Revised 8 / 06 (Monroe 6th ed.)

The Pre-Cambrian Earth


Pre-Cambrian (0.6 to 4.6 bya)

The PreCambrian represents the first 4 billion years of earth history (approx. 85%)

Pre-Cambrian rocks make up the bulk of continental crustal rocks

Mostly obscured by a relatively thin veneer of Phanerozoic sedimentary, igneous, and metamorphic rocks

These "rafts" of Pre-Cambrian igneous/metamorphic rocks are called the continental 'cratons'

The exposed areas are called the 'shields'

These shields are only partially exposed on the continents

DIGRESS TO: shields vs. platforms

These limited outcrop areas form the basis for our knowledge of the Pre-Cambrian earth


The primordial crust

Review of the generalized theories of the formation of the earth

Condensation - collapse of the nebula by gravitational attraction

Nucleation - the initial "crystallization" of solid matter as a result of the condensation

May have resulted in partial differentiation of an iron-nickel "core" material and a silicate "mantle" material (inhomogonous accretion)

Accumulation - the concentration, due to gravitational attraction, of the solid nucleated matter into the pre-earth

Most assume that this process resulted in a cold body

But a "profound" heating event must have occurred early in the earth's history

The source of the initial heat uncertain (isn't it all?):

Impacts of objects at high velocities

Radioactive decay

Friction from the movement of differentiating material

This heating resulted in the "final" differentiation of the iron/nickel core

Final? Seems to me that it could still be going on, at least to a limited extent

Certainly the majority of the present core must have differentiated relatively quickly

There's evidence of a magnetic field in the earliest rocks

The formation of the core was (is?) EXOTHERMIC

A large quantity of heat was liberated

Possibly enough to melt the "outer several hundred kilometers" of the earth's surface

Resulted in the formation of the "primordial crust"

What was the composition of this original crustal material

All possible composition have been suggested

Felsic, Mafic, Ultramafic, Anorthositic - define

Most assume a mafic to ultramafic composition was the most likely

This actually supports what I've been saying about the differentiation of the crust!

ie. That the initial earth was relatively un-differentiated

Felsic crustal material has only become available through time due to differentiation

"Possibly this first crust was rather unstable" for a number of reasons

Rapid convection within the mantle due to the extremely high temperatures

This upwelling of new mantle material and decent of the initial crustal materials would speed up the initial differentiation process of the crust

We're talking about the initial differentiation of relatively felsic rocks out of the original mafic to ultramafic source material

In any event...

None of this original crust has been positively identified

If any exists, it would probably be a part of the high-grade metamorphic cores of the continents

Or as xenoliths in Archean intrusives

Meteorite impacts probably had quite an effect on the earth's crust

Up to 3.9 billion years ago

This probably represents the last major stage of the earth's accumulation

Continued impact of large meteorites resulted in disruption of the original crust

Possibly instrumental in the formation of the earliest felsic continental rocks

Define proto-continent

It is assumed that from 25 to 30 "major" impacts occurred in the last 200 million years of the major bombardment phase

We're talking about some BIG chunks - "a few hundred to a few thousand kilometers in diameter"

How did these impacts result in the formation of the initial proto-continents?

Let's start with a meteorite 50 to 60 km in diameter (a small one!)

Upon impact, the meteorite would explode and create an impact crater up to 1000 km across

As well as penetrate into the earth for up to 100 km

The rocks around the crater would be "greatly heated"

The excavation of all this material would result in a decrease in pressure at depth and partial melting of the upper mantle in the immediate area below and lateral to the impact

Isostatic rebound and uplift would occur due to the loss of material

In addition, an upward readjustment of the local isotherms and increased volcanism would occur

This upwelling within the mantle is called a "mantle plume"

Describe the Hawaiian plume - is this the result of an impact in the Pacific 60 million years ago?

The impact basin would begin to fill with volcanic flows and volcaniclastics

The weight of this material would lead to basin subsidence

Partial re-melting of the volcanics at depth within the basin would lead to the formation of felsic to intermediate magmas

Remember how Bowen's Reaction Series (reversed) will result in the melting of the felsic minerals before the mafic minerals

These differentiated magmas would intrude into the overlying mafic volcanics

This sequence may have caused the initial formation of the continental sialic masses

Basically stir up the crust & allow further differentiation


The oldest rocks

Most of the exposed PreCambrian rocks consist of either metamorphic rocks or intermediate to felsic intrusives

Most represent the roots of ancient mountain systems

As do all metamorphic/intrusive terrains

Other portions of PreCambrian terrains include sections of relatively un-metamorphosed volcanics and sediments

These sedimentary sequences reflect the surface conditions at the time of formation and are important to our understanding of PreCambrian terrains and events

Many are "several thousands of meters thick"

PreCambrian rocks "contain no useful fossils"

Useful by what definition - maybe we just haven't deciphered them yet

This lack of fossil correlation makes an understanding the timing of PreCambrian events extremely difficult

There is a general division of PreCambrian rocks into two VERY broad categories

Archean - the earliest PreCambrian rocks

2.5 to 4.0 billion years old

Proterozoic - late PreCambrian

570 million to 2.5 billion years old

The rocks in most PreCambrian shield regions can be grouped into broad "structural provinces"

Individual provinces have distinctive structural characteristics

Separated by "sharp metamorphic or fault boundaries"

Greenstone Belts

Archean sequences of relatively un-metamorphosed sedimentary rocks, and slightly metamorphosed interlayered volcanic flows and breccias

The term greenstone is derived from the chloritic alteration of the mafic minerals within the predominantly basaltic lavas

Many of the basalts exhibit pillow structures

Represent seafloor extrusion

Are often associated with extensive granitic intrusives

Common in the Canadian Shield

Occur in all PreCambrian shield areas in "huge elongate downwarps"

More or less define the "broad structural provinces"

Archean sedimentary rocks associated with the greenstone belts

Predominantly greywacke, with interbedded shale & conglomerate

Greywacke indicates rapid uplift and erosion of a nearby highland area

Not much highland area during the Archean?

So short transport distances

Banded Iron Formations

Basically an iron-rich chert deposited during times of "relative volcanic and tectonic quiescence"

We'll discuss these in detail later

Features common to greenstone belts include:

DIAGRAM: generalized X-section (look familiar?)

All appear to be relatively thick

Ranging from 6,000 to 18,000 meters

Similar sedimentary features

Dominated by greywackes which were formed by turbidity currents

The amount of sedimentary material increases up-section

Bases are composed of flows and intrusives of mafic to ultramafic composition

Becoming more felsic (and clastic) up-section

Ultramafic to mafic flows and sills at the base

Intermediate flows and volcaniclastics in the middle

Felsic volcaniclastics at the top

Young greenstone belts (<3 billion years old) commonly repeat this progression several times

Younger greenstone belts also seem to have a greater amount of continental (sialic) debris associated with them

Indicates that the amount of felsic crustal material increased with time (?)

All contacts are faulted

Structurally emplaced assemblages

No idea what they rest on (ie. sima or sial)

Does all this greenstone stuff sound familiar?

It seems to me that similar suites of Phanerozoic rocks are associated with ophiolites

Refer to Back-arc model - handout


Plate tectonics and the Pre-Cambrian formation of the earth

Back to the small proto-continents which were formed by meteorite impacts in the early Archean

These mini-rafts probably "drifted" around on the underlying material looking for something similar to accrete to (mate with?)

There must have been many localized "spreading centers" in which ophiolitic types of rocks could have been formed

Therefore, I feel that the majority of the greenstone belts were probably deposited not on sial or sima, per se, but directly on the upper mantle

Again, similar to Phanerozoic ophiolite sites

This is assuming, of coarse, that plate tectonics was active in the Archean!

Obviously, I think it was

Remember, I'm the guy who proposed that the moon went through a minor period of active plate motions early in its history

So how does plate tectonics relate to the Pre-Cambrian formation of the earth?

The heat flow from the earth was probably greater in the early Archean

This probably resulted in "vigorous" plastic flow in the upper mantle

Also, the amount of felsic 'scum' was far less than today

Crustal differentiation hadn't been going on for all that long

Therefore, any felsic plates would have been relatively small and thin

And there may have been a whole herd of them (possibly as a result of the meteorite impacts discussed above)

Much of the early history of the Pre-Cambrian may have been dominated by the 'accumulation' of these minor platelets into larger plates

How does plate tectonics relate to the development of the greenstone belts?

Within individual greenstone belts, the oldest (lowest) rocks are compositionally similar to MORB's

The younger (stratigraphically higher) units are similar to island arc type volcanics

This is similar to the Rogue Volcanics problem in Josephine County

And again indicates that greenstone belts help define the margins of pre-Cambrian continental blocks


Archean Granitic Rocks

These units generally surround the greenstone belts and form the basic cores of the sialic cratons

The granitics were intruded into the greenstone belts during the latest stages of their development

Emplaced as "huge batholiths"

Therefore, it can be assumed that they came from below and essentially underlie the greenstone belts as well as surrounding them

These forms the actual stable basement complex of the cratons

Can exceed 40 km in thickness

Must have differentiated from the upper mantle and/or be the product of re-worked mafic crust

Mafic and ultramafic rocks have "a proportionally minor but volumetrically huge quantity of felsic minerals (chiefly quartz and feldspar)"

Felsic minerals have a lower density, and melt at lower temperatures, than do mafic minerals

Partial melting of mafic rocks can provide a source for felsic magmas

Theory - Could eclogite be what is left after the lighter felsic minerals have been removed from the descending basaltic crust?

The metamorphic halo surrounding Archean granitic plutons is substantially smaller than those associated with Phanerozoic plutons

Indicates a shallower depth of burial and more rapid dissipation of the heat

Shouldn't this mean that the Archean granitics are generally finer-grained?


Mobile Belts, Cratons, and Ocean Basins

As we discussed... The continental masses probably began in the early Archean as small proto-continents

The eventual enlargement of the proto-continents could have occurred in two ways

Suturing of individual proto-continents through random collisions

Peripheral accretion of newly-formed sialic material

The actual enlargement is probably a combination of both processes

Let's talk "peripheral accretion" - Platform Sequences and Geosynclinal Belts

The continental cratons became "stabilized" at the close of the Archean

Following a "worldwide episode of igneous activity" which marks the Archean/Proterozoic boundary

This had a great effect on the types of sedimentary rocks which were formed

The Archean was dominated by thick sequences of deep-water marine greywacke and shale

Proterozoic sedimentary rocks resemble Phanerozoic sediments

Except for the lack of fossils

This includes both marine and continental sediments, as well as deposits associated with transitional environments

Beaches, tidal flats, deltas, etc.

Proterozoic sedimentary rocks can be grouped into 2 broad categories

"Platform sequences"

The stable internal portions of the cratons

The elevations of the cratons appear to have been generally close to sea level since the beginning of the Proterozoic

They were covered by shallow "epicontinental" (or epieric) seas during times of subsidence

Other periods are marked by emergence and erosion

Because of these periodic relative fluctuations, Proterozoic sedimentary sequences are characterized by numerous unconformities

"Geosynclinal sequences"

Thick sedimentary sequences in excess of 10,000 meters

Nearly continuous sedimentation, so unconformities are rare

The stratigraphic record is essentially complete

Geosynclines commonly form on the trailing edges of continental plates

Can be broken into two distinctive types of sedimentary environments


Shallow-water sediments deposited on the continental shelf

Include limestone, dolomite, shale, and sandstone

These form in relatively stable settings which are free from igneous activity

This means that there shouldn't be any volcanics associated with them


Turbidites (greywacke) and pelagic sediments (shale) deposited on the continental slope and abyssal plain

Carbonates are rare (limestone and dolomite)

Volcanic flows and clastics may be present

Eugeosynclines sound just like the types of sedimentary units common to the Archean

Maybe the only difference in the Proterozoic is the addition of the 'stable' continental land masses and their associated sedimentary units

Commonly, a change in the relative plate motions has resulted in the compression of these geosynclinal sedimentary belts

Results in large-scale folding and thrusting, and ultimately mountain building

Partial melting of the base of the geosynclinal sediments results in the formation and emplacement of granitic magma

These active regions along the plate margins are called "Mobile Belts"

The Appalachian Mountains are an excellent example of a compressed Paleozoic mobile belt


Note: the terms Geosyncline, Miogeosyncline, and Eugeosyncline seem to have fallen out of favor with many geologists, and that is fine with me - they are, after all, only words representing earth processes and environments, which continue to exist no matter what we call them.


Evolving PreCambrian Environments

Banded Iron Formations and the increase of free oxygen

Refer to separate discussion


Early Life

Oxygen Metabolism and Evolution of the Metazoa

Oxygen metabolism is the way in which the higher forms of animals and plants break down compounds to produce energy

This is an aerobic process

Many primitive "bacteria" are anaerobic, and don't use oxygen to produce energy

This is a fairly inefficient process and is not suitable for complex organisms

Ex. glucose - anaerobic fermentation produces 57 kilocalories/mole

Oxidation of glucose produces 637 kilocalories/mole

Important Note: As we all know, the fermentation process is critical in the production of certain life-sustaining beverages, so let's not get too uppity about this matter!

Oxygen metabolism is required for complex, multi-celled organisms

Eucaryotic cells as opposed to prokaryotic cells

Some modern single-celled organisms can do it both ways (in humans this is illegal!)

Will normally metabolize aerobically, but when oxygen levels drop they can use anaerobic processes

Example : Yeast

It's uncertain when eucaryotic cells developed

Data indicate that there was sufficient free oxygen by 1.4 BYA to support widespread aerobic metabolism

The First Fossils

The oldest "convincing" fossils are bacteria and algae from cherts 3.2 billion years old

The first "abundant" remains are Stromatolites (a form of blue-green algae)

Some workers use stromatolite shapes to establish broad divisions in the upper Proterozoic

Biotic Change and the End of PreCambrian Time

A major expansion of life forms occurred 600 million years age

Marks the PreCambrian / Cambrian boundary

True animal fossils began to appear in the latest portions of the Proterozoic

These earliest organisms were fairly thin because of the need to disperse oxygen by diffusion through cell walls

Only later were "fatter" organisms possible due to the evolution of respiration systems

These were soft bodied organisms without hard calcium-based shells

The end of the PreCambrian (and start of the Phanerozoic) is "traditionally" when hard calcium-based shells began to appear

The first Cambrian fossils indicate a rapid evolution into complex organisms

The reason for the "sudden" appearance of abundant fossils in the Cambrian is unknown

Remember, life had existed for 3 billion years already

Why the sudden expansion of life forms?

Was it a "truly abrupt evolutionary proliferation"

Or was it that the development of hard shells allowed pre-existing life forms to be preserved?

The book seems to feel that it is unlikely that there were abundant soft bodied organisms in the PreCambrian

They would have at least left traces of burrows and trails

There is evidence of these traces increasing 700 million years ago

The book supports "a burst of metazoan evolution" at the close of the PreCambrian

What caused this rapid diversification of life forms?

The book breaks the possibilities into 2 broad categories

Biological adaptations

Ex. - the development of sexual reproduction ( as opposed to asexual 'division')

Allows a greater potential for evolutionary changes (and mutations)

Physical changes in the ecosystems

The increase of free oxygen levels in the atmosphere



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