Revised 10 / 10 (Monroe 6th ed.)
Including...
Accumulation and differentiation of the earth
Origin & evolution of the crust
Origin of the atmosphere and hydrosphere
How does God fit into all this? (Divine vs. Natural processes)
Just like our discussion of the universe last time
It's important to keep in mind that the processes discussed in this chapter happened a long time ago
These represent theories based on current levels of understanding
Any discussion of the ultimate origin of the earth must include the oceans and atmosphere
As well as the apparent differentiation of the earth's interior
VERY important!
No one theory proposed fully addresses all the points satisfactorily
And it's probable that we'll never really be sure how it all started
There are no direct methods to observe the formational process of the earth, and its oceans, atmosphere, and moon
All methods are essentially indirect and involve some pretty major assumptions
Leaps of faith?
So, let's keep the uncertainty factor in mind while we discuss the early formational period of the earth
Cut the earth open
Like an egg (Monroe, fig. 11.1, pg. 366)
The earth is apparently differentiated by density
(Based on seismic studies to be explored in greater detail in several weeks)
Differentiation is a very important concept so pay attention
Heaviest (iron/nickel) in the center and the lighter materials closer to the surface
DIGRESS TO: Density and Specific Gravity
Demonstrations/labs related to density:
Mercury and water
Candle in the Wind
Rock, water, oil, and air
Pepsi & Diet Pepsi
Summary of earth's interior
Note: temperature, pressure, and density increase with depth
Crust: silicates, 3 to 50 km thick
Lithosphere: silicates, down to 100 km.
Relatively cold and brittle
Asthenosphere: silicates, down to 350 km.
Relatively weak layer, plastic-like
Mantle: silicates, extreme pressure and temperature
Outer Core: iron/nickel, liquid
Inner Core: iron/nickel, solid
There are two basic theories as to how this density differentiation could have happened
Inhomogenous accretion of nebular materials
This theory rests on the assumption that the earth's core existed from the beginning
Iron and nickel would have to have condensed out of the nebula first
If accumulation began immediately, then an iron/nickel core would be formed
Additional cooling would condense the lighter silicate minerals which would accrete and form the mantle
Problems with the inhomogenous accretion theory
The order of condensation wouldn't permit Fe/Ni to nucleate first
The first to condense would be calcium and aluminum oxides
The condensation temperatures of the Fe/Ni core and the mantle silicates overlaps
This would not result in the (apparent) clear-cut definition of the core/mantle boundary
Would require quite a bit of post-accretion differentiation
The core doesn't appear to be completely Fe/Ni
10% to 20% appears to be lighter elements
Incomplete differentiation
Same with the crust - isn't 100% light stuff
These problems also plague the...
Homogenous accumulation of the nebular material
The iron/nickel core formed later by differentiation
This would require that the "initially cool" earth was heated up fairly quickly
This partial melting could have been the result of the decay of radioactive elements "which were in far greater abundance" early in the earth's history
Also from the pressure and density as contraction continued
Sinking of the heavier elements would displace the lighter materials toward the surface
DIGRESS TO: There are less than 100 elements found on nature
Not evenly distributed, either in location or amount
The 8 most abundant account for 99% of all crustal materials
Weight per cent: O (46.6); Si (27.72); Al (8.13); Fe (5.00); Ca (3.63); Na (2.83); K (2.59); Mg (2.09)
Note: Silica & Oxygen account for nearly 75%!
Oxygen is 65% of living things and the top 4 account for 96%
Anyway: The heavies go to the core - elemental iron and nickel
The lightweights float to the surface - oxygen, silica, sodium, calcium, potassium, aluminum
Problems with this theory: same as the other and...
Much of the argument against this theory rests on the need to heat up a "cool" earth
Was the earth really cool at the beginning, and if so, for how long
There is certainly ample evidence for volcanism, tectonic activity, and metamorphism early in the earth's history
Additional arguments are based on the assumed rate of differentiation from a totally homogenous earth to the present layered earth
All in all, I like a partially homogenous accretion theory
Probably a combination of the two processes (and maybe others)
Initial "local" differentiation within the nebula may have tended to concentrate a portion of the heavies near the center
But, basically, the predominant process has been post-accretion differentiation
No matter what, the earth must have been essentially solid as opposed to liquid when it initially formed
A liquid earth would have allowed complete freedom of ion migration
All the iron/nickel would have sunk to the core early
There's still too much in the crust (and probably the mantle) for this to have happened
This is important - it's where we live
At the same time all the heavy stuff is trying to sink to the center, all the light stuff is trying to rise to the surface
If heat and pressure force the inner portions of the earth to be molten or plastic, then exposure at the surface will allow this molten material to harden
Earth is a big sphere of magma which has cooled where exposed to space
DIGRESS TO: 4 states of matter
Based on energy level
Anyway, the lighter scum differentiates to the surface
Exposed to space
Cools and hardens
We call this hardened scum "rock"
DIGRESS TO: Basalt (mafic) and Granite (felsic)
Works a lot like Chicken and Dumplings, or Susie's Peach Cobbler
Scum (chicken fat) floats to surface and surrounds the dumplings
In the case of the earth, the C&D are basically made of silica and oxygen
DIGRESS TO: the importance of the silicate minerals
This crust of scum is really thin
Much thinner than an eggshell
Moves around and breaks up a lot
Discuss this in great detail soon!
Was there a primitive atmosphere?
Selective nucleation of the heavier elements early in the earth's formation could have caused the remaining nebula to become enriched in the lightest elements
These "left-overs" would include
Hydrogen - the most abundant element in the solar system
Inert gasses - do not combine with anything
Much of this remaining gas could have been trapped by gravity to form the initial atmosphere
If this happened, most of this primitive atmosphere must have escaped
The present atmosphere is significantly depleted in hydrogen and the inert gasses
Where did these gasses go?
Possibly driven into space by the solar wind
All the inner planets (Mercury to Mars) are similarly depleted
Outer planets seem to be enriched in these materials
This supports the solar wind theory
It's also possible that the gasses were never there in the first place
If the original available gasses were driven off, where did the present atmosphere and hydrosphere come from?
Several possibilities: terrestrial vs. extra-terrestrial origins
Terrestrial origins
Many geologists feel that the elements must have been included in the material of the earth
And have been brought to the surface by volcanic activity
DIGRESS: Some of the oldest dated rocks are metamorphosed sedimentary rocks
There must have been oceans in "pre-geologic time"
Argues for some pretty major volcanic activity in the earliest period of the earth's formation
This supports the assumption that increased subsurface temperatures must have been available from the beginning to permit differentiation
The early atmosphere probably mirrored whatever volatiles were included in the early volcanic rocks
Present-day volcanoes generally give off water vapor and carbon dioxide in abundance
Also many other gasses and compounds in smaller amounts
If the early-day volcanoes had a similar content (no guarantee!), the water vapor would condense to form the primitive ocean
Leaving an atmosphere essentially composed of carbon dioxide (and the other gasses and compounds)
Extra-terrestrial origins
Brought to earth by comets (giant snockballs from space)
Lots of recent support for this theory
I like a combination of the two
An initial accumulation by comets, with continuing minor additions by both processes
The overall composition of the ocean has probably changed very little through time
"It is still 96.5% water"
The remaining 3.5% is composed of dissolved minerals
This chemistry may have changed (and continue to change)
Certainly the percentage is slowly increasing as chemical weathering and transport continue
We'll touch on some of these changes in greater detail later
The rate of increase of the volume of water is unknown
Is the hydrosphere a closed system?
In all probability, major bombardments by comets and increased volcanism in the earliest days of the earth resulted in the formation of a relatively large ocean fairly quickly
With (relatively) minimal additions since
Click here for additional information on the earth's hydrosphere
Unlike the "fairly constant" composition of the ocean, the atmosphere has almost certainly evolved with time
The earliest atmosphere was probably dominated by carbon dioxide
No animal life to convert carbon dioxide to oxygen
Carbon dioxide still makes up the majority of the atmospheres of Venus and Mars
Two processes have helped to evolve the earth's atmosphere to what it is today
78% nitrogen, 21% oxygen, and traces of several other elements and compounds
Carbon dioxide is a very small (but important) constituant
Inorganic processes
Result in the removal of carbon dioxide and other "reactive" gasses from the atmosphere
Essentially through chemical reactions with rocks and water
Forms new solid materials (Carbonates, Sulfates, Nitrates, etc.)
Organic processes
Tied to the origin and expansion of life
Both nitrogen and oxygen are directly tied to animal and plant respiration
Both as a source of energy and as a by-product (or waste product, if you prefer)
DIGRESS TO: fossil fuels and global warming
Click here for additional information on the earth's atmosphere
Let's start with some basic ground rules
What constitutes "life?" (Get responses & put on board)
Some possibilities...
Grow
Die
Reproduce
Use energy
Convert "food" to energy
Produce waste
Adjust to surroundings
Adapt / evolve
Carbon based
Water based
Note: While water may be required for life, according to the 4th Law of GeoFantasy it is ultimately destructive to all non-organic materials found on the earth
The appearance of abundant fossilized remains is a rather late event in the earth's history
The Phanerozoic Eon represents only 15% of the earth's overall history
However, there is evidence of primitive life dating back to the oldest rocks
This indicates that the ultimate beginnings of life may have occurred during the "pre-geological interval"
How did all this happen
Obviously, no one knows, but several assumption can be made
Basically the same 2 choices as for the air & water
Formed in place
All life, as we know it, is composed of combinations of four elements
Hydrogen, carbon, nitrogen and oxygen
These elements "were undoubtedly" present in large quantities throughout the earth's history
Biologists say that: (that's right, blame it on the biologists)
Interactions between "simple compounds" of these 4 elements with ultraviolet radiation, "and other energy sources", caused the formation of "complex compounds"
Quite a mouthful!
Recent laboratory simulations have supported this theory, somewhat
What they have demonstrated is that the proposed interactions COULD have produced the necessary building blocks from which a living organism MAY have been formed
Also, that any oxygen present MUST be combined with other elements
There can be no free oxygen in the system
The organic building blocks don't appear to form in the presence of free Oxygen
Remember, free oxygen constitutes 21% of our present-day atmosphere
This indicates that free oxygen is a relatively recent addition to our atmosphere
The occurrence of the banded-iron formations also indicates that there was little or no free oxygen in the early atmosphere
DIGRESS TO: Banded Iron Formations
Hydrothermal vent sites: chemosynthesis
How about an extra-terrestrial origin
Recent studies indicate organic compounds in comets!
Life may be much more "universal" than some of us would like to believe
Brings us to a potentially sticky discussion...
This may be a good place to address His input, if any
I use the masculine out of tradition and convenience only
God could well be female, or some other gender we can't comprehend.
There is little doubt that the origin and evolution of the earth and life have progressed along lines similar to those proposed by modern science
The geologic record is what it is!
But, something had to add that initial spark to set it all in motion
I don't feel that God and the geologic record are necessarily at odds
Some feel that God, if He exists, is watching and controlling all that we do
I prefer a more "regional" view
Something got all this going (call it God if you want to)
And, once set in motion, the sequence of events will themselves dictate their course
It's not at all clear to me whether God created us in his image, or we create Him in ours.
If there is a God, I'd like to think that He's as curious, and likes surprises, as much as I do
Absolute control over all aspects of our lives would be terminally boring, and I really don't think that MY God is into boredom
I'd like to think that He's sitting back with a six-pack of His favorite beverage and enjoying the show
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