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Transcript
Topic 3 A Internal Structure of the Earth Lecture by Dr. Ken Galli, Boston College EESC116301 Environmental Issues and Resources July 7, 2016 Please do not distribute beyond the EESC116301 Class. Case History: Two Major CA Cities • San Andreas fault: a transform plate boundary between the North American and the Pacific plates • Two major cities on the opposite sides of the fault: Los Angeles and San Francisco •Many major earthquakes related to the fault system •Loss of many lives and billions of property damages due to earthquakes •New construction and retrofitting of infrastructures has become more expensive •When will be the next “big one” and what to do? How to deal with the potential consequence? Internal Structure of Earth • The Earth is layered and dynamic: Interior differentiation and concentric layers • Chemical model by composition and density (heavy or light): Crust, mantle, core, and Moho discontinuity between the crust and mantle • Physical property model (solid or liquid, weak or strong): Lithosphere (crust and upper rigid mantle), asthenosphere, mesosphere, liquid outer core, inner solid core Theory of Plate Tectonics • The upper mechanical layer of Earth (lithosphere) is divided into rigid plates that move away from, toward, and along each other • Most (!) deformation of Earth’s crust occurs at plate boundaries Earth’s Layers 3 main layers defined by composition: • Crust - Outer • Mantle - Middle • Core - Center http://www.physicalgeography.net/fundamentals/10h.html Study of Earth’s Interior Structure • Knowledge primarily through the study of seismology • Seismology: Study of earthquakes and seismic waves • Examining the paths and speeds of seismic waves through reflection and refraction • Magma likely generated in the asthenosphere • Slabs of lithosphere have apparently sunk deep into the mantle • Variability of lithosphere thickness reflects changes in its age and history Composition - How Do We Know? Best Guess! Whole Earth • Meteorites - Fe, Ni (same age as Earth) • Information from velocities of seismic waves indicate material Crust (5-40 Km) • Samples (mountain building helps!) Mantle (5/40 to 2885 Km) • Kimberlite pipes - intrusive igneous rock from the mantle • Lava / volcanic rock • Mountain building Core (2885 to 6371 Km) • Inference – Earth’s mean density = 5.5 g/cm3 – Crust 2.5 to 3 g/cm3; mantle 3.3 g/cm3 to 5.5 g/cm3 – Density of core at least 10 to 11 g/cm3 (iron and nickel) Seismic P Wave • Primary or push-pull wave, travels like sound wave • Direction of rock particle vibration parallel to that of wave propagation • Fastest rates of propagation, first arrival to the seismograph • Body wave travels through Earth interior and all media—solid and liquid Seismic S-Wave • Secondary or shear waves • The direction of particle vibration perpendicular to that of propagation • Propagates slower than P waves • Body wave, propagating through Earth’s interior, but not its liquid layers Seismic Waves and Internal Structures • Earth’s interior boundaries: Sudden changes in the speed of seismic waves • Different characteristics: Different rates and paths of wave propagation • Asthenosphere: Low velocity zone, major source of Earth magma • Outer Core: Liquid, no S wave transmits through it Model of Earth’s Crust ————— Moho Mantle Interior Figure 2.2b Crust • Two types of crust: – Continental • 30% of crust • Granites and Diorites - rich in silicates and feldspars (lighter materials) • 35-40 Km thick • Oldest is 3.8 billion years (90% solar system age; missing ~700 m.y.) – Oceanic crust • Basalt - Mg, Fe (heavier materials) • 5-10 Km thick • 200 Ma oldest; 100 Ma average Our deepest hole: 9 Kilometers ….. we have a long way to go! Mantle • MOHO - Mohorovicic Discontinuity • Core mantle boundary - change in mineralogy • Density - getting heavier • 3.3 - 5.5 g/cm3 • Probably material such as peridotite (lots of heavy olivine - Fe, Mg) • Samples from kimberlites, xenoliths in volcanic eruptions, basalt composition; lab experiments Our deepest hole: 9 Kilometers ….. we have a long way to go! Core • Outer core – Molten, near solid point (does not transmit certain seismic waves) – Density of pure iron or nickel/iron – Includes ~ half of diameter of Earth – 2x density of mantle • Inner core – Solid (higher pressure than outer core) – Density of pure iron or nickel/iron – ~ Size of moon Our deepest hole: 9 Kilometers ….. we have a long way to go! Earth’s Layers 3 layers defined by mechanical properties (strength): • Lithosphere • Asthenosphere • Mesosphere http://earthguide.ucsd.edu/mar/dec5.html • Lithosphere – – – – PLATES in Plate Tectonics Upper 100 km Crust and upper mantle Rigid • Asthenosphere – 100 km to ~350 km (to 660km in places) – Upper mantle – Near melting point; little strength; ductile - NOT A LIQUID! – Plates moving on this – Magma generation • Mesosphere – Extends to core – Also hot; strong due to pressure Internal Dynamics of Earth • Evidence – Earth’s landscape – Dynamic phenomena: earthquakes, volcanoes • Plate Tectonics: Hypothesis and Theory – Continental drift – Seafloor spreading – Plate tectonics – a unifying theory Dynamic Earth—Evidence • Mountain belts (continental mountain ranges and oceanic ridges) • Earthquake distribution: Concentrated zones • Earthquake occurrences over time • Volcanism in space: Concentrated zones • Volcanism over time