Lecture #3 - Building the Earth: Earth's Interior (Dr. Spotila GEOS 1004)
Lecture #3 - Building the Earth: Earth's Interior (Dr. Spotila GEOS 1004) GEOS 1004
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This 9 page Class Notes was uploaded by Dylan Notetaker on Thursday September 1, 2016. The Class Notes belongs to GEOS 1004 at Virginia Polytechnic Institute and State University taught by Dr. Spotila in Fall 2016. Since its upload, it has received 8 views. For similar materials see Physical Geology in Science at Virginia Polytechnic Institute and State University.
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Date Created: 09/01/16
GS1004 Spotila 29 August, 2016 Lecture 3: Building the Earth: Earth's Interior Relevant reading: Interlude D, p. 282-294; Chp. 11, p. 365-373 Motivating questions: It took ~100 million years to grow the Earth by planetessimal accretion, including a “giant impact”. But, it didn’t look like Earth now. What did it look like and why? How did it evolve, to have the structure it has today? How do we know this structure? Gravitational accretion by meteorite impact converted potential energy into kinetic energy (heat). A meteorite traveling 20 km/s produces same energy as an equal mass of TNT. What did all of this heat do to the Earth? Softens and melts the Earth. Large bodies (even the Moon) are spheres because they are soft (hot) when they form (and gravity). What did we call this early Earth? “Hadean Earth” Differentiation: Earth differentiates by gravity by ~4.4 b.y. ago – dense liquid sinks to the center, light floats near the surface Change from homogenous to compositionally layered. INNER CORE: From center to ~1200 km (1% by volume), solid, consists of iron and nickel, ~5000 °C. OUTER CORE: From ~1200 to ~3500 km radius (16% volume), liquid, iron and nickel composition, 4000 °C. GS1004 Spotila 29 August, 2016 Why is the hotter inner core solid, while the outer core is liquid? Because of the phase relationship of iron alloy (state of phase versus temperature and pressure) Melting point of iron alloy increases with pressure; pressure increases with depth. The crossover point is at the inner–outer core boundary. How do we know the structure of the core? Radius of earth = 6370 km Deepest well = only 15 km deep 1) Earth has a magnetic field emanating from the core The geomagnetic field is generated by convection of the outer core around the inner core. Two conductive metals moving in the Sun’s electrical field, with high energy (heat). We call this a “dynamo”. LOOK-UP ITEMS: define "geomagnetic dynamo" or “dynamo theory”. Know the difference between modes of heat transfer; convection, conduction, radiation. The earth’s magnetic field drifts, and reverses (flips) every ~1 Myr. Aurora borealis; seeing the magnetic field. 2) Seismic waves: Waves produced by earthquakes transmit through the earth. By recording them on the other side (whether they show up, how fast they travel), we can tell about the nature of the layers at depth (i.e. liquid, solid, how soft it is, etc.). GS1004 Spotila 29 August, 2016 S-waves shadow: S-waves cannot go through liquid, and so cannot penetrate outer core and define where outer core starts. P-waves shadow: P-waves can go through liquid, but they bend as they go down (as velocity increases; this is called refraction). Since velocity decreases in the liquid core, they bend the opposite way, leaving another shadow zone. LOOK-UP ITEMS: what are P-waves and S-waves? What is different about them? 3) Material science; experiments at high temperature and pressure When we can simulate temperatures and pressures that exist lower down in the Earth, in the core. Enough about the core. What about the mantle? MANTLE: From 3500 to 6350 km radius (83% volume), solid, silicate (Si, O, Fe, Mg) composition, 3000 °C. Mantle rock = “peridotite”, made of Fe and Mg rich silicate minerals (Si and O 2 olivine and pyroxene. Mantle is solid, but ductile (i.e. easily flows and deforms) soft, and flows slowly over time A consequence of this is convection; transfer heat from bottom (~4000 °C) to Earth’s surface (10 °C). Mantle convection can be simulated by supercomputers. How do we know the structure of the mantle? Imaging seismic velocity structure; global “tomography” How is mantle convection possible (if it’s solid)? It’s a very slow process, and it is a viscous solid (cm/yr) GS1004 Spotila 29 August, 2016 What is the viscosity of the mantle, and how do we know? Almost like window glass, cm/yr LOOK-UP ITEMS: define "viscosity". Mantle motion is slow; cm/year, over a very long lengthscale (i.e. thousands of km). What you visualize as “convection” (like a lava lamp) actually takes hundreds of millions of years in the mantle. Consequences of convection: 1) the Earth cools, 2) Plate tectonics The Earth should have cooled in about 100 million years by conduction, but it’s obviously still hot (e.g. the core), and therefore dynamic. Why is the Earth so hot? Where does the heat come from? Radioactivity Question of the day: Which of the following is the current (i.e. 0.78 million years ago to today) “chron” for the geomagnetic reversal time scale? a. Olduvai b. Bruhnes c. Matuyama d. Gauss GS1004 Spotila 29 August, 2016 GS1004 Spotila 29 August, 2016 GS1004 Spotila 29 August, 2016 GS1004 Spotila 29 August, 2016 GS1004 Spotila 29 August, 2016
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