Presentation on theme: "Origin and Structure of the Earth"— Presentation transcript:
1 Origin and Structure of the Earth Marshak – Chapter 1(plus an introduction to Chapter 2)
2 The Earth is part of the solar system and thus most likely formed at the same time… So, what do we know about the solar system and it’s structure?These are the observations which are needed to come up with an idea (hypothesis) for how the solar system (and Earth) formed.
3 Formation of the solar system and differentiation of Earth Hypotheses must satisfy observations: planets orbit sun in one direction, axes of rotation nearly perpendicular to orbit, most planets rotate in same direction as orbit about sun, >99% solar system mass in sun, ~99% solar system angular momentum in planetsInner Terrestrial - Mercury, Venus, Earth, MarsOuter Jovian - Jupiter, Saturn, Uranus, Neptune, Pluto?Terrestrial – dense, rocky, >3 g cm3, Mg, Fe, Si, K, Ca, metals combined with OJovian - “gassy” <~1.5 g cm3, ice, H, He, CH4 methane CO2Asteroid belt between Mars and Jupiter, source of meteorites
4 Origin of our Solar System: The Nebular Hypothesis
8 The Sun is ~99% of the mass of the solar system ~99% of the angular momentum is in the planetsInner planets are rocky and dense – terrestrial planetsOuter planets are gassy – gas giant planetsWe know the Earth is composed of layers – Why?
9 Planetary Differentiation Why?There is a motiveLayers of different chemical composition can have different density, and gravity provides a driving force whereby planets can lower their potential energy by sorting the denser material towards the center.There is a meansSolids are hard to sort mechanically, but liquids are easily separated gravitationally. Partial or complete melting allows large-scale differentiation.There was an opportunityHeating beyond the melting point of most components of undifferentiated solar material during planet formation is inevitable for bodies above a certain size (> approx. 1,000 km radius) that formed early enough or fast enough.
10 Chemical Differentiation of the Earth Early EarthEarth TodayEarly Earth likely entirely molten – gravitational segregationof dense metals (mostly Fe) to the center is the result.
11 Origin of the moon by planetary impact on Earth This occurred ~4.5 billion years ago (4.5 Ga)(very early in Earth history as age is only ~4.6 Ga)
12 Whole Earth has significant Fe - due to the core However, outer layers of Earth are much different
13 Earth’s crust (thin outer layer) mostly Si and O Earth’s mantle (between core and crust) issimilar to the crust, but with lower Si, andhigher Fe and Mg
15 Plate Tectonics: Structure of Earth’s surface is largely caused by the formation, movement, and destruction of large rigid plates…Major conclusions of Plate Tectonics:The lithosphere (outermost shell of Earth) is composed of 13 or more large rigid plates and numerous smaller onesThe plates move with respect to one another and thus continents are mobile (imbedded in plates)Continents are relatively old, ocean basins relatively youngGeologic activity (earthquakes, volcanoes) is concentrated along the boundaries between plates
16 January 20, 2011 – Earthquakes in the past 5 years from
17 Earthquakes mark outline of Earth’s tectonic plates.
18 Known volcanoes of the world – do the locations look familiar? from the Smithsonian Global Volcanism Project
19 Note that earthquakesand volcanoes generallyoccur in the same locations.Where are Earths largemountains found?Are all of these generallyfound in the same places?
20 These movements are now measured by GPS and VLBA techniques. Earth’s outermost layer comprises plates which move relative to each other.These movements are now measured by GPS and VLBA techniques.
21 The Theory of Plate Tectonics Earth’s outer layer broken up into 13 major tectonic plateswhich are made of the crust and uppermost mantle beneath.
22 Plates may contain oceanic or continental crust or both contain both continentalOthers are mainly oceanic crustSome platesand oceanic crust
23 Earth is Composed of Multiple Layers from Core to Crust. Crust and Upper Mantle (Lithosphere) = Locked Together as Rigid Plate.In terms of overall radius of Earth the plates are only 1-2%.
24 The lithosphere is cold, rigid and solid. What about the asthenospheric mantle beneath?
25 Oceanic crust (mostly basalt) Continental crust(mostly granite)Now follows a brief discussion of the main points or postulates of the modern theory of plate tectonics.First point: the rigid lithosphere rides on the ductile asthenosphere.Continental lithosphere is comparatively light and thick, and oceanic lithosphere is comparatively thin and dense, but both ride or “float” on the asthenosphere.Note again that the asthenosphere is ductile but not molten.GRAPHIC: Cross-section of a portion of the outer solid Earth, showing oceanic and continental lithosphere riding on the asthenosphere and deeper mantle layers below (Tasa Graphic Arts).Cold, rigidLithosphere(mostly olivine)MantleAsthenosphereHot, ductileThe rigid lithosphere slides on the ductile asthenosphere, which is partially molten.
26 Crust, mantle, and core refer to composition (what is it made of?). Crust: mostly granite on continentsmostly basalt on oceans (we will talk aboutMantle: made mostly of the mineral olivine these later…)Core: mostly iron and some nickelLithosphere and asthenosphere refer to the strength (Is it hot, or is it cold? Is it rigid, or does it flow like toothpaste?)Lithosphere : Strong, rigid, cold outer shell of rock which includes the crust and part of the upper mantle.Asthenosphere: The hotter, weak, ductile layer of solid rock below the lithosphere that flows plastically. Analogy – cold toothpaste.
46 Mantle tomography – provides images similar to ultrasound.
47 Mantle tomography – hot material in red (slower seismic wave velocity), cold material in blue (faster seismic wave velocity).Earthquake locations shown by white dots.Clearly shows the subducting oceanic lithosphere (cold) beneath the Japan volcanic arc system (hot).
48 More detailed image of subduction zone beneath Japan.