in chemical and physical properties Discontinuities in the Earth’s interior Earth’s Magnetic Field /Geomagnetic field ▪ Fe, Ni, Co – magnetic A magnet's north pole points to Earth’s magnetic north pole. How can this happen?  Geodynamo The mechanism responsible for the generation of the Earth's magnetic field An interior dynamo requires the circulation of liquid ferrous metals in the core – where do you find them? Why not convection in the mantle - why outer core? The ferrous metals most easily magnetized are iron, nickel, and cobalt – all present in Earth’s core The geomagnetic poles are not the same as the North and South Poles – Is this true? Rotational axis passes through N and S poles Axis of earth’s magnet is aligned at about 11.5 to the rotational axis. Is it a permanent magnet inside Earth? Why not? Earth’s magnetic poles often shift, due to activity far beneath surface. Rocks preserve position of E’s geomagnetic poles in the past – How? The magnetic field is preserved when lava cools and becomes solid rock - strongly magnetic particles within the rock become magnetized by Earth’s magnetic field. These magnetic records also show that the geomagnetic poles have reversed – changed into the opposite kind of pole— hundreds of times since Earth formed. Magnetosphere  Magnetosphere Solar wind, charged particles from the sun, presses the magnetosphere against Earth on the side facing the sun and stretches it into a teardrop shape on the shadow side. Northern Lights  How are auroras formed? The magnetosphere protects Earth from most of the particles - some leak through it. When particles from the solar wind hit atoms of gas in the upper atmosphere around the geomagnetic poles, they produce light displays called auroras. Northern Lights or … Southern Lights or …