Revised 10 / 10 (Monroe 6th ed.)

The Earth - an overview




Accumulation and differentiation of the earth

Origin & evolution of the crust

Origin of the atmosphere and hydrosphere

The Ocean

The Atmosphere

The beginnings of life

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


Accumulation and differentiation 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


Origin & Evolution of the Crust

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!


Origin of the atmosphere and hydrosphere

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


Evolution of the Ocean

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


Evolution of the Atmosphere

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


The beginnings of life

Let's start with some basic ground rules

What constitutes "life?" (Get responses & put on board)

Some possibilities...




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...


How does God fit into all this? (Divine vs. Natural processes)

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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