Birth of the Solar System.
Immanuel Kant 1755 - Nebular Theory
- spinning cloud of gas, contracting and flattening into a disk.
- explains orbits and rotation of the planets nicely

*** new version by Weiszacker and Kuiper - solar nebula theory
- basically same as Kant, turbulence in the eddies led to the planetesimals condensing
- refractory elements (heavier elements that are stable at higher temperatures) condensed in the inner nebula.  Explains Terrestrial Planets.
- volatile elements (lighter elements that are stable only at lower temperatures) condensed in the outer nebula.  Explains Jovian Planets.

 Planetary Formation - 3 steps
1) Condensation in nebula created small globs of mafic silicate minerals, like olivine.
2) Accretion into Protoplanets -- small globs => big globs, protoplanets swept up all the globules into a real planet.
3) Differentiation - all the terrestrial planets have an iron core and a silicate mineral rind.  The process of producing an internally zoned planet is called Differentiation.  How? I don’t know....
 

Meteorites and their message
- Meteorites are bits of solar system that enter earth's atmosphere.  Most burn up from friction (never hit). ~500/year baseball-sized.
- Rare bigger ones hit, including a big one about 65 Ma (10 km diameter in Mexico).
- Big ones cause local melting of any rocks that they hit.  Explosive craters.

 5 types: Stones, irons and stony-irons:  DESCRIPTION

1) Chondrites - 79% - most common
 - tiny balls (chondrules) of mafic minerals formed by rapid cooling.
 - Age = 4.6 Ga.  Oldest rocks of solar system.

2)  Carbonaceous Chondrites - 5% - chondrites, plus minor organic compounds such as amino acids.
 - Composition same as Sun (for non-volatile elements like C, Si, Al, Fe, Mg).

3)  Achondrites - 8% similar to terrestrial mafic igneous rocks, some with brecciated texture.

4)  Iron Meteorites - 6% - intergrowths of iron-nickel alloys.
 - Large crystals indicating slow crystallization.

5)  Stony-irons - 2% - mixture of iron-nickel and silicate minerals.

 5 types: INTERPRETATION
1)   Carbonaceous Chondrites - chondrites + carbon.
 - Represent protoplanetary material formed at condensing of the solar nebula and never remelted.

2)  Iron Meteorites - iron-nickel alloys.
 - Core of differentiated protoplanets.

3)  Achondrites - mafic igneous rocks, some brecciated.
 - Represent somewhat younger pieces of igneous rocks produced on larger asteroids.

4)  Stony-irons - mix: iron-nickel and silicate minerals.
 - Interpreted to represent the transition zone between an iron-nickel core and a silicate mantle.

5)  Chondrites - mafic silicates formed by rapid cooling.
 - May represent crystallization as drops from melted silicates as a result of asteroid collisions in the accretionary phase.

 The Moon -  first mapped in 1609 by Galileo.  One of the largest moons in the solar system.
- Low density (3.3).  - small core.
- Dry County: No water, volatiles.
- Old highlands (cratered), young basalt “seas”.

Origin of the Moon
1) Moon accreted along with the earth as part of a two-planet system.
 - should have similar compositions.

2)  Moon accreted separately, was captured by earth.
 - problems in mechanics of orbital capture.

3)  Moon spun off from the earth during earth’s differentiation.  1898 by George Darwin.
 - Pacific Ocean is hole left by Moon.
 - No: Moon could not reach escape velocity.

4)  Impact by a Mars-sized protoplanet.
 - Explains difference in composition, relative sizes.
 
 

Precambrian - geologic time before the appearance of hard-shelled organisms (clams and snails and friends).
Precambrian divided into:
Proterozoic -     540 - 2500 Ma
Archean -     2500-3800 Ma
Hadean -    older than 3800 Ma

Hadean  - (the pre-Archean) planetary accretion,
differentiation, HOT Earth.  Why so Hot?
1)

2)
 

- Very little geologic record of Hadean.  Why not?

Craton:  Strong, stable portion of continents.  Stable means little orogenic activity in past 1 Ga or so.  For North America, the craton is in Canada. For example - Canada has the oldest known ophiolite sequence (about 2.0 Ga).  What does this finding tell us?

Archean rocks - Greenstone Belts (small orogenic belts) separated by G-G complexes

- Granite-Gneiss Complexes - high grade gneisses (commonly Granulite grade) from regional metamorphism of intermediate to felsic igneous rocks including tonalites (diorite + 10% quartz)
- Greenstones - low to intermediate grade metamorphism of
 - sediments - shales, sandstones, banded iron formations
 - mafic (and some felsic) volcanic rocks - lots of pillow basalts
 - ultramafics - igneous rocks called komatiites (high Mg, Ca, low SiO2 igneous rocks, formed from extensive 60-80% partial melting of mantle peridotite).

Why do we call them Greenstone Belts? - low to int grade Greenschist Metamorphic Facies - mafic igneous rocks (in particular) grow chlorite micas as an early metamorphic mineral, gives a dominant green color to the rocks.

Archean Tectonics - Subduction and back arc spreading
- Overall setting is Hot - probably convection more rapid than today - Why Hotter??
- Oceanic crust dominated by komatiite not basalt as today - more dense, more easily subducted
- Diorites (Tonalites) formed at ocean-ocean subduction zones, formed small felsic terranes
- Several felsic terranes collided to form bigger protocontinents

3 Possible Models
1) - Back-Arc Spreading produces Extension
 - ultramafic komatiite volcanism
 - mafic (and felsic) volcanism - partial melting of crust
 - sediments fill in the basins as the volcanism ceases and the crust cools
  - turbidites record some deep water sedimentation
  - shallow water sediments eroded from volcanic arcs
  - includes Witwatersrand Supergroup in S. Africa - famous for placer gold.
2) - Fore-Arc Basin - - Collision with other terranes causes deformation, regional metamorphism
3) - Continental Rifting -
*** Very Little Data - No Consensus ***  Protocontinents grew by terrane accretion.  Superior province greenstones stacked in there closely as a result of repeated opening and closings of back-arc greenstone basins.
 

Life of the Archean:  Not a pleasant place. 
- Methane, Ammonia, Hydrogen, Water vapor.
- Very little oxygen.  Might be a good thing, since it can be poisonous in large doses or to very simple organic compounds.
- No ozone screen in the upper atmosphere.  This prevents the penetration of UV radiation that could inactivate DNA molecules.
 

Types of Archean Fossils:
- Procaryotes - no nucleus, no organelles, DNA loose in cytoplasm, poor tolerance for oxygen, resistant to UV.  Blue-green algae.  Asexual reproduction.
1)  Micro-organisms:  spheroids, rod-shaped, filamentous.  Similar to forms of today's bacteria.  Oldest microfossils found are at ~3.5 Ga.  Some carbon with an organic isotopic signature found in banded iron formations of Greenland indicate 3.8 Ga is a possibility.....
2)  Stromatolites - laminar, organic sedimentary structures formed by trapping of sedimentary particles and precipitation of calcite by algal mats.  Oldest are 3.5 Ga, but not really widespread until the Proterozoic (availability of suitable marine shelf areas?)
 
 

Origin of Life?  A big question, but Not One Answerable By Science!  We can hypothesize that:

1)  organic molecules could be created by natural processes in the early hydrosphere.
This is Testable and has been, starting in 1953 by Stanley Miller and Harold Urey.  Sparks in a proto-atmosphere produced amino acids and other organic molecules.
 
2)  organic molecules Were Created by natural processes in the early hydrosphere.
This is UNTESTABLE.  Not a scientific theory.
 
 

Possibility of Black Smokers as sites for the origin of life???
1)  present day: rich animal communities built on abundance of sulfur-reducing bacteria
2)  in past: constant temperature, nutrient source, no danger of UV....
 

1. Hadean (4.6 to 3.8 Ga)
a. accretion, differentiation, formation of Moon
b. too hot to preserve significant continetal material

2. Archean (3.8 to 2.5 Ga)
a. small proto-continents grew by terrane accretion (greenstone belts & granite-gneiss complexes)
b. early life appeared: bacteria-like organisms and stromatolites, oxygen introduced to atmosphere

3. Proterozoic (2.5 Ga to 540 Ma)
a. suturing of proto-continents into full-fledged continents (like Laurentia)
b. Grenville Orogeny: formation of first recognized "supercontinent" (Rodinia)
c. Rifting and breakup of Rodinia
d. Early life matured: rise of eukaryotes and multicellular organisms (Ediacara fauna)
 

1. More terrane accretion - Fig 10.2 in text
- Archean protocontinents all clanging together
- connected by Proterozoic orogenic belts.

2. Slave province as an example: Archean proto-continent, contains lots of stromatolites
 
 
 
 
 
 
 

a. Rocks say: thick metamorphosed sedimentary rocks, some basalt, faults: normal, thrust and strike-slip. (these are in Thelon and Taltson belts, see map).

b. 1970 Plate Story: Rifts developed around Slave on south and east - then rifting halted ("failed rift" or Aulocogen).  Textbook example!
c. 1990 Plate Story: Slave Province (see map) slides northeast along LARGE transform (strike-slip) fault, collides with Rae Craton
 

- This creates Thelon Orogen - what kind of collision would you call it?
 

- No rifting involved!  Rewrite the textbooks!

d. West Side Story: Wopmay Orogen (Fig. 10.4)
Sketch this: (dates: 2.2 Ga to 2.1 Ga)
 
 
 
 
 
 
 
 
 
 

Evidence for what parts of Wilson Cycle?
 

2. Along with Grenville collisions, see Keweenawan rocks of the Midcontinent region (Fig. 10.7):

narrow band of thick
basalts, sandstones,
shales with copper and
oil, normal faults….

Tectonic Origin?

_______________
 
 
 

Evidence for Rifting and Breakup???
- Belt Supergroup of Montana (lots of thick sed rocks: carbonates & sandstones, some basalt)
 

- Grand Canyon Supergroup (similar to Belt rocks)
 

- Chilhowee Supergroup (in Appalachians: normal faulted basins with thick conglomerates, overlain by thick sed rocks)
 
 
 
 

Eukaryotes - Complex.
  - Nucleus with membrane, organelles, DNA in chromosomes, more sensitive to UV.  Capable of sexual reproduction.
 

One Model: Prokaryotes + Symbiosis => Eukaryotes.
 

Look up Symbiosis if you don’t know what it is.

Read Perspective 10.1 on p. 280-281.
 

- 1947 discovery of fossils in the Ediacara Hills

- B. on overhead = Sprigina floundersi - possible precursor to the Trilobite

- F. on overhead = Tribachidium heraldicum - possible precursor to the echinoderms

- Possibly NOT ancestors of anything, just deadends.

- Proposed a new Period be added to the Paleozoic;  the Ediacarian Period (~ 670-540 Ma)
-- What do you think?

- No trace fossils, especially the burrowing, grazing type.  What does this mean?

- Did they evolve quickly when oxygen became abundant?  Or had they evolved in locally oxygen rich areas and just proliferated when oxygen became more abundant?  Don't know.