Lectures 3 and 4:
Origin and Differentiation of the Earth


"One gets such wholesale returns of conjecture
out of such a trifling investment of fact"
                                    Mark Twain

 
"Refrain from illusions, insist on work and
not words.  Patiently search divine and scientific truth"
                                    Mendeleyev's Mother (not known for her humor)

Summary of Topics Covered:

Some questions to consider:

Stages of the Earth's Accretion and Differentiation that affected its thermal structure:

  1. Bombardment
  2. Gravitational Compression
  3. Radioactive Heating
      4. Just after the accretion, the Earth is a homogeneous, undifferentiated planet with an average temperature of ~1000 C.

As the Earth began to warm up, in response to 1, 2, and principally 3 above, it moved through the following stages

  1. Iron Catastrophe  (formation of Iron core as geotherm heats up beyond the melting point of iron).
  2. Possible formation of a magma ocean through voluminous volcanic eruptions which provide gases for formation of the atmosphere and eventually the oceans.
  3. Continued differentiation leads to "chemical zonation"

    4.  
      This last stage, chemical zonation, continues today as the Earth convects and melts.
  Note about Chemical Zonation:  Chemical and Behavioral Zonation are often different beasts!  Keep this in mind throughout the course!

 

Some further notes on the early formative history of the Earth.

Following the formation of the universe (about 12-20 billion years ago), one of the early stars began to run out of Hydrogen fuel. This star expanded to a red giant and then collapsed on itself and exploded in a supernova. In this supernova, like billions that have occurred elsewhere in our Universe, all of the other elements were created.

I. The mass of new matter again collapsed into a disk shape mass of dust and gas
(a). The center became superheated and formed a new star, our sun
(b). From this disk of matter the planets began to condense
(c), according to the widely supported nebular hypothesis of Immanuel Kantand Pierre-Simon Laplace. The two strongest points in favor of this idea are: 1) that the disk began by rotating in one direction and the rotation of all of the planets around the sun follows the original disk; and 2) that because the disk flattened out as time progresses, all of the orbits of the planets (except Pluto) lie more or less in the same plane
(d). Pluto is possibly a captured giant asteroid.
 
 

III. The earth condensed in four basic steps. 1) It began to accrete from
 the nebular cloud as particles smashed into each other forming so-called
planetesimals. These in turn collided with each other and as their mass
grew began to gather material from the nebular disk. 2) As the mass of the
Earth grew so did it's gravitational force and the Earth began to compress
itself into a smaller and denser body. This happened about 4.5 billion
years ago. 3) In the third step the compression itself began to heat the
interior of the Earth; also there was heat generated by radioactive decay.
The interior of the earth began to melt. Because iron is the heaviest of the
common elements that make up the Earth, as the Earth began to melt
droplets of melted iron began to sink towards the center of the earth, where
they condensed. 4) Proceeding slowly at first it sped up to catastrophic
 proportions - hence it is called the iron catastrophe. Note that 3 and 4 in
 the figures to the right are cross sections