Birth Date of a Planet?

Transcription

1 Save the whales. Collect the whole set Plan to be spontaneous tomorrow Life is too short not to be in a hurry Oceanography Lecture 4 Defining Boundaries: 2) Plate Tectonics I 1. Review 2. Intro to Plate Tectonics 3. Isostasy Review Defining Boundaries: 1) Marine Provinces 2) Intro to Plate Tectonics 1. Marine Provinces Continental Margins (Shelf,( Slope, Rise) Deep-Ocean Basins (Abyssal( Plains, Abyssal Hills, Seamounts, Trenches) Mid-Oceanic Ridges 2. Intro to Plate Tectonics Differentiation and formation of distinct layers Isostasy (Ocean vs.. Continental Crust) Continental Drift and Sea Floor Spreading Evidence Birth Date of a Planet? Based on long-lived radioisotopes the! Age of Earth is about B. years! Age of meteorites: B. years

2 So we have a pebble in the sky So now we know its density Whereby density = M/V M = 5.95!10 27 g V = 1.08! m 3 So average density of Earth = 5.5 g/cm 3 From astronomy we know its mass: M And its volume: V (Erastothenes,, ~200 B.C., calculated its circumference almost to the dot!) So what s s next? You are a curious scientist: You establish a hypothesis and set off to put it to the test Hypothesis testing Density of surface rocks? Continents = 2.7 g/cm 3 Oceans = 2.9 g/cm 3 Does the surface density equal that of the Earth? Hypothesis testing We are assuming that all of Earth is homogeneous. Can we test this? We need to define seismic waves: Compression Shear Measurement " Prediction Hypothesis is WRONG! (unless there is something wrong with premises )

3 Seismic waves The velocity of waves will change depending on the state of the medium Velocity (speed)! as density! Note: : for most substances, density! along the gas " liquid " solid continuum. Valid for compression waves but not for shear waves Shear waves do NOT propagate through liquids So let s s look at out hypothesis then: Earth is homogeneous in composition. Seismic waves (that s s true for most substances water being a fundamental exception: Ice floats!) So we might also say that velocity of a wave! from liquid to solid state Yes and No! 1) Homogeneous composition Source 2) Central higher density (but solid) Source Propagation Propagation

4 3) Changes in density and phase Propagation Source Speed and direction of propagation The change in Speed and direction of propagation tell us something about the 1) State of internal parts of the Earth 2) Density of these parts We call these parts Layers (envision a big onion) Density So we have a physical layering that indicates density changes But how can we make up the high density of Earth? (remember Measurement " Prediction) Density and chemistry: Volume of atoms change relatively little ( cm) Mass changes a lot (1->250) The more nuclear particles in an atom the higher its density! Density Then, if we discriminate by concentrating - heavy elements inside the Earth - light ones at the surface Then we may explain why 1) We only find low density rocks at the Earth surface 2) The average density of the Earth is so much greater than that of its surface! Average Layer Depth (km) Density (10 6 g/m 3 ) Mass (g) Volume (m 3 ) % Total Mass % Total Volume Whole Earth E E % % Crust E E % 0.80% Mantle E E % 83.00% Core E E % 16.20%

5 Meteorites! Two main compositional types Iron meteorites: 93% (Fe and Ni) Stony meteorites: 7% (Fe-Mg silicates) Combined composition very similar to Earth Differentiation We then need a mechanism to explain what we call differentiation. Why do liquids of different densities (oil and vinegar) separate? " Gravity But can we differentiate two solids? If we turn them into liquids! As temperature increase " melting point (For example, Fe melts at around 1530 C) Differentiation But we have a little problem: Pressure! As Pressure increase!, melting point! (at higher internal pressures, Fe melts at >2000 C) Density (10 6 g/cm3) T of Earth Interior vs. melting point of Iron Depth (km) Energy Sources 1) Accretion: : Impact of planetessimals by which motion energy (kinetic) is turned into thermal energy (thermal). 2) Self-Compression: : As the Earth gets bigger, the gravity forces the mass to contract into a smaller volume " produces heat. 3) Differentiation: : Conversion of gravitational potential energy into heat during core formation. 4) Short-lived (now extinct) radioisotopes: surrounding material absorbs the energy released during radioactive decay (e.g. nuclear power plants)

6 An Introduction to Plate Tectonics Originally, the elements segregated during the formation of Earth (density fractionation ) " Differentiation and formation of distinct layers seemed to have taken very little time 100My Core 31.5% (Ni, Fe) Mantle 68.1% (Mg, Fe, Si,, O) Crust 0.4% (Al, Si,, O) Physical differentiation? But remember, not all of it is molten As Pressure increase " melting point! (at higher internal pressures, Fe melts at >2000 C) So liquid iron only occurs where temperatures are high enough but pressures are low enough But what does it mean to have a liquid core? But what does it mean to have a liquid core? The movements of liquid metal in the outer core (eddies) generate a process akin to a dynamo that generates a magnetic field on Earth (and reversals ) As the result of the Earth s rotation, the molten iron flows continually around the solid inner core " forming a magnetic field

7 And how does that change our lives? - First of all it protects us from solar winds - But then it provides evidence for differentiation and molten core - Also provides evidence for Plate Tectonics What does it mean to have a plastic Mantle? Principle of Isostasy It s a matter of balance Ocean Crust vs. Continental Crust. For next time!! Continental Drift! Plate Tectonics! Read Chap. 3