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)

4. Meteorites

a) Chondritic = core

b) Stony = outer parts

5. Geophysics:

a) Density

b) Seismic Waves

c) Magnetics

d) Earth precession

 

Historical Perspectives:

Earthís Interior:

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:

I. Core

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

III. Lithosphere

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)

 

Continental Drift:

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

 

 

Plate Tectonics:

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

Trenches

Island Chains

Transform Faults

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

Confirmation:

Explained all continental drift data

Polar Wander paths

Magnetic Stripes

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

Orogenic Belts

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

Transform margins

Triple Junctions

What drives plate motions?

Mantle Convection

Ridge Push

Slab Pull

 

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