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Lecture 3: EARTH 2: Structure, origin, age

Powerpoint Slides

Internal zones
Origin and age of Earth
Origin of the oceans and atmosphere
Important events in Earth history

(Brief notes start here)

Layer Name  Depth (km)  State  Composition Density 
 Temp.
 Crust (Continental)  55 (25-90)  solid (rigid)  "granite"  2.7  <1000
 Crust (Oceanic)  10 (5-10)  solid (rigid)  basalt  3.0  <1500
 Mantle  to 2900  solid (ductile) Mg, Fe silicates 4.5 1500-3000
 Core, Outer 2900-5300 liquid Fe, Ni metal 11.5 4000
Core, Inner 5300-6370 solid Fe, Ni metal 13.0 5000

We can also divide the earth according to rheology (i.e., if the material is rigid or plastic)
 Layer  Depth (km) Rheology
 Lithosphere 150 (continents), 70 (oceans) rigid, elastic
 Asthensophere  bottom of lithosphere to ~250 ductile, plastic

Origin and Age of Earth

"Big Bang"
Earth and Solar System

Origin of Oceans and Atmosphere

Oceans

Earth History

Geologic Time Scale: based on record in rocks of "events" (mountain building, fossil evolution); radioactive dating gave "absolute" ages (in years).
Important events:
Origin of Earth: 4,600 m.y.
Appearance of "oceans": ~4,400 m.y.
Oldest preserved rock on continents: 4,200 m.y.
First bacteria: >3,800 m.y.
Photosynthesizing algae appear: 3,500 to 2,500 m.y.
O2 in the atm.: ~2,000 m.y.
Multicellular organisms: ~600 m.y.
First "hominids": ~4 m.y.


(Detailed notes begin here)

Origin and Age of Earth: (partially from Lecture 2)

"Big Bang:" We know that the galaxies of the universe are rushing apart, i.e., the universe is expanding, and thus, the "birth" of the universe is assumed to have started with a dense, hot "exploding" moment. During expansion, the matter of the Universe "segregates" into lumps to form galaxies and their stars. The region between stars and galaxies contains gas and dust.

Earth and Solar System: The most widely accepted theory involves the following stages of development:

(1) Gravitational collapse of a rotating mass of interstellar gas
(mostly H and He) and dust beginning about 5 billion years (b.y.) ago.

(2) Formation of flat, rotating disk with matter concentrated into central "protosun" -- heating by compression and nuclear reactions in interior

(3) In the enveloping disk of gas and dust, cooling allowed solids to condense and accrete into larger and larger bodies:
dust -->"pebbles"--> asteroid-sized bodies = "planetismals",
parents of meteorites ---> planets

The origin of the Earth, other planets, and meteorites is dated at 4.6 b.y. = 4600 m.y. (million years)

The Earth and other planets continued to be bombarded intensely by planetismals and comets to about 4400 m.y. ago; impacts continue to occur. This early, intense heating of Earth (impacts, compression, radioactive decay) lead to (a) violent volcanic activity and (b) segregation of interior into silicate mantle and metallic core.

Internal Zones: core, mantle, crust

Various lines of evidence indicate that the interior of Earth consists of three concentric shells: core, mantle, and crust. The following table lists the important physical and chemical properties of those internal shells

Layers of the earth's interior
Layer
Name
 Depth
(km)
 State  Composition Density 
(g/cc)
 Temp.
deg. C
 Crust (Continental)  55 (25-90)  solid (rigid)  "granite"  2.7  <1000
 Crust (Oceanic)  10 (5-10)  solid (rigid)  basalt  3.0  <1500
 Mantle  to 2900  solid (ductile) Mg, Fe silicates 4.5 1500-3000
 Core, Outer 2900-5300 l iquid Fe, Ni metal 11.5 4000
Core, Inner 5300-6370 solid Fe, Ni metal 13.0 5000

The best evidence for this "model" of Earth's interior comes from the passage of seismic waves (earthquake-generated vibrations) through the Earth.

o Earthquakes are due to the sudden release of energy stored in the crust and mantle due to movement and deformation in those layers. When the amount of deformation becomes high, rocks are displaced violently, usually along a fault zone.

o Energy released by earthquakes is carried by different kinds of waves. The two most common are P-waves and S-waves.

Wave velocity changes and waves are bent (refracted) as they pass through different layers.

With the development of plate-tectonic theory, earth scientists realized that the outer portion of Earth (crust and upper mantle) behaves as a rigid lithosphere (broken into plates) underlain by a ductile (plastic) layer, the asthenosphere.

Internal Zones: lithosphere, asthenosphere

 Layer  Depth (km) Rheology
 Lithosphere

150 beneath continents,

70 beneath ocenas

 rigid, elastic
 Asthensophere bottom of lithosphere to ~250 ductile, plastic

Origin of Atmosphere

The composition of the atmosphere changed (evolved) as (a) reactive gases were oxidized and removed by reaction with surface rocks and (b) CO2 removed and O2 accumulated as life developed and evolved.

Origin and Evolution of the Oceans

As earliest Earth surface cooled (first few 100 m.y.), water vapor condensed ("rained-out") to form the early ocean

Acidic gases (like HCl and H2SO4) were dissolved in this "rain" and reacted with surface silicate rocks to form (a) sediments and (b) dissolved products carried by rivers to the early ocean.

How have the oceans changed over their ~4,000 million years history? This is rather speculative, but our best estimates are:

(a) Salinity and area covered have remained about constant.

(b) Depth and volume probably increased over the first 2,000 million years of Earth history, as continental crust thickened and ocean basins became deeper.


Earth History

Geologic time scale is like a "terrestrial calendar," in which geologic time is subdivided into units called Eons, Eras, Periods, etc. just like our annual calendar is divided into months, weeks, days, etc. The time scale was based initially on world-wide "events" recorded in rocks, such as fossil evolution (including mass extinctions) and the appearance of large mountain chains ("tectonic" events). Dating of rocks by radioactive methods gave ages in "years" to the "relative" geologic time scale.

Hallmark events in geologic history:

Origin of Earth: 4,600 m.y.
Appearance of "oceans": ~4,400 m.y.
Oldest preserved rock on continents: 4,200 m.y.
First bacteria: >3,800 m.y.
Photosynthesizing algae appear: 3,500 to 2,500 m.y.
O2 in the atm.: ~2,000 m.y.
Multicellular organisms: ~600 m.y.
First "hominids": ~4 m.y.


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