Topic 13 - Earth Structure/Plate Tectonics
How Can we Determine the Structure and Composition of Earth:
1. Go look at surface materials (0-30 km and rarely100 km, China)
2. Drill holes (15 km, Khola Peninsular, Russia)
3. Magmatism (0-200 km; deepest = diamond bearing kimberlites)
a) Chondritic = core
b) Stony = outer parts
b) Seismic Waves
d) Earth precession
Pre-1600: Myth and Religious view of Hell
Greeks: Center of Earth = Hades
14th Century: Danteís Inferno ñ map of Hell
Newtonís Laws Start of Geophysics
1600ís Earthís magnetic field = two-pole bar magnet
Late 1700ís: Density of earth known - 5.5 g/cm3.
1785-1795 James Hutton (Scottish farmer) ìFounderî of modern geology
Dynamic view of earth as internal heat engine rather than static.
Magmatism = melted rocks coming from depth
1873 J.D. Dana: meteorites = inner core of earth.
1900ís By turn of century recognized three-fold division of earth into core, mantle, and lithosphere.
1928-40 Arthur Holmes - speculated on mantle convection and that this drives continental drift.
1934 W.M. Elsasser: motion of outer core causes magnetic field.
Present View of Earth Structure:
a) Inner: solid (Fe, Ni, + Sulfer or Oxygen??)
1. Spins 4 times faster than outer and 100,000 times faster than continents move.
b) Outer: liquid (Fe, Ni, + Sulfer or Oxygen??)
II. Mantle (Fe, Mg silicates, peridotite)
a) Mesosphere: no melt, convecting??
b) Asthenosphere: 6%-10% melt, convecting
a) rigid outer plates of earth = crust
b) includes ocean floor = 2-10 km thick
c) includes continents 20-70 km thick
d) includes some rigid mantle above asthenosphere
IV. Lithospheric plates
a) Lithosphere divided into plates by zones of active deformation
b) Names and distribution of plates (see handout)
1596 Abraham Ortelius (Dutch Cartographer): first suggested that continents joined together then separated.
1620 Sir Francis Bacon: Often thought to be the first to suggest continental drift, but really only noted similarity in shapes of continents.
1858 Antonio Snider ñ made map of floating continents
1908 F. B. Taylor suggested that moon once closer to earth and that gravitational forces would move continents.
1910 Alfred Wegener and others found evidence of Africa and South America being together.
a) Glacial and other Deposits,
b) Animal and Plant fossils
c) Climatic zones
d) Wegenerís reconstruction of Pangea (Greek= all land)
e) Assumptions in Wegenerís continental drift:
1. Shapes of continents donít change with time.
2. Continents move over or through ocean basins.
f) Above idea not generally accepted because no mechanism.
1937 Alexander Du Toit (South African Geologist): Also argued for continental drift.
1928-40 Arthur Holmes - speculated on mantle convection and that this drives continental drift. Still knew little about oceans.
1900-1960 There were many other issues that geologists were trying to explain such as the distribution of mountain ranges and change in the types of sediments near continental margins, and the evidence for continental drift. Explanations increasingly unsatisfactory ñ science of geology in state of ìcrisis.î
During World War 2: lots of submarine travel so topography (bathymetry) of oceans very important. Harry Hess was a submarine commander. Used sonar (sound waves) to record bathymetry.
Imagine Draining water out of oceans - MAPS of OCEAN FLOOR!
Two average levels of crust on earth, continents & oceans
MORs = Mid-oceanic ridges
1962 Hess Proposed Idea of PLATE TECTONICS (new paradigm)
MORs = locations where plates formed and spread apart
Trenches = locations where plates slide under one another
Continents sit on plates and move with them
Holmes idea of mantle convection drives plate motion
Supporting Evidence for this new Paradigm:
1960's Vine, Mathews, Cox - magnetic anomolies symmetrical around ocean ridges
1967-68 J. TuzoWilson & W. Jason Morgan- outlined rigid plates, triple junctions, transform faults, Wilson cycle
1968 Isacks, Oliver, Sykes - unified theory
1970 John Bird & John Dewy- related orogenic belts (deformed mountain belts) to plate tectonics
a) underthrusting (Andean) thermally driven
b) collision (Himalayen) mechanically driven
1980 Coney, Jones, and Monger - suspect terranes
Explained all continental drift data
Polar Wander paths
Island Chains = Hot spots fixed in mantle below plates
(e.g., Pacific plate changed direction 43 Ma ago).
Also Explains MANY OTHER Geologic Features
Mio and Eugeosynclines
Collisions like India
Coupling along subduction zones
Earthquakes occur in belts along plate boundarys
Magmatic activity focussed along MORs or arcs
Island Chains = Magmatic hot spots fixed in mantle below plates
Movement of plates VIDEO?
Directions (GPS, Geologic Data like hot spots, old faults, arcs)
How fast 0-14 cm /yr, SAF = 6 cm/yr; Atlantic 2 cm/yr
Types of plate boundaries
MORs, Continental Rifts
Subduction zones and Arcs
What drives plate motions?
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