The Earth February 26, 2013
The Planets 2
How long ago did the solar system form? Definition: Cosmic Rays High-energy particles that constantly bombard objects in space Mostly they are hydrogen nuclei (protons), helium nuclei and electrons traveling at close to the speed of light Many produced by Supernovae Interact with particles in Earth s atmosphere, cause a cascade of other particles This interaction produces Carbon-14, 14 C which will decay (is unstable) 4
Radioactive Decay Radioactive decay - atomic nucleus spontaneously loses energy Parent nuclei transforms to daughter nuclei We know the decay rate
Radiometric Dating Calculating the absolute age of material by measuring the abundance of a radioactive isotope relative to a stable isotope Example: Radiocarbon dating
Exponential Decay and Radiometric Dating Each type of radioactive nucleus decays at its own characteristic rate, called its half-life Half life is the time when 1/2 of the original amount of material is left
Some Naturally Occurring Radioactive Isotopes and Their Half-Lives Radioactive Isotope (Parent) Radioactive Isotope (daughter) Half-Life (years) Uranium - 238 Lead - 216 4.5 Billion Potassium - 40 Argon - 40 1.26 Billion Uranium - 235 Lead - 207 0.7 Billion Carbon - 14 Nitrogen - 14 5730
Radiocarbon Dating Assuming Cosmic Ray flux is constant Know how much 14 C is produced Compare with 12 C (stable) In the atmosphere: Ratio ( 14 C/ 12C) atmosphere = constant = 1 part per trillion Measure ( 14 C/ 12 C) sample The ratio of (sample/constant) give age
Radiocarbon Dating at UA Dead Sea Scrolls 200 B.C. to 68 A.D. Shroud of Turin 1300 A.D.
Radiometric Dating of Rocks Can tell when a rock solidified: Uranium has a half life of 4.5 billion years Oldest rocks on Earth: Acasta gneiss in Canada s Northwest Territories are 4.03 billion years old (Gyr) Oldest mineral grains on Earth: Zircon grains from Western Australia 4.3-4.4 Gyr Because plate tectonics are active Earth Rocks are not a good indicator for Solar System age
Radiometric Dating of Meteorites Oldest rocks in the solar system are meteorites (pieces of asteroids that land on Earth) Radiometric dating of meteorites reveals that they are all nearly the same age - 4.56 Gyr 4.5 Gyr taken to be the age of the Solar System (and Earth)
Age of Earth: 4.5Gyr how do we know? radiometric dating of meteorites (formed when solar systems was forming) 13
Earth s Internal Structure Temperature and pressure increase with depth Inner core is solid and made mostly of Iron Outer core is molten (liquid) and made of Iron Mantle is solid and made of iron, magnesium and other minerals
Earth s Internal Structure Inner Core -solid 80% iron, 20% lighter (Ni,Ox,S) Pressure keeps it solid Molten outer core -liquid Same composition as inner core Lower mantle - convective Upper mantle - solid but weak Asthenosphere upper part of upper mantle, more brittle Lithosphere - crust and upper asthenosphere Floats on top of upper mantle Region of earthquakes, mountain building, volcanoes, continental drift
Surface of Earth - A Jigsaw Puzzle? Alfred Wegener (1915) Observations Continents on either side of Atlantic ocean fit together like pieces of a puzzle Fossils of particular species found in both South America and Africa Glacial sediments in South America, Africa, Asia, India, Theory: Continental Drift - later Plate Tectonics - not accepted until 1960s
Plate Tectonics Theory to explain movement of Earth s lithosphere Lithosphere broken into 8 tectonic plates Boundaries of plates are regions of earthquakes, volcanoes, mountain building, seafloor spreading Movement of plates few cm/year Three types of boundaries: -Divergent -Convergent -Transform
Plate Boundaries Divergent Boundaries - regions are spreading sea floor spreading Convergent Boundaries - boundaries are colliding subduction zones Transform - slipping
Plate Tectonics -Modern Evidence Mid Ocean ridges continents do not just move through the sea sea floor spreads carrying continents with it
Plate Tectonics -Modern Evidence Subduction Zones Pacific Ring of Fire- Region of large numbers Of Earthquakes and Volcanoes 75% of Worlds active And dormant volcanoes 80% of World s largest earthquakes
Plate Tectonics - Modern Evidence Earthquakes at plate boundaries - San Andreas Fault - transform
Convection 22
What Drives Plate Tectonics? Convection in Earth s mantle Slab pull as plates subduct
Media Reported Science Get in groups of 4-5, discuss each of your answers to the MRS questions, discuss any differences, what you thought of the study... 24