Section 1: Introduction Formation of the Solar System Modern View: As nebula cooled, cloud flattened into rotating lens-shaped disk.

When core dense enough, gravitational collapse created proto-Sun.

Hydrogen combined to form helium, releasing much energy.

Material in disk cooled and condensed as small grains, which coalesced into planetesimals.

Asteroid-like rocky planetesimals formed near Sun, icy planetesimals away because volatiles such as water and methane driven off by Sun's radiation.

Planetesimals collided to form planets.

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Formation of the Earth Earth accreted from planetesimals ~4.6 Ga. Heated up from kinetic energy released by impacts, compression of planet under its growing weight, and radioactivity:

Early Earth was probably homogeneous, but as it heated up to ~2000 oC, iron melted and sank to form a central iron core:

Lighter molten materials rose to the surface, and cooled: • Magma ocean ~100 km deep may have existed • Eventually, surface solidified and formed a primitive crust. Origin of presentday stratified Earth 2

Earth's Orbit and Rotation Orbital Eccentricity Slightly elliptical orbit with eccentricity that varies between 0.001 and 0.060 over 109,000 yr period due to influence of other planets.

Also 413,000 yr periodicity.

Increased eccentricity results in greater contrasts in solar radiation through year

Orbital Precession Earth's orbit is not a closed ellipse: successive positions of perihelion and aphelion differ slightly.

Effect is similar to precession of rotational axis and their combination results in alteration in times at which seasons occur with period of 20,500 yr

Rotational Axis Rotational axis varies between 21.9o and 24.3o to pole of ecliptic with 41,000 yr period due to effect of other planets.

Increased obliquity enhances contrast between summer and winter.

Milankovitch Climatic Cycles Climatic effects related to cyclical changes in Earth's orbital and rotational parameters with periods of 20.5, 41, 109, and 413 kyr are called Milankovitch cycles after Yugoslavian astronomer.

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The Earth's Internal Structure In early 20th centuries, studies of seismic waves from distant earthquakes showed that Earth has a layered structure with sharp contrasts in seismic properties at interfaces: Each layer has distinct physical properties determined by composition, pressure and temperature:

Density Stratification Crust: Rich in silica as derived from mantle by repeated

melting: on average ~7 km thick beneath oceans, ~40 km beneath continents.

Mantle: Rich in Mg silicates. Outer Core: Mainly iron, which is molten due to high temperature. Inner Core: Iron, solidified by intense internal pressure of Earth.

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Lithospheric Plates Stratification of Deformational Properties Lithosphere: Outer part of Earth rigid down to ~70-100 km beneath

oceans, ~100-150 km beneath continents. Asthenosphere : ~150 km thick partially molten layer that behaves like

a viscous liquid or plastic solid over geological time scales. Permits relative motion of overlying plates.

Mesosphere: Semi-solid mantle that deforms in a plastic manner Lithospheric Plates • Rigid lithosphere is brittle and fractures when strongly stressed,

creating earthquakes. • Seismic zones subdivide lithosphere into tectonic plates:

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