GEOS218 Geological Disasters & Society: Earth Formation PDF

GEOS218 Geological Disasters & Society Unit 2: Earth – formation and structure Formation of the Solar System Internal Energy Radioactivity The Layered Earth 1 On-Line Lecture n Part 1a – Energy (which drives disasters) in...

GEOS218 Geological Disasters & Society Unit 2: Earth – formation and structure Formation of the Solar System Internal Energy Radioactivity The Layered Earth 1 On-Line Lecture n Part 1a – Energy (which drives disasters) in the Earth System n Part 1b – Formation of the Earth & Solar System n Part 1c – Earth differentiates into Layers n Part 1d – Layers: Chemistry vs Physics 2 1  The physics: follow the energy n Natural disasters result from rapid release of concentrated energy. n Where does the energy come from, how is it concentrated and where does it go? 3 Origin of the Sun and Planets Solar system began as rotating spherical cloud of gas, ice, dust and debris Cloud contracted, sped up and flattened into disk due to gravity Sun and Planets formed at same time, ~4.6 billion years ago: 4 2  Origin of the Sun and Planets Solar system began as rotating spherical cloud of gas, ice, dust and debris Cloud contracted, sped up and flattened into disk due to gravity Sun and Planets formed at same time, ~4.6 billion years ago: 5 Origin of the Sun and Planets Sun and Planets formed at same time, ~4.6 billion years ago: Processes of planet formation created huge amounts of heat 6 3  Earth History Earth began as aggregating mass of particles and gases – Aggregation took 30 to 100 million years – Occurred nearly 4.6 billion years ago Processes of planet formation created huge amounts of heat – Impact energy – Decay of radioactive elements – Gravitational energy – Differentiation into layers 7 Three main sources of Earth’s internal heat - summary n Accretion heat – kinetic energy of bolides converted to heat on collision. Not clear how much of this is left (probably not much). n Gravitational heat – gravitational energy converted to heat when the core separated & sank to the middle. Also not clear how much of this heat is left (again, probably not much). n Radioactive heat – from the slow decay of Uranium, Thorium & Potassium-40 inside the Earth. This is probably the main source of heat through later geologic time & today. 8 4  Internal Sources of Energy 1. “Primordial” (early) heat a. impact energy: as things hit Earth, Earth heated up b. gravitational energy: as heavy things sank and compressed (esp. forming core) Earth heated up Heat (a little, at least) from both of these still flows to surface today, conducting very slowly through rock. Most of heat flowing to surface today is from … 2. Radiogenic heat... n Isotopes: same element, different numbers of neutrons (n) n Radioactive isotopes are unstable and decay to stable isotopes, releasing heat & energy n Types of nuclear decay: n Alpha decay, Beta decay, Electron capture 9 Radioactive decay n Radioactive atoms decay and release heat n U, Th, and K are the most Half-life important n In very early Earth, many short-lived radioactive elements were around—so it was (a lot) hotter! § half-life: length of time for half the radioactive parent atoms to decay to daughter atoms 10 5  Radioactive decay n Substitute coins for atoms. n Throw one hundred coins, remove all those that come up tails, place them in a pile, repeat—you've got yourself a hands-on model for radioactive decay. n The piles graphically show the meaning of the term “half-life.” § 100 coins to start, ~50 after 1 flip, ~25 after 2 flips, etc. where each “flip” is a half-life 11 Study Question Radioactive decay After 2 half-lives, there will be how much of the radioactive element left? t A. 1/2 l ef 0% B. 1/4 =5 lf l- ife l ef t C. None 1 ha 5% =2 D. 3/4 es lf -l i v a 2h 12 6  Study Question Radioactive decay After 2 half-lives, there will be how many of the daughter atoms will there be? A. 1/2 B. 1/4 If 25% of the “parents” are left, then 75% have C. None turned into daughters. D. 3/4 13 In Greater Depth: Radioactive Isotopes Elements are defined by the # of positively charged protons Every atom of Hydrogen has 1 proton, all Helium atoms have 2 protons, every Lithium atom has 3, etc. Up to Uranium which all have 92 protons Some elements can have different numbers of neutrons – these are called isotopes Some isotopes (but not all) are radioactive 14 7  In Greater Depth: Radioactive Isotopes All atoms of hydrogen have 1 proton: – The 3 isotopes of hydrogen have different numbers of neutrons (zero, one or two) 1H 2H 3H Normal Hydrogen and Deuterium are NOT radioactive – they do not decay – each atom will last forever! Tritium atoms are radioactive – half of these decay into helium atoms every 12 years 15 In Greater Depth: Radioactive Isotopes Elements are defined by the # of positively charged protons – Isotopes are different forms of the same element with different numbers of neutrons: – e.g. 235U has 92 protons and 143 neutrons – while 238U has 92 protons and 146 neutrons – Radioactive isotopes are unstable and release energy through their decay process to more stable isotopes Knowing the half-life of radioactive isotopes allows us to use their quantity as a clock to date rocks uranium 238 U thorium protactinium 16 8  n Oldest Solar System materials are 4.57 billion years old n Oldest Earth rocks (found in northwest Canada) are 4.055 billion years old n Oldest Earth materials (zircon grains from Australian sandstone) are 4.4 billion years old 17 Earth History: geology vs human Analogy with Mother Earth, 46-year-old woman: (1 Mother Earth year = 100 million geologic yrs) (1 ME day = ~274,000 yrs; 4 ME days = 106 yrs = 1 million yrs) n First 6-7 years unaccounted for (zircon “baby pics” at ~2yr, 4.4 bya) n 11 years old: single-cell life appeared in ocean (3.5 bya) n 26 years old: oxygen accumulates in ocean and atmosphere (2 bya) n 36 years old: first animals evolve in ocean (1 bya) n 42 years old: life appeared on land (incl. amphibians, ~400 mya) n 43.8 yo = 2.2 years ago: dinosaurs evolved (220 mya) n 44.7 yo = 1.3 years ago: flowering plants (130 mya) n 7.8 months ago: dinosaurs died out (65 mya) n About 2 weeks ago: human ancestors evolved (4-5 mya) n Yesterday: modern humans evolved (250,000 years ago) n Last hour: discovered agriculture, settled down (11,000 years ago) n 1 minute ago: Industrial Revolution (190 years ago) n About 1.5 seconds ago = pandemic à 1 second = ~3.2 years 18 9  Three main sources of Earths internal heat - review n Accretion heat – kinetic energy of bolides converted to heat on collision. Not clear how much of this is left. n “Gravitational heat” – gravitational energy converted to heat when the core separated & sank to the middle. Also not clear how much of this heat is left. n Radioactive heat – from the slow decay of uranium, thorium & potassium-40 inside the Earth. This is probably the main source of heat through later geologic time & today. 19 Internal Sources of Energy n Initially (4.6bya) the sum of internal energy from impacts, gravity and radioactive elements was big n Internal temperature has been declining since early Earth maximum, but still causes plate tectonics, EQs and volcanic eruptions 20 10  On-Line Lecture n Part 1a – Energy in the Earth System n Part 1b – Formation of the Earth & Solar System n Part 1c – Earth differentiates into Layers n Part 1d – Layers: Chemistry vs Physics 21 Earth differentiates into: crust, mantle, and core heavy (more dense) elements/minerals sank toward middle light (less dense) elements floated up toward surface 22 11 The Earth is Differentiated into layers of increasing density But what is density? Old Riddle: Which weighs more: a ton of bricks or a ton of feathers? 23 What is density? A ton is a ton – both weigh the same! The pile of feathers is just much bigger. Density is the relationship between Mass and Volume 24 12  Density is the ratio between mass & volume or number & volume mass/volume or #/volume (more dense = less space between) 25 Density (kg/m3) = Mass/Volume Mass (kg) (kg/m3) = Density * Volume Volume (m3) (kg/m3) * m3 = Mass/Density kg / (kg/m3) 26 13  The Layered Earth (density) Physical Differences Chemical Differences 27 Isostasy Less dense materials float (are buoyant*) on top of more dense materials (e.g., iceberg floats in/on ocean) When partially melted, low-density rock will float up through higher density rock *Buoyancy = density relative to the environment 28 14  Earth is a series of density- Layering/Isostasy n stratified layers n Atmosphere – least dense n ~0.00125 g/cm3 n Oceans – water n ~1.03 g/cm3 = 1030 kg/m3 n Continental crust – granite n ~2.7 g/cm 3 n Oceanic crust – basalt n ~3.0 g/cm 3 n Mantle – olivine n ~5.7 to 3.3 g/cm 3 n Core – metal Note the 2 different crusts! n up to 16 g/cm3 29 Comparison of OC & CC Earth’s Crusts Oceanic Continental Ave Mafic Felsic Composition Fe, Mg & Ca Si, K Density 3000 kg/m3 2700 kg/m3 ~4 km below ~1 km above Avg Elevation sea level sea level Avg Thickness 8 km 30 – 35 km What are the implications of this difference? 30 15  Isostasy - the balance between gravity & buoyancy Oceanic crust floats lower than continental crust They float on more dense mantle. Mountains on continents are regions of thicker lower-density crust. Mountains have roots! 31 Depth (m) The Layered Earth: 0 Crust is 5-60 km thick Composition/Chemistry n Core: metallic, very dense 2900 (10-16 gm/cm3); ~1/2 of Earth radius n Mantle: solid rock, 3 to 6 6371 gm/cm3; ~1/2 of radius n Crust: solid rock, lighter (2.7 to 3 gm/cm3; very thin (5-8 km oceanic, 10-60 km continental) This density layering is important, but physical strength layering controls plate tectonics 32 16  Behavior of materials Elastic - recovers Ductile (plastic) - deforms Brittle - breaks 33 Earth Structure: Mechanical behavior Brittle: Material breaks or fractures Ductile: Material flows or bends (Properties depend on composition and temperature) 34 17  Layered Earth https://www.iris.edu/hq/inclass/animation/layers_of_the_earth https://www.youtube.com/watch?v=UD7GHzIRI-s Layers of the Earth—What are they? How were they found? 35 Plate vs Crust? https://www.iris.edu/hq/inclass/animation/take_2_plate_vs_crust https://www.youtube.com/watch?v=zQLN-SigAv4 Crust vs. the Tectonic Plate (What's the difference?) 36 18  What is a Plate? Geochemical/Density Layers Physical Layers Temperature Surface Crust = low density minerals Lithosphere = crust + uppermost (mostly O & Si) mantle à RIGID layer = PLATE! Cool ~ 100km thick Asthenosphere = w/in the mantle MUSHY (like silly putty), can flow! ~ 100km thick Depth Mantle = high density minerals (O & SI + Fe & Mg) Lower mantle = RIGID ~2500 km thick Core = metals (iron/nickel) Outer Core = LIQUID HOT Inner Core = SOLID Layer thickness NOT to scale! Layer thickness NOT to scale! 37 Strength Layering: Plate Tectonics Lithosphere (strong!) § Crust + Uppermost Mantle § Brittle – will break under stress § ~100 km thick Lithospheric plates overlie (float on) asthenosphere § Asthenosphere is solid (weak, not molten!) § Plastic – flows like Silly Putty n Asthenosphere is 100-300 km thick n Asthenosphere is entirely within mantle 38 19 Continental Crust vs Oceanic Crust LITHOSPHERE (~100km) LITHOSPHERE (~100km) Continental Crust Ocean Crust 30-35 km thick ~7km thick Granite Basalt Lower density: ~2.7 g/cm3 Higher density: ~3 g/cm3 Mantle Mantle Asthenosphere 39 The Layered Earth: Summary Plate Tectonics Layering Chemical Layering n strong lithosphere over n Crust weak (not molten!) n Mantle asthenosphere n Core 40 20  How does the internal energy of the earth cause disasters? It drives plate tectonics! Next Time! 41 21