ELEMENTS OF OCEANOGRAPHY

 

TOPIC 3 - EVOLUTION AND STRUCTURE OF THE EARTH [Lecture ppt | pdf]

Overheads:

Geocentric Model of the Universe - the Earth is central. This model was developed by the ancient Greeks and accepted until the time of Copernicus and Galileo (~1580-1620).

The Heliocentric Model is the current Model of the Solar System.

Solar System Characteristics
(which any theory of formation must explain)
Almost all planets orbit the sun in a common plane
Planet all orbit in the same direction
Planetary orbits are almost circular
Planets display a regular progression in orbital period
The Sun has most of Solar System mass
Inner planets are 'rocky' and solid
Outer planets are mostly gaseous
Meteorites are similar to the 'rocky' planets in composition

Prevailing Theory for Solar System Formation
Initial spinning disk of dust and gas (about 5 Ga)
Gravity caused most mass to move toward center, increase spin, and create turbulant eddies
Protosun formed from central densest eddy and became star when high density started fusion reactions in core
Protoplanets developed from smaller eddies

Formation of the Earth - I
Protoplanet Earth swept its orbit clean of dust/gases; added then to its mass
Continued gravitational collapse, radioactive decay, and impacts cause increased heating of the planet interior
Heavier elements (Fe, Ni) migrated toward the Earth's center as partial melting occurred

Formation of the Earth - II
Lighter elements (gases, Si, Al, Mg) migrated towards the Earth's Surface
Planetary differentiation was complete by about 4.6 Ga
Oldest known rocks about 4.1 Ga

Structure of the Earth - 1
CORE (2900-6370 km deph)
­ composed mostly of iron
­ inner core solid; outer core liquid
­ Temperatures greater the 5000*K
MANTLE (~100-2900 km depth)
­ Composed on Fe and Mg rich silicates
­ Solid rock but ductile
­ Temperatures from ~5000*K-1000*K

Structure of the Earth - 2
CONTINENTAL CRUST
­ Al Silicates (Granite)
­ 30-100 km thick
­ Makes up all continents and many large islands
­ Up to 3.5 Ga in age
Oceanic Crust
­ Fe, Mg Silicates (Garbbro. Basalt)
­ ~20 km thick
­ underlies all major ocean basins
­ <200 Ma in age

Lithospheric Plates
Aesthenosphere lies 10-50 km below the crust within the uppermost mantle
Aesthenosphere is a region or partially melted mantle
Uppermost mantle and crust above the Aesthenosphere is termed lithosphere
The lithosphere can be considered to be 'floating' on the aesthenosphere

The Earth's Magnetic Field
The earth's magnetic field is generated by dynamo action in the liquid-Fe outer core
The field at the Earth's surface look s like that of a bar magnet (dipole) located at the Earth's center and aligned with the rotation axis
The field at the Earth's surface generally points toward the North Pole and points up or down depending on latitude
Paleomagnetism tells us that the dipole field has changed polarity (bar magnet gets flipped) many times (last time was 780 ka

Initial Formation of the Earth's Outermost Fluid Layers
The Earth's oceans and atmosphere developed last
As the sun started to shine its solar wind blew most volitile elements (gases) outward where the now make up most of 'Jovian' planets
Gases and water on Earth came from 'Degassing' of the crust and mantle after planetary differentation

Oceans and Atmosphere
The Oceans
­ Composed of water and dissolved solids
­ Makes up 71% of Earth's surface
­ Up to 10km thick
The Atmosphere
­ Now composed mostly of N (78%) and O (21%)
­ has several distinct layers
­ about 100km thick
­ Originally had no free oxygen - developed after origin of photosynthesis

 

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