Lecture 3: Birth of Earth, 3S, Geoid PDF

Previous Class – This Class – 1. Earth System 1. Earth and its birth 2. Introduction 2. The 3 S to Earth Science 3. Geoid 3. Uniformitaria 4. What lies under nism our feet? 4. Earth and its birth Books ▪ Press, F., and Siever, R., 2001,...

Previous Class – This Class – 1. Earth System 1. Earth and its birth 2. Introduction 2. The 3 S to Earth Science 3. Geoid 3. Uniformitaria 4. What lies under nism our feet? 4. Earth and its birth Books ▪ Press, F., and Siever, R., 2001, Understanding Earth (3rd Edition), Freeman and Co. Ltd. ▪ Grotzinger, J., and Jordan, T., 2014, Understanding Earth (7th Edition), Freeman and Co. Ltd. ▪ Fowler, C. M. R., 2004, The Solid Earth: An introduction to Global Geophysics (2nd Edition), Cambridge University Press. Exam dates ▪ Quiz 1 – Sep 17-20, 2024 ▪ Mid-semester exam – Oct 14- 21, 2024 ▪ Quiz 2 – Nov 5-8, 2024 ▪ End-semester exam – Nov 24 – Dec 2, 2024  Earth as a planetary system 1.Third planet 2.Only planet “known” to have life 3.Age ~ 4.54 billion years 4.Distance from Sun ~ 1.496x108km 5.Radius ~ 6371km 6.Avg. density ~ 5.513 g/cm3 7.Some more??? Let’s watch this video by NatGeo!!! https://www.nationalgeographic.org/video/earth-101/ Origin of planets? Planetesimals Protoplanets Planets You know about the theories of origin of Earth (or planets)? 4 main - Nebular Planetesimal Tidal Big Bang Nebular Hypothesis Immanuel Kant, 1755 and Pierre Laplace, 1796 Solar System formed from rotating cloud of hot gases – nebula Gas cooled gradually nebula shrank – rotated – disk shaped Centrifugal force and gravitational force – rings of gases (forms planets and satellites) and centre of the nebula (forms the sun) Planetesimal Hypothesis Forest Moulton and Thomas Chamberlin, 1900 Solar System formed when passing star almost collided with Sun Gravitational attraction created a bulge and mass ejected from Sun – dirty snowballs - planetesimal Planetesimals accreted forming planets Tidal Hypothesis James Jeans and Harold Jeffreys, 1900s Solar System formed when an ‘intruding star’ came close to the ‘primitive sun’ Gravitational force – tide created on surface of ‘primitive sun’ – cigar-shaped filament ejected – thicker centre, thinner ends Cooling and contraction – broken down to several pieces - formed planets – bigger planets at centre and smaller ones towards ends Rest of primitive sun made the present-day Sun Big Bang Hypothesis Aleksander Friedmann and Georges Lemaître, 1920s Solar System formed from a hot, dense point 13.7 billion years ago – violent explosion – since then universe is expanding Eventually sub-atomic particles created with cooling Later, elements ……starting with He and later H and other elements Videos related to The Big Bang https://www.youtube.com/watch?v=HdPzOWlLrbE https://www.youtube.com/watch?v=nVsHjnY-o9s Earth’s Size Radius ~ 6371km Difference between Dp and De? Earth’s Shape Oblate spheroid/ellipsoid – Earth shape bulges at the equator and flattens at the north and south pole. Why this shape? The centrifugal force generated by the Earth’s rotation pushes mass away from the axis of rotation, leading to a flattening effect at the equator Earth’s Surface ▪ Not at all smooth ▪ Highest point – Mt. Everest (8848m) ▪ Deepest point – Have you read about the most Challenger’s Deep recent dive to Challenger’s (10935m) Deep? Dawn Wright and Victor Vescovo So, Earth is not a perfect spheroid or ellipsoid!! Many mountains and trenches Geoid The equipotential surface of the Earth’s gravity field which best fits the global mean sea level Perpendicular to the direction of the gravitational pull of the earth. Since the mass of the Earth is not uniform at all points, the magnitude of gravity varies, and the shape of the geoid is hence irregular. For more on the concept of geoid https://www.britannica.com/science/geoid/The- concept-of-the-geoid Which is the farthest point from Earth’s center? Which is the farthest point from Earth’s centre? But, wait…!! Why Mt. Chimborazo? Where is it? When did we get to know the height of In 1802, Humboldt and Bonpland Mt. Everest? And ascended Chimborazo, a volcano who measured it? just below the equator that was believed at the time to be the highest mountain 1850s by Radhanath Sikdar  Earth’s Structure What lies under our feet? 4 main layers- Crust Mantle Outer Core Inner Core Tarbuck and Lutgens, 2012 Crust – 0.4% of Earth’s mass contains us and all known life in the universe!!!  Crust Continental crust Oceanic crust t = 35-40 km t = 7-10km Granitic composition Basaltic composition ρ = 2.6-2.7g/cm3 ρ = 2.8-3.0g/cm3 Older crust Comparatively younger Age of oldest continental crust and that of oceanic crust? Why this density difference? Mantle (t~2900km) 1. Upper Mantle 2. Transition zone 3. Lower Mantle Mantle - 84% of Earth’s volume  Mantle Upper Mantle Upto ~410km Mostly solid Crust + upper mantle = lithosphere (up to 200km)– tectonic plates/lithospheric plates, brittle, fractures at low temperatures Asthenosphere lies below lithosphere – ductile behaviour Temperature range 500-900ºC  Mantle Transition zone 410 to 660kms under the Earth’s surface Solid-solid phase changes - crystalline structure changes here Composition remains same but the crystalline structure changes.  Mantle Lower Mantle From 660km to 2900km Hotter and denser – less viscous than upper mantle and transition zone Temperature ~4000ºC  Core Outer core Inner core 2900 – 5100km 5100 – 6371km Liquid Solid Magnetic field Why is the inner core solid? 4400ºC - 6000ºC Main source of heat in the core – Decay of radioactive elements Physical layering and compositional layering  Elemental abundance in bulk earth– Fe>O>Si>Mg>Ni>Ca>Al>S Elemental abundance in earth’s crust – O>Si>Al>Fe>Ca Elemental abundance in solar system – H>He>O>C>N Temperature inside Earth increases with depth – Geothermal gradient ~25 deg Celsius/km