Download The Origin of the Earth The earth, then, grew

The Evolving Universe
And The Building of Matter
The Planets Form
•Inner Planets: Mercury, Venus, Earth, and Mars
•Giant Outer Planets: Jupiter, Saturn, Uranus, and Neptune
The Origin of the Earth
•The earth, then, grew from the accretion of planetismals out of the original
gas and dust cloud. The initial dust cloud was cold, thus the evolving protoearth must have been cold.
•Initially the mass accumulated by gravitational collapse.
•In time earth grew by gathering planetismals (accretion)
Took place about 4.7 billion years ago.
Under these conditions the earth would originally have begun as a homogeneous
mass of silicate compounds, iron oxides, magnesium oxides, and other (especially)
gaseous elements.
Yet a homogeneous mass of rock is not what we see. We have an earth stratified into
density layers.
What was the source of this stratification?
It results from the dissipation of energy which allows the fractionation of the
homogeneous earth into its layers.
The rapid heating of the earth early in its history took place via three sources.
1. Heat of impact
2. Heat of compression (gravitational collapse)
This is enough to raise the average internal temperature of the earth to
1000º C.
3. Radioactive decay
Uranium, thorium, radioactive potassium, and were enough to get the
required T.
The results of all the generation of heat during the earth's formation is that the earth
becomes a molten hot ball of iron and rock.
•The heavier materials start to sink toward the center to form the earth's core.
•Lighter material float toward the surface and form the mantle.
•The lightest basic igneous rocks form a thin scum on the surface which will
become the crust.
Instead of a homogeneous mass of rock we now have a body stratified into layers
by density - a fractionation process.
Rock is a very poor conductor of heat.
Thus, the earth cooled to a semi-solid state fairly quickly.
The core, however, remains hot and liquid.
It will require more than 4 by to cool off.
It is the heat coming from the core that keeps the earth geologically active to this
day.
It is hard to say why the other planets did not do through this cycle, but they quickly
became closed systems.
From what we have been able to gather earthquakes are rare or non existent indicating
they are indeed dead.
May have to do with relative sizes
There has been little explanation so far. That is, the mechanisms by which these
changes take place have not been justified.
Earths Unique Character
1.
Differentiation stratified earth chemically
a.
a metallic core (Fe, Ni)
b.
a rock mantle (Mg, Fe, Si)
c.
and crust (O, Si, Al, Fe, Mg, Ca, K, Na)
2. Differentiation, caused by repeated melting of the crust and mantle, stratified the
Earth physically. This allowed less dense material to rise to the surface
a. This less dense material collected into primitive continents that grew and
were reshaped by processes of erosion, sedimentation, and plate
tectonics
3. Differentiation caused oceans and
atmosphere to develop as the earths
surface cooled and volcanic activity
continued:
a. volcanic eruptions today
release steam, H, CO2, and
N
b. steam condenses to water,
which became the oceans.
The remaining gases
evolved into the present
atmosphere
c. Principle of
Uniformitarianism: Present
is the key to the past
4.
NOTE: Differentiation is responsible for the creation of earths "Dynamic
Systems" (powered by heat engines)
a.
External Processes - powered by the sun
i.
b.
(1)
Hydrosphere
(2)
Atmosphere
(3)
Lithosphere (earths surface)
(4)
Biosphere
Internal Processes - powered by decay from radioactive elements
i.
5.
Four Realms (Envelopes) of the Earth
melts rocks, moves continents, forms mountains
About 4 billion yrs ago, Earth had come to resemble its present structure