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Transcript
Dynamic Earth Class 12 16 February 2006 Volcanic Imagination (Chapter 4) Exploring the Earth’s Interior How do we know about the Earth’s Interior? By studying Meteorites Direct observation (rocks originating from depth) Experiments at high pressure By studying earthquake waves (Seismology) Meteorites have struck the Earth in the past. Many are probably pieces of proto-planets similar in composition to Earth. Meteorites Accumulate Daily Meteorites Stony meteorites are rich in olivine and pyroxene Similar to Earth’s lithosphere Meteorites Iron meteorites are made of iron and nickel Earth’s interior (core) is similar Lafayette Meteorite Types of Meteorites - I Stones Primarily silicates (like Earth’s crust and mantle) >90% of all meteorites Irons Iron-nickel alloys Stony irons Combination of stony and iron meteorites Types of Meteorites - I Types of Meteorites - II Falls Meteorites observed falling to the ground Primarily stones (suggests they are more common) Finds Meteorites discovered on the ground Primarily irons (collected because they are unusual looking) Composition of Meteorites Chemical Composition of Earth QuickTime™ and a Animation decompressor are needed to see this picture. How do we know about the Earth’s Interior? By studying Meteorites Direct observation (rocks originating from depth) Experiments at high pressure By studying earthquake waves (Seismology) Large Volcanic Eruptions Voluminous volcanic eruptions Sample significant part of mantle Can infer something about mantle composition Kimberlites Rapidly injected rock Volatile-rich Often contain diamonds Known to form at high pressure - deep in mantle (>400 km) Hosted by mantle rock Kimberlites Kimberlites Sample Mantle Peridotite How do we know about the Earth’s Interior? By studying Meteorites Direct observation (rocks originating from depth) Experiments at high pressure By studying earthquake waves (Seismology) Diamond-anvil cell Diamond-anvil cell Multi-anvil Press Multi-anvil Press Seismology Study of the propagation of mechanical energy released by earthquakes. When energy is released, waves of motion (like the effect of a pebble tossed into a pond) are set up in the Earth. Structure of the Earth Seismic velocity (how fast earthquake waves travel through rocks) depends on the composition of material and pressure. We can use the behavior of seismic waves to tell us about the interior of the Earth. Seismic waves Waves are started because of initial tension or compression in the rock. Path of waves are curved because different rock types at different depths change speed at which waves travel Most common types of earthquake waves: P-waves and S-waves – Body waves Primary waves travel the fastest in the crust and usually are the first waves to arrive Secondary (or Shear) waves are slower and therefore take longer to arrive Three Main Types of Seismic Waves P-waves travel faster than S-waves, so they arrive at the recording station sooner Types of Seismic Waves Difference in traveltime for P and S waves tells us how far away the earthquake is from the recording station Fig. 16.8 Seismic Travel-time Curve Structure of the Earth Seismic velocity (how fast earthquake waves travel through rocks) depends on the composition of material and pressure. We can use the behavior of seismic waves to tell us about the interior of the Earth. Changes in P- and S- wave Velocity Reveal Earth’s Internal Layers Velocities generally increase in each layer Surface waves Rayleigh waves Love waves Refraction Reflection Refraction and Reflection of a Beam of Light Refraction and reflection of seismic body waves P-and S-wave Pathways Through Earth Travel paths for shallow seismic waves P-wave Shadow Zone S-wave Shadow Zone S wave shadow zone P wave shadow zone Seismology and Earth structure Layers of the Earth Earth’s CORE Outer Core - Liquid Fe, ~2200 km thick, No S-waves transmitted -> S-& P-wave Shadow Zones Inner Core - solid Fe (some Ni, Co, S, C), ~2500 km thick How do we know? Meteorites, Seismology, Magnetic field Earth’s Geodynamo Origin of Earth’s magnetic field: the geodynamo The basic idea: an electric motor is a dynamo Motion of the liquid outer core -- a conductor -in a magnetic field generates current The current generates a stronger magnetic field Origin of Earth’s magnetic field: the geodynamo Modeled Geodynamo Ocean crust records magnetic reversals Magnetic Reversals in the Ocean Magnetic Reversals …the inner core rotates faster than the mantle. Isostasy: Another key to Earth’s Interior Buoyancy of low-density rock masses “floating on” high-density rocks; accounts for “roots” of mountain belts First noted during a survey of India Himalayas seemed to affect plumb bob The less dense crust “floats” on the less buoyant, denser mantle Mohorovicic Discontinuity (Moho) Crust as an Elastic Sheet Continental ice loads the mantle Ice causes isostatic subsidence Melting of ice causes isostatic uplift Return to isostatic equilibrium Uplift Formed by Removal of Ice Sheet Northern hemisphere during the last glacial age Evidence of isostatic uplift after melting of ice sheet Uplifted beach ridges Earth’s internal heat Original heat Subsequent radioactive decay Conduction Convection Earth Formation QuickTime™ and a Animation decompressor are needed to see this picture. Temperature vs. Depth