Lecture3 - Prelude to Earth Materials will it last why does...

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Unformatted text preview: Prelude to Earth Materials will it last? why does it degrade? what is it made of? what is steel? what is concrete? is it unique? how is it different? how is it put together? what are the mechanics behind its functionality? Prelude to Earth Materials Rock is an aggregate of 1 or more minerals Mineral is a natural, inorganic solid 10-15 mineral types; 3 rock types Igneous: originally molten material Sedimentary: originally weathered and broken preexisting rock that is compacted and cemented together Metamorphic: any pre-existing rock that has undergone T, P or structural changes 1 Prelude to Earth Materials Rock cycle Prelude to Earth Materials One more important concept to be aware of thru the course (which we will ultimately build towards): PLATE TECTONICS Centuries of observations have concluded that the Earthʼs make-up is not random. Pattern recognition: earthquakes occur where mountains rise & volcanoes explode. 2 Plate Tectonics can be summarized with 4 concepts: 1) Outer portion of the Earth is composed of rigid layers called plates. 2) The plates move …… slowly*. 3) Most large-scale geologic activity occurs at plate boundaries. 4) Interior of plates are relatively geologically quite. *rate = fingernail growth cm/yr 3 Layers of Earth based on density & composition differences: composition crust: oceanic & continental; solid, strong, rigid; 30-70 km thick mantle: solid, weak, ductile; 2900 km thick core: outer (liquid) & inner (solid); 3480 km thick physical/mechanical crust & upper mantle = lithosphere (~100 km) below lithosphere = asthenosphere (~140 km) below asthenosphere = mesosphere core outer (liquid) & inner (solid) Divergence - Rifting / Extension 4 Convergence - Subduction / Collision But: HOW do we know? 5 ..quick intro to seismology .. more in the coming weeks.. What is an earthquake? vibration of Earth produced by the rapid release of energy stored in rock subjected to stress (plates rubbing past each other) • energy released radiates in all directions from its source (like sound) • energy is in the form of waves • body & surface waves Transmission of P & S waves thru a solid Primary Secondary 6 7 Significant characteristics of body waves velocity is proportional to density & elasticity velocity increases with depth P-waves propagate thru all mediums S-waves propagate thru solids Pʼs travel faster than Sʼs density/composition change in medium = waves are refracted (bent) &/or reflected (mirrored) How can we "use" earthquakes? (not only quakes, but also nuclear testing) allows us to "x-ray" image the Earth (flashlight) variations in travel-times which are not accounted for by distances traveled Remember: waves propagate in 3D shown as rays paths, not waves 8 Planet interior what would seismic waves thru a uniform, homogenous planet look like? Planet interior But we know in Earth: pressure increases with depth = increase density P = ρgh 9 P increases with d = increase density = increase velocity But we also know: Earth is chemically (vertically) differentiated Waves can refracted across boundaries Angle of wave refraction dicated by Snell's Law: 10 Using travel times to measure the depth of the 'layers' Waves can also reflect off boundaries Law of reflection says that the angle at which the wave is incident on the surface equals the angle at which it is reflected. Using travel times to measure the depth of the 'layers' 11 Planet interior Discovering Earth’s major boundaries crust-mantle boundary • aka: Mohorovičić Discontinuity (Moho) • discovered by A. Mohorovičić (1909) • based on the observation that seismic velocities are slower in crust (6 km/s) than the mantle (8 km/s) Planet interior Discovering Earth’s major boundaries core-mantle boundary • discovered by B. Gutenberg (1914) • based on the observation that P waves die out at 105° from the earthquake and reappear at about 140° - this belt is named the P-wave shadow zone 12 The P-wave shadow zone Planet interior Discovering Earth’s major boundaries core • I. Lehmann (1936) proved seismic waves travel in/out of core • characterized by bending (refracting) of the P waves • the fact that S waves do not travel through the core provides evidence for the existence of a liquid layer beneath the rocky mantle (S-wave shadow zone) 13 The S-wave shadow zone Planet interior Seismic waves & Earth’s structure abrupt changes in seismic-wave velocities that occur at particular depths helped seismologists conclude that Earth must be composed of distinct shells • because of density sorting during an early period of partial melting, Earthʼs interior is not homogeneous layers are defined by: • composition / chemistry • mechanics / physics 14 Planet interior Compositionally layered Earth continental crust • 4000 Ma • 3-70 km thick (0-100+ km) avg: 40 km • Si-O, Al (K, Na, Ca) • average: granodiorite; ρ: 2.7 g/cm3 oceanic crust • 180 Ma • 3-15 km thick (0-20+ km) avg: 10 km • Mg & Fe (Si-O) • average: basalt; ρ: 3.0 g/cm3 15 Planet interior Compositionally layered Earth mantle • 3000 km thick • Mg & Fe • peridotite; ρ: 3.3 g/cm3 core • 3500 km radius • Fe & Ni; ρ: 11 g/cm3 • T: 6700°C Planet interior Composition? layered Earth So, Earth is layered but… • competing P&T forces complicate things: • increase depth = increase T = melting • increase depth = increase P = increase rock strength 16 Planet interior Physically layered Earth Layers defined by physical properties • depending on the temperature & depth, a particular Earth material may behave like a brittle solid, deform plastically, or melt and become liquid • main layers of Earthʼs interior are based on physical properties and hence mechanical strength Planet interior Physically layered Earth lithosphere • cool & strong; brittle • continental: 100-200 km thick • oceanic: 5-100 km thick ---- detached ---asthenosphere • partially melted; ductile • extends to depth of 660 km 17 Planet interior Physically layered Earth mesosphere • strong & hot • extends 660 km to 2900 km in depth core • outer: liquid, metallic Fe, 2300 km thick • inner: solid, Fe, 10% Ni (S, O), 1200 km radius 18 anchorage man tle + core ottawa regina mantle Planet interior Earth’s internal heat engine Earthʼs temperature gradually increases with an increase in depth at a rate known as the geothermal gradient • varies considerably from place to place • averages between about 10°C to 20°C/km in the crust (rate of increase is much less in the mantle and core) 19 Planet interior Earth’s internal heat engine contributions to Earthʼs internal heat: 1) heat emitted by radioactive decay of isotopes of uranium (U), thorium (Th), and potassium (K) 2) heat released as iron (Fe) crystallizes to form the solid inner core 3) (latent) heat released by colliding particles during the formation of Earth (10-15 Ga) 20 Planet interior Earth’s internal heat engine heat flow in the crust • process called conduction • rates of heat flow in the crust varies mantle convection • no large change in temperature with depth in the mantle • mantle must have an effective method of transmitting heat from the core outward Transfer of heat in the Earth by mantle convection 21 Planet interior Core Earthʼs magnetic field requirements for core to produce magnetic field: 1) composed of material that conducts electricity 2) it is mobile liq outer shell convecting around solid inner shell = magnetic field - inner core rotates faster than the Earthʼs surface - the axis of rotation is offset about 10° from the Earthʼs poles Possible origin of Earth’s magnetic field 22 -mag field has a North & South magnetic pole -mag field allows use of compasses -mag field occasionally 'flipsʼ -Normal polarity (now) & Reverse polarity (time of opposite poles) ..how do we know? -magnetic minerals (magnetite) in igneous rocks record orientation of mag field as they cool 23 24 Plate movements & boundaries Divergence - Rifting / Extension (Great East African Rift; Atlantic Basin; Basin & Range) Convergence - Subduction / Collision (Appalachians; Himalaya; Andes) Conservation - Translation / Transform (San Andreas Fault, Calif) Briefly, why do plates move? thermal convection cells within the mantle beneath the solid plates 'drags' the plates as it rises and cools Convergence - Subduction / Collision 25 Divergence - Rifting / Extension more in the coming weeks.. 26 ...
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