Origin and Differentiation of the Earth

Current hypotheses have the Earth (and the other planets) formed from material in the solar disk as described in the previous lecture.  The sequence of events leading to a differentiated Earth are presented below.
 
The Earth formed by the process known as Accretion.  This process is "gravity-driven", and involves gravitational attraction of material in the vicinity of the solar disk to the growing Earth.  The process continues to this date with meteorite impacts, but must have occurred at a very high rate while the planet was initially forming.  Release of energy during impact of the accreting bodies produced heat.  Owing to the fact that the rocky material which comprises the Earth is a good heat insulator, heat from the accretion process was maintained below the Earth's surface, as each hot layer was subsequently covered by, and insulated by overlying accretion debris. 
 
 
 
As the Earth grew larger, it's gravitational field increased and it began to compact as a result of the growing mass of largely unconsolidated material.  The process of compaction also produced heat, which further served to increase the temperature within the still-forming planet.  At this stage of planetary formation, the Earth was "warm" with internal temperatures probably reaching as high as 1500 degrees Centigrade. 
 
 
In addition to heat generated by the processes of accretion and compaction, the newly formed Earth also contained relatively small quantities of radioactive elements (esp. isotopes of U, K and Rb). [Some of these isotopes, such as U(238) have a half life of about 4.5Ga, and consequently about twice as many radioactive atoms existed at this time relatively to the present.]  As these radioactive atoms spontaneously disintegrate by nuclear fission, energy is released in the form of heat.
 
 Immediately following accretion, the Earth must have been uniform or homogeneous in composition. Furthermore, it would have lacked both an atmosphere and hydrosphere.

The three main processes involved in planetary formation (I, II and III above) each contributed heat to the newly formed planet.  The figure below (Fig. 1-9 in your textbook) represents results of calculations of interior temperatures following planetary formation.  The lowermost curve (a) gives the temperature vs depth profile immediately after accretion and compaction (i.e., without radioactive heat production).  Radioactive decay resulting in a warming of the interior as indicated by curves b and c.  At some point in time, radioactive heating resulted in temperatures exceeding the melting point for iron (curve d).

Once temperatures were high enough for iron (and other lower-melting-point minerals) to begin to melt, a profound event occurred.  This event is known as the Iron Catastrophy.  Once the interior began to partially melt, high density, iron-rich liquids began to sink toward the center of the earth, while low-density liquids rose toward the surface.  In other words, the planet began to differentiate (seperate) into a density stratified body.  As dense material sank toward the center, more energy was released in the form of heat, causing the temperatures to increase even more, resulting in more melting, more sinking of iron, and more heat being released ( a positive-feedback process).  It is likely that much of the Earth melted in this process.  There is strong evidence to suggest that the outer portions of the Moon became completely molten, thus forming a magma ocean.  The same may have also happened on the Earth.  The sequence of events and final result are illustrated in the figure below (Fig. 1-9 in your textbook):
 
 

The differentiation resulted in formation of an iron-rich core (ca. 90% Fe, 10% Ni), a low density crust rich in silicon, aluminum and oxygen, and an intermediate density mantle rich in magnesium, silicon and oxygen. The efficiency of the differentiation can be judged by the following table showing the relative abundance of elements in the whole Earth compared to that in the crust:
 
Element Wt.% in Whole Earth Wt. % in Crust
Oxygen 30
46
Iron
35
6
Silicon
15
28
Magnesium
13
4
Nickel
2.4
<1
Calcium
1.1
2.4
Aluminum
1.1
8
Sodium + Potassium
<1
4.4
 
 
Another profound result of the Iron Catastrophy was the initial formation of the primitive atmosphere and hydrosphere.  As the Earth began to melt, gases would be released in a fashion similar to that observed in contemporary volcanoes. Release of enormous quantities of volatiles during wholesale melting, resulted in the generation of the primitive atmosphere and hydrosphere. I use the term  primitive, because these volcanic gases would have been very toxic, would have contained little or no free oxygen, and certainly would have been incapable of supporting any life.  It would take an additional billion years or so, before primitive organisms could develop and begin the process of photosynthesis necessary to generate an atmosphere at all like the one we enjoy and rely upon today. 
 
 
Another consequence of the Iron Catastrophy is that it resulted in a very hot planetary interior.  The large temperature gradient between the interior and the surface results in heat transfer from the interior to the surface.  The dominant mode of Earth's heat transfer is convection, primarily in the still-hot, plastic portion of the mantle known as the asthenosphere. This convection is responsible for the many dynamic characteristics of the Earth today.  Indeed, it can be said that the past 4.6 Ga of Earth's history can be described as the dynamic consequences of the Earth's internal mechanism of transferring heat from the interior.  These consequences include, but are not limited to the following:

 
The diagram to the left presents a very simplified model showing how convection in the mantle  might be the driving force of plate tectonics.  Hot matter from deep within the interior rises  beneath divergent plate boundaries (rifts) and flows apart carring with it the overlying, rigid lithospheric plate.  Plates sink with cold matter along convergent plate boundaries (subduction zones). 
 
 
Terms and Concepts which you should understand:  
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