Evidence for Earth's Structure

• Crust beneath our feet (Direct):

  • most of land surface is well mapped; analysis of surface geology maps suggest what lies below surface (especially in areas where older crustal rocks bought to surface by earth movements/exposed by erosion)
  • from this we know upper continental crust is varied showing a range of sedimentary/igneous/metamorphic rocks (with overall granitic composition)
• Volcanic magma from depth (Direct):

  • magma originates in lower crust/upper mantle carrying up samples of rock from these layers to surface (eg. composition of upper mantle estimated by chemical analysis of basalt lavas and their volatiles)
  • igneous material in volcanic pipes occansionally includes diamonds; crystallise under high pressures in upper mantle (250km depth) + enclosing igneous Kimberlite includes mantle xenoliths of peridotite + magma carries up fragments of country rock from walls of deep volcanic vents
• Volcanic magma from depth (Direct):

  Kimberlite pipes are result of explosive volcanism from deep mantle sources; within 2km of surface, highly pressurised magma explodes upwards/expands to form conical vents + Kimberlites are fine, ultramafic igneous rocks.

  
• Mines/Boreholes (Direct):

  • direct access to higher levels of crust through mines; drainage, ventilation, temperatures and lifting ore to surface limit depth of mines (deepest mines penetrate 4km of crust)
  • boreholes penetrate deeper; samples of rock/microfossils can be brought up from boreholes and remote sensing undertaken (deepest oil well penetrates 10km into crust)
• Deep Boreholes (Direct):

  • Project Mohole (1961-66); American enterprise aimed to drill through thin oceanic crust into mantle off Mexico + reached depth of 183m where core samples of basalt taken + abandoned due to cost/failed to reach Moho.
  • Kola Superdeep Borehole (1970-94); Russian project aimed to drill item through thicker continental crust on Kola Peninsula + one braching borehole reached 12,262m depth (1/3 into crust) + 300$^oC$ temperatures made rock plastic and drill ineffective.
• Ophiolite Suites (Direct):

  • during collisions between lithospheric plates, sections of oceanic crust may be broken off and thrust onto edge of continental plates (ophiolites then exposed by erosion)
  • section of ancient oceanic crust can be examined on land without borehole through ocean floor - peridotite at base of ophiolite sequence from upper mantle
  • if structure of ophiolite sequence returned to original undeformed orientation, total thickness 7km (thickness of oceanic crust)
• Ophiolite Suites (Direct):

  eg. Ophiolites at Lizard Peninsula and Anglesey + Troodes Mountains, Cyprus; millions years old and similiar to modern ocean crust/upper mantle.

  A) Pillow Lavas

  B) dykes

  C) Gabbro

  D) Peridotite

  
• Stony/Iron Meteorites (Indirect):

  • siderophiles within iron meteorites are equivalent to composition of core; not changed composition (formed 4560Ma) as not subject to weathering, metamorphism etc. (meteorite evidence suggest core is iron with some nickel)
  • meteorites account for rare siderophiles in crust (prevented from descending into solid rock); eg. iridium a rare/dense transition metal in crust (usually would descend) + iridium around impact crater implies extra-terrestrial origin, eg. K-T Boundary, New Mexico
  • stony meteorites represent silicate rocks of mantle
• Geomagnetism (Indirect):

  • core made of iron/nickel (2 main magnetic metals) + inner core solid at 5700$^oC$, and outer core liquid with temperature at mantle-core boundary of 3500$^oC$ (temperatures above Curie Point where materials lose magnetism)
  • earth's magnetism must be generated; temperature difference sets up convection currents within outer core + convecting mass of molten iron generates electricity/induces magnetism + balance between generation/destruction allow magnetic field
  • convection currents affected by earth's rotation
• Mean Density Calculations (Indirect):

  • density of rocks of continental crust 2.7g/cm$^-3$ and oceanic crust 2.9g/cm$^-3$; density of core must be higher to balance lighter crust
  • on surface, density of iron 7.9g/cm$^-3$ and nickel 8.9g/cm$^-3$; at core these would be higher (at least 12g/cm$^-3$ under extreme pressures)
  • by calibrating seismic wave velocities in rock of known physical properties, we can estimate densities for layers of Earth's interior (taking into account pressure/temperature)
• Conductivity (Indirect):

  • Electromagnetic Surveys (EM) used to detect presence of fluids in subsurface rocks, eg. partial melting in upper mantle/below mid-ocean ridges means increase in conductivity of rock (can be mapped) + only 1-2% of partial melting can be detected (especially if magma hydrated)
• Conduction - process which thermal energy is transferred through a substance with no overall movement of that substance (energy transferred atom to atom down thermal gradient).
• Gravity (Indirect):

  • gravity anomalies used to investigate interior of Earth; typical variations on gravity value are 20 miligals
  • gravitational attraction depends on respective masses and square distance between them; value of gravity measured over area of dense rock (gabbro) is different from value over less dense rock (granite) -average is 9.81ms$^-2$
• Gravity Anomalies:

  • Latitute; earth slightly flattened at poles/bulges at equator due to earth's spin so poles slightly closer to centre of mass meaning gravity is slightly higher (98322>978100 miligals)
  • Altitude; if a location is 100m above sea level it is 100m further away from centre of earth so gravity value adjusted by adding 0.31 miligals every metre
  • Bouguer Anomaly; final value must account for extra 100m of mass above sea level (1 miligal per 10m subtracted from altitude anomaly to remove effect of mass)
• Seismology (Indirect):

  • Seismic Waves - vibrations from earthquakes detected by seisometers; analysis of earthquake history has helped build a picture of earth's structure.
  • P (Primary)/S (Secondary) are seismic waves that travel through earth; when rock more rigid/incompressible they speed up + when rock more dense they travel slower (vibrations occur more frequently so more energy lost).
• Changes in Velocity through Earth:

  • P/S waves slow down in asthenosphere as 1-5% partial melting reduces rock rigidity
  • P/S waves speed up through mantle as pressure increases/rock more incompressible
  • P waves slow at Gutenburg dicontinuity/2900km as enter liquid outer core where rigidity is low + S waves stop as cann't travel through liquid
  • at solid inner core/Lehmann discontinuity/5100km, P waves speed up as rigidity increased + S waves propogated at 90$^o$ to P waves
• Shadow Zone:

  • large distances from focus at which P/S waves can't be detected; P waves (103-142$^o$) + S waves (103-103$^o$)
  • seismic P waves refracted at Gutenbury discontinuity marking edge of outer core + waves between 103-142$^o$; P waves refracted and S waves stop (can't transmit through liquids) at discontinuity created zone with no body waves
  • beyond zone of 142$^o$, S waves can't be detected (can't pass through liquid outer core) + P waves slow down due to loss of rigidity/incompressiblity in liquid out core