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Tales of a Seasick Naturalist

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Western University
Earth Sciences
Earth Sciences 1083F/G
Cameron Tsujita

Brenna Llewellyn 250700534 ES 1023 October 01 2012 Know: How think the layers are, but you don’t need to know other numbers n Review of Last Lecture Telesiemic Nomenclature: Waves are renamed when they pass through a different medium (Ex. a P wave that passes through the core is designated a K wave) Liquid Outer Core: P-Waves slow down but do not disappear, as they pass through liquids this causes them to curve - S- waves disappear entirely - This causes shadow zones that show up when we record earthquakes around the world - Discovered in 1906 by Oldham from these seismic shadows - Vigorous convection in the outer core causes Earth’s magnetic field - Because P-waves slow down and curve, they are passing through a different medium - Because S-waves disappear entirely and we know that shear waves cannot pass through a liquid, it was concluded that the earth’s core must be liquid Solid Inner Core: Discovered in 1936 by Danish seismologist Inge Lehmann, using fain to observations of P-waves that bounced off of or were sped up by inner core and emerged in unexpected locations - Current debate: is the inner core rotating faster than the rest of earth? - Know PkpIp wave The Mantle: Composed of silicate material, but chemically distinct from the crust - Dominant minerals are olivine and pyroxene - Upper mantle is divided into the lithosphere and asthenosphere - The mantle transition zone: Region where lithosphere separates from the asthenosphere. -Olivine undergoes transitions between 440 and 660 km - At 660 km (Top of lower) olivine transfroms to mineral pervoskite - These transformations result in sudden velocity, density, and pressure changes. Gutenberg Discontinuity (The Core-Mantle Boundary) - Depth: 2891 km - Rock: liquid metal (most dramatic change in physical properties in the planet) - Layers immediately above the CMB is called D” - D” has been referred to as the graveyard of the plates The Core: composed mainly of liquid Fe and Ni, with small amounts of light alloy elements (S,O) - The outer core flows as easily as water and is vigorously convecting - The inner core is solid metal and has a radius of 1229 km - The region of the earth above the core is divided into several layers, the top thin layer is the crust. It is primarily elastic, about 35 km thick beneath the continents, and only 7 km thick in the ocean basins - The interface at the base of the crust is called the Mohorovicic discontinuity (Moho) and separates the crust from the mantle, it is thought to represent a compositional change from more silicon rich crustal material to more iron and magnesium rich mantle material - The mantle, that region between crust and outer core is thought to have several internal layers as well (2900 km thick) - It is generally agreed that the mantle is subject to high pressures and temperatures and under these conditions can flow over long enough time periods - The upper mantle is designated as the lithosphere and asthenosphere with boundires that vary slightly from those of the crust and mantle but are still based on seismic discontinuities such as a low-velocity zone at about 50-100 km in depth - The lithosphere is elastic - The asthenosphere is plastic/viscoelastic - There is a 440 km discontinuity not shown. - The upper mantle stops at the discontinuity Seismic Waves - Earth’s properties change with depth - Ex. the velocity of a P-wave (Vp) or S-wave (Vs) changes as it passes through the crust, mantle and core. - Why can’t S-waves travel through liquid? - Body wave velocities are dependent on the properties of the material they pass through, specifically the bulk modulus, K, the shear modulus, μ, and the density, p, as follows: Bulk modulus is related to how much a material deforms under pressure. P is a pressure wave! Shear modulus is related to rigidity – how much the material resists internal shear forces. S waves are shear waves! But, for a liquid, the shear modulus, μ, is equal to zero! So Vs = 0 in a liquid and S waves cannot travel through a liquid. Also Vp is much slower in a liquid, as the second term of the numerator is zero. Formation of the Moon: Theories The fission theory: The moon was once part of the earth and separated in early in its history The capture theory: The moon was formed independently and captured at a later time by the gravitational field of the earth. The condensation theory: The moon and the earth condensed together in a dual system from the original nebula of the solar system The colliding planetesimals theory: The interaction of Earth orbiting planetesimals early in their formation led to their breakup. The moon condense from this debris. The giant impactor theory: A planetesimal the size of Mars struck earth ejecting large volumes of matter. A disk of orbiting material was formed and this matter eventually condensed to form the moon Formation of the moon: Evidence - Earth has
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