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Transcript
The Structure of the Earth The Earth’s Interior: Methodology Earth’s Interior • How do we know what it is like today? – Drilling • Most drilling includes the upper 7 km of crust • Deepest well: Soviet – 12 km deep – 5 yrs, 7 km; 9 yrs, 5 km; then it got stuck – Cost $100 million dollars – Temperature 190oC – – – – Volcanic Activity Laboratory experiments Meteorites Seismic Wave studies Meteorites: Evidence about the Interior • Meteorites – Fragments of asteroids and small early planets (planetesimals) that broke up as a result of collision with other solid bodies • Composition of Meteorites – Divisions • 1. Iron-nickel meteorites – From core of original bodies?? • 2. Stony meteorites – Original body area like our mantle • 3. Stony-iron meteorites – 3 subclasses: » Chrondrites » carbonaceous chondrites » achondrites – From the cores and mantle of asteroids & planetismals Iron-nickel meteorites 90% iron • 9% nickel • . Stony Meteorite Stony-iron meteorites Seismology: Evidence of the Earth’s Interior • Seismology – The scientific study of earthquake – The key to understanding the Earth’s interior Seismic Waves • What are they? – Vibrations in the body of the Earth • How are they generated? – Naturally • Earthquakes • Volcanoes • Impacts – Artificially • Explosions • Mechanical devices Seismic Waves: Measuring • Seismograph – Thousands of stations are set up worldwide – Used to locate earthquake – Simple pendulum • When the ground shakes, the base and frame of the instrument move with it • Inertia keeps the pendulum in place • It will then appear to move; relative to the shaking ground • As it moves, it records the pendulum displacement • The tracing is called a seismogram – Networks of seismograph stations are used to determine the location of the earthquake • Seismology and the Science of Earthquakes – Waves – Magnitude – and the Richter Scale • (3:22 minutes) Seismic Waves: Terms • Earthquake – Release of stress build up along a fault – Slipping and moving of rocks along a fault • Focus – The origin of seismic waves – The energy of seismic waves is released from this point • Epicenter – The point on the earth’s surface directly above the focus • Fault – Crack or break in the earth’s crust – Surface along which rocks move • Waves: – P waves – S waves – L waves Seismic Waves • 3 Main Types released by Earthquakes: 1. P-waves (Primary / Compression waves) • Longitudinal / compressional wave – Alternately push (compress) and pull (dilate) the rocks through which they pass » Squeezes and stretches the material in the same direction – Generated by the Focus • Fastest of the waves • Propagates through solids, liquids and gases 2. S-waves (Secondary / Side-to-side waves) • Transverse waves – Generated by the Focus • Slower wave • Moves material at right angles to the wave direction • Propagate only solids Animation of P and S Waves • http://www.classzone.com/books/earth _science/terc/content/investigations/es 0402/es0402page05.cfm • http://www.classzone.com/books/earth _science/terc/content/visualizations/es 1002/es1002page01.cfm?chapter_no= visualization Visualization of an Earthquake • http://www.pbs.org/wnet/savageearth/a nimations/earthquakes/flash.html 3. L-waves (Long waves or Love Waves) • Surface waves that cause horizontal shearing of the ground – Travel on the surface of the earth and shake rocks sideways as they move across the surface – Generated by the epicenter – Particles travel in a rolling motion – Note: • Surface waves cause the most damage • Not used to determine Earth’s interior Locating The Epicenter • P and S waves travel at different rates – This helps determine the distance to the epicenter • P waves will always arrive at a seismic station first – The distance of the P wave ahead of the S wave depends on how far away the earthquake is • Key: The further away the epicenter, the wider the gap between the P and S waves. Earthquake • http://earthquake.usgs.gov/eqcenter/ • Predicting Earthquakes • Video 2 minutes • Earthquakes • Video 3 minutes Seismic Topography Volcanoes: Evidence of the Earth’s Interior • Referred to as “Windows into the Earth’s Interior.” Why? • Volcanoes erupt material that is tens to hundreds of kilometers into the Earth – Deepest rock samples of the Earth’s interior are volcanic rock Importance of Earth Hot Interior • Seismic activity – earthquake, volcano • Magnetic field (Geomagnetism) – Generated by the movements of the molten iron and nickel layer in the outer core – Convection current in this electrically conducting fluid act as a Dynamo* • Fluid acts to generate and maintain the magnetic field • * Dynamo theory – Process through which motion of a conductive body in the presence of a magnetic field acts to regenerate the magnetic field – Note: Without this Dynamo effect, the magnetic field would die out within 10 000 years Importance of Earth Hot Interior • Interior heat powers convection current of the asthenosphere which in turn allows the lithosphere to move (plates moving) – Result: • • • • formation of mountain ranges deep sea trenches Volcanic belts Earthquake zones • May be linked to long-term climatic changes