Presentation on theme: "Origin and Structure of the Earth Marshak – Chapter 1 (plus an introduction to Chapter 2)"— Presentation transcript:
Origin and Structure of the Earth Marshak – Chapter 1 (plus an introduction to Chapter 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.
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 planets Inner Terrestrial - Mercury, Venus, Earth, Mars Outer Jovian - Jupiter, Saturn, Uranus, Neptune, Pluto? Terrestrial – dense, rocky, >3 g cm 3, Mg, Fe, Si, K, Ca, metals combined with O Jovian - “gassy” <~1.5 g cm 3, ice, H, He, CH 4 methane CO 2 Asteroid belt between Mars and Jupiter, source of meteorites
Origin of our Solar System: The Nebular Hypothesis
The Sun is ~99% of the mass of the solar system ~99% of the angular momentum is in the planets Inner planets are rocky and dense – terrestrial planets Outer planets are gassy – gas giant planets We know the Earth is composed of layers – Why?
Planetary Differentiation Why? –There is a motive Layers 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 means Solids are hard to sort mechanically, but liquids are easily separated gravitationally. Partial or complete melting allows large-scale differentiation. –There was an opportunity Heating 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.
Chemical Differentiation of the Earth Early EarthEarth Today Early Earth likely entirely molten – gravitational segregation of dense metals (mostly Fe) to the center is the result.
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)
Whole Earth has significant Fe - due to the core However, outer layers of Earth are much different
Earth’s crust (thin outer layer) mostly Si and O Earth’s mantle (between core and crust) is similar to the crust, but with lower Si, and higher Fe and Mg
Introduction to Plate Tectonics
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 ones –The plates move with respect to one another and thus continents are mobile (imbedded in plates) –Continents are relatively old, ocean basins relatively young –Geologic activity (earthquakes, volcanoes) is concentrated along the boundaries between plates
January 20, 2011 – Earthquakes in the past 5 years from www.iris.edu
Earthquakes mark outline of Earth’s tectonic plates.
Known volcanoes of the world – do the locations look familiar? from the Smithsonian Global Volcanism Project
Note that earthquakes and volcanoes generally occur in the same locations. Where are Earths large mountains found? Are all of these generally found in the same places?
Earth’s outermost layer comprises plates which move relative to each other. These movements are now measured by GPS and VLBA techniques.
The Theory of Plate Tectonics Earth’s outer layer broken up into 13 major tectonic plates which are made of the crust and uppermost mantle beneath.
Plates may contain oceanic or continental crust or both contain both continental and oceanic crust Some plates Others are mainly oceanic crust
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%.
The lithosphere is cold, rigid and solid. What about the asthenospheric mantle beneath?
The rigid lithosphere slides on the ductile asthenosphere, which is partially molten. Lithosphere Asthenosphere Cold, rigid Hot, ductile Continental crust (mostly granite) Oceanic crust (mostly basalt) Mantle (mostly olivine)
Lithosphere 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. Crust, mantle, and core refer to composition (what is it made of?). Crust: mostly granite on continents mostly basalt on oceans (we will talk about Mantle: made mostly of the mineral olivine these later…) Core: mostly iron and some nickel
Mantle tomography – provides images similar to ultrasound.
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).
More detailed image of subduction zone beneath Japan.