Lecture 2 Structure of the Earth PDF

Structure of the Earth 1 After listening to this lecture, you should be able to...  Discuss the Earth’s shape and size  Distinguish between density and mass  Discuss how and why the density of rocks change as we move from the surface of Earth towards the center  Identify and describ...

Structure of the Earth 1 After listening to this lecture, you should be able to...  Discuss the Earth’s shape and size  Distinguish between density and mass  Discuss how and why the density of rocks change as we move from the surface of Earth towards the center  Identify and describe the two sets of layers that comprise Earth’s interior  Explain how seismic waves can be used to study Earth’s interior  Explain the theory of continental drift and discuss the different types of evidence thereof  Describe the sequence of the break-up of Pangaea Shape and Structure of the Earth (Tarbuck, Lutgens and Tasa, page 346) ! Earth’s shape NOT a sphere Causes flattening at the poles and bulging Earth’s equatorial radius = 6378 km at the equator Earth’s polar radius = 6356 km causing the shape of earth to be an oblate spheroid/ellipsoid Equatorial diameter LARGER than polar diameter (Oblate Ellipsoid) Gravity is slightly weaker at the equator than at the poles Earth’s Density (Tarbuck and Lutgens, page 334) Density is mass  Density = Mass/Volume that has been  Measures how much mass is in a given volume standardized for volume  Expressed in units of mass/volume, e.g. g/cm3  Explains why ice floats because its density is less than water  Density of Earth as a whole ~ 5.5 g/cm3  Typical rocks at the surface of Earth have a density of ~ 2.0-2.5 g/cm3  What does that require of the density of material in the Earth’s interior? Main division of Earth’s layers (Tarbuck, Lutgens and Tasa, pages 23 to 24)  Compositional layers   Physical/mechanical layers  distinguished based on distinguished based on chemical properties physical properties  Crust  Lithosphere  outer, rigid  Composition ranges from shell felsic (continental crust) to  Asthenosphere (plastic-like/ mafic (oceanic crust) ductile) solid, but mobile  density: 2-3 g/cm3 layer of the mantle directly  Mantle beneath lithosphere  ultramafic rock called peridotite (Very rich in iron)  Solid rock, other than Felsic rocks are also called granitic rocks asthenosphere which is plastic composed almost entirely of light-coloured  density: 3-6 g/cm3 silicate minerals (quartz and feldspar)  Core while mafic rocks contain dark silicate  iron-rich sphere minerals and are rich in iron  density: 10-13 g/cm3 Main division of Earth’s layers  Compositional layers  Physical/mechanical layers  Crust  Lithosphere  Oceanic  Crust  Continental  Uppermost part of upper  Mantle mantle  Upper mantle (does not have  Asthenosphere one uniform physical consistency – it is both rigid and hard but is ductile)  Uppermost upper mantle (forms part of lithosphere)  Asthenosphere (forms the rest The compositional layers and of the upper mantle)  Transition zone physical layers are not mutually exclusive  Lower mantle  Core  Outer core  Inner core  Temp Pressure crust lithosphere Low uppermost upper mantle upper mantle asthenosphere transition zone lower mantle outer core inner core Solid Earth High (Tarbuck, Lutgens and Tasa, pages 23 to 24, pages 337 to 342) Lithosphere (Tarbuck, Lutgens and Tasa, page 24) Lithosphere ≠ crust!  The lithosphere  Consists of the both the crust and a portion of the uppermost mantle  Extends to an average depth of 100 km  Is a relatively cool rigid shell  Moves independently of asthenosphere Crust (Tarbuck, Lutgens and Tasa, pages 23 and 337)  Continental crust  Underlies the continents  Less dense than oceanic crust  Thickness ranges from 10 to 70 km  Large areas older than 1500 Ma and some even as old as 3500 Ma  Composition varies, but mostly felsic (granites)  Oceanic crust  More dense than continental crust  Average thickness 7 km (extremely uniform)  Nowhere older than 200 Ma  Mafic composition (mostly basalts) Mantle (Tarbuck, Lutgens and Tasa, page 24  Nearly 2900 km thick  Upper mantle  Extends to a depth of about 660 km  Lithospheric mantle  rigid, solid  Asthenosphere  Soft comparably weak, but still solid, layer  think of the consistency of “Putty” or “Playdough”  Top portion has temperature/pressure conditions that result in small amount of melting  Within this zone the lithosphere is detached from the asthenosphere  Transition zone  Zone immediately below the asthenosphere (between 410 and 660 km), characterised by a sudden increase in density  Lower mantle  More rigid layer than the asthenosphere  capable of “gradual flow” because of heat, but still solid, because of pressure  Largest layer – 56% of the volume of the planet Core (Tarbuck and Lutgens, pages 24 and 341 to 342)  Outer Core  Iron-nickel alloy  Liquid state, because of high heat  Reaches to a depth of 5150 km  Inner Core  Iron-nickel alloy  Due to immense pressure it behaves like a solid  Smaller than drawings suggest  Did not exist early in Earth’s history when planet was hotter  Continues to grow as the planet cools, at a rate of 1mm per year  Reaches to the centre of the Earth (6370 km) How do we know what we know about Earth’s interior? Tarbuck, Tasa and Lutgens, pgs. 334 to 337  Deepest drill into Earth  12.3 km  Kola Peninsula, Russia  1/500 of the way to the center  Too hot and pressure too high to go further  Not very practical way to study Earth’s interior  Seismic waves provide a more practical alternative to study Earth’s interior! What are seismic waves? Tarbuck, Tasa and Lutgens, pgs 334 to 337  Waves of energy produced by earthquakes  Many travel all the way through Earth and are detected on the other side  Planet Earth’s x-ray system!  Seismic waves are recorded by a seismograph on a seismogram! Seismograph Seismogram What does the seismogram tell us? Tarbuck, Tasa and Lutgens, pgs 334 to 337  Where there is oil and natural gas  seismic waves reflect off boundaries between rock layers, where petroleum tends to get trapped  What properties the different layers of Earth possess  Seismic waves travel faster in stiff (rigid), less compressible rock  Seismic waves therefore travel faster in cooler rock than hotter rock  The speed of the seismic waves also depends on the composition of the rock  Within a given layer of Earth’s interior, the speed of seismic waves increases with depth  pressure increases, squeezes rock into a more compact rigid material Continental drift  The mechanical detachment of the rigid outer layer of Earth (lithosphere) from the layer underneath (asthenosphere) means that the lithosphere can move on top of the asthenosphere  forms the basis of what is known as the theory of continental drift. Continental drift (Tarbuck, Lutgens and Tasa, page 40)  Alfred Wegener, 1915  The Origin of Continents and Oceans  Present continents were once joined and formed a supercontinent, Pangaea  Pangaea began to fragment into smaller landmasses, which drifted to their present positions over the span of millions of years  Theory referred to as continental drift  Evidence  Coastline fit evidence  Fossil evidence  Rock types and geologic features  Ancient climate evidence  Breakup of Pangaea signalled 195 Million Years Ago by opening rifts (Divergent boundaries) 66 Million Years Ago The South Atlantic had opened and widened (Divergent boundaries) Modern World India pushed into Asia (Convergent boundary) leading to the formation and continued growth of the Himalayas Africa collided with Europe (Convergent boundary) closing the Mediterranean and leading to the formation of a new mountain range In another 55 Million Years Where did the information in this presentation come from?  Tarbuck, E.J., Lutgens, F.K. and Tasa, D. 2011 Earth: An Introduction to Physical Geology. Tenth Edition. Pearson Prentice Hall (ISBN No 978- 0-321-69903-3)  Chapter 1: An Introduction to Geology  Chapter 2: Plate Tectonics  Chapter 12: Earth’s Interior  Also see:  https://www.youtube.com/watch?v=WwiiOjyfvAU  http://publish.illinois.edu/alfredwegener/evidence