Geology 102 - Outline for Lecture 8
ORIGIN OF THE EARTH
THE ORIGIN OF THE SOLAR SYSTEM
The Sun is one of about 250 billion stars in the Milky Way Galaxy.
One of about 100 billion galaxies in the observable Universe.
The birth of all matter began 13-14 billion years ago with the BIG BANG.
The universe began to expand. We know this due to the Redshift
Three minutes old - formed protons and neutrons
100,000 years old – 3000°C
light elements began to form
As expansion continued it became patchy - galaxies
stars - NUCLEAR FUSION
• Hydrogen burning (sun)
• Helium burning
• Then heavier elements
Stars die.
Supernova - Form heavier elements
OUR SOLAR SYSTEM
Our solar system formed from remnants of other stars 4.54 billion years ago
How do we know it is 4540 million years ? - no rocks that age
1) alignment of planets
2) meteorites
3) moon rocks
1) alignment of planets
SOLAR NEBULA THEORY
PLANETISMALS THEORY - (planet seeds)
PLANETARY PATTERNS - oddities
- Pluto
- Uranus is on its side
- Earth - Moon
2) meteorites
meteorite - extraterrestrial object
comets - "dirty snowball"
asteroids - MOST from the Asteroid Belt - a failed planet??
Meteorites - Why study meteorites ?
Believed to be representative of the primitive material of the solar system - similar to the SUN!
a) irons - Fe-Ni alloy
b) stony irons - silicates and Fe-Ni
c) stones - silicate material
STONES
• 90% of all found meteorites
• 1) chondrites
• 2) achondrites
• 3) carbonaceous chondrites
Most meteorites are found in Antarctica.
ACHONDRITES - with different dates - 1300 ma
SNC meteorites
Shergotty, India,
Nakhla, Egypt,
Chassigny, France
Are these meteorite from Mars ?
Life on Mars?
3) Moon Formation
• moon rocks
craters are 3.8 – 4.54 ba
Theories of moon formation need to account for
• lack of water especially
• small core
• maria - basalts
• high lands - feldspar rich
impact of the Earth by a Mars-sized planetismal
INNER PLANETS - TERRESTRIAL PLANETS
Mercury, Venus, Earth, Mars.
small and dense
OUTER PLANETS - JOVIAN PLANETS
Jupiter, Saturn, Uranus, Neptune
large and gaseous
except Pluto
THE EARTH
Slightly flattened sphere
3957 miles radius - 6370 km
orbiting the Sun at about 93 million miles
AT FIRST - a homogeneous mixture
relatively cool,
lifeless ball
No surface water
No atmosphere.
Temperature began to rise due to
(i) impact energy
(ii) radioactivity
FORMATION OF LAYERS (iron catastrophe)
• Before 4000 ma
• 250 miles down - iron melted
• denser material sank - lighter material rose.
• core, mantle and crust were formed
Origin of Oceans and the Atmosphere
OCEANS
• First Ocean from water vapor
• Must have had an ocean by 3.5 ba - Why?... ??4.4 ba
• Seas more acidic than today. Why?
• Effects on weathering?
• Are they all due to outgassing and condensation?
• Salinity – same as today by early Archean
ATMOSPHERE
First atmosphere
• probably rich in hydrogen, methane and ammonia.
• rich in noble gasses such as argon, neon and krypton
• First atmosphere was lost - why?
• Why do we think we had it in the first place.
Second atmosphere
• formed from the outgassing of volcanoes.
• rich in carbon dioxide, water vapor, carbon monoxide, hydrogen, and hydrogen chloride
with maybe some ammonia and methane.
• Compare to recent volcanic gasses.
• Oxygen as a gas was not present.
• Why do we say that the early atmosphere has no oxygen?
Look at rocks of 3.5. ba - what do they tell us?
• no oxidized minerals
• Pyrite, iron sulfide is the usual iron mineral.
• Uraninite UO2
• Lack of limestones and dolomites.
• Rain more acidic
ORIGIN OF CONTINENTS
The Precambrian - from January to November
• 20% of the Earth's surface
• history is complex.
• Most of it is metamorphosed - many times
• lack of fossils
• Radiometric dating
First Crust - Most must have been basic lava and be recycled
The moon is a 'fossilized earth' in its early stages.
Partial melting of ultramafic rocks (with water) give - basaltic - intermediate and some felsic rocks.
• e.g. Iceland on MOR - 10% is felsic
• repeated partial melting produces more and more felsic rocks
• any felsic material remained at the surface and increased in volume.
probably formed first at "HOT SPOTS"
Weathering produced sedimentary rocks - first oceans by 3500 ma. (or earlier)
sedimentary rocks are also more "felsic"
this felsic material formed
PROTOCONTINENTS – small continents
ARCHEAN p. 251
• earth was hotter - radioactive decay – calculated as twice as hot.
• numerous hot spots
• many small protocontinents with rifting, subduction and transforms.
• Earth had some permanent felsic crust - which then remained at the surface.
7% of continental crust is Archean in age
• no large continents – unstable
• At 3800 we had 5-40% of the volume of crust today.
• At 2500 we had 60-100% of the volume of crust today.
• 2700-2300 ma widespread metamorphism. All Archean rocks are metamorphosed
Protocontinents were small with no continental shelf
• sedimentary rocks (now metamorphosed) are deep water
• lacks sandstones and limestones
Greenstone Belts
Greenstone belts form rounded masses surrounded by granulites
GRANULITES are
• metamorphic grade
• deformation style
• original rock type
• depth of rocks
GREENSTONES - are
• metamorphic grade
• deformation style
• original rock type
• distinctive features
• minerals now
• depth of rocks
• restricted to infolded tracts between areas of high grade basement.
Other rock types with them include chert and banded iron formation
So.....let's look at these rocks - is there a modern analogy? - uniformitarianism.
can we put them in a plate tectonic setting?
PROTEROZOIC
p. 266
over time, temperatures cooled and continents became larger.
By 2500 ma, continental crust occupied at least 60% of the area
had a thickness comparable with today’s continents.
Change in tectonic style -
a more modern style of orogeny
Proterozoic rocks are
• Less metamorphosed
• radiometric dating - easier
• large cratons appeared
• sediments like today appeared
Pongola & Witwatersrand p. 254
• clastic sediments
• intertidal sequences
• Witwatersrand, Gold
• still no life on land by end of Proterozoic
WE WILL NOT DO LIFE HERE (p. 258 - 262)
WOPMAY OROGEN
• 2000 ma
• Oldest preserved Wilson Cycle
• sedimentary sequence of passive margin rocks.
• sandstones and limestones
• fold and thrust deformation
• associated igneous intrusions
• assoiciated metamorphism
• Slave craton collided with an island arc - CONTINENTAL ACCRETION
Compare Wopmay and modern orogenic system.
Continental Accretion.
p. 280
CRATONS p. 243, 252, 254
Each continent has an exposed PreCambrian nucleus or PreCambrian Shield.
Shield -
Platform
Craton -
Cratons can be subdivided into smaller cratons
sutured together by deformed rocks or belts OROGENS
In N. America this is The Canadian Shield
part is dated at 3800-4000 million years!!
Shields are
• intensely deformed,
• metamorphosed,
• stable.
• very low lying,
• flat, featureless,
• isostacy keeps them above sea level.
Precambrian in North America
p. 281
LAURENTIA a large land mass -
N. America, Greenland, NW Scotland, ? parts of Scandinavia.
Growth of Laurentia by continental accretion
by 1.8 ba
• LAURENTIA craton - Greenland, Canada + North Central USA
• accretion of at least 5 microcontinents
• very rapid accretion
1.8 - 1.6 ba
• Accretion along the southern margin
• Antarctica, Australia, Africa
1.2-1.0 ba.
• Middle Proterozoic Rifting
• Much igneous intrusion into the center of the craton.
• a failed rift
1.2-1.0 ba ago
• A major episode - the GRENVILLE OROGENY
• Continental-continental collision on East Coast
• Recorded in the Appalachians and Adirondack Mountains
• Recorded in the Blue Ridge. 1 billion year old granite.
Position of major plates by the end of the Proterozoic??
• Rodinia supercontinent
• Breakup of Rodinia
When did oxygen become important?
ARCHEAN p. 261.
very little oxygen....then increasing slowly with photosynthesis.
End of Archean.
• oxygen 1%
2.3 - 1.9 b.a. p. 278
• Oxygen became more abundant
• a change in the oxidation of iron.
• rusty red bands of iron oxide alternating with grayish layers called Banded Iron Formation (BIF)
• Iron ranges of Minnesota, N. of Lake Superior.
1.9 b.a. onwards
• Iron is uniformly oxidized and typical red beds occur.
• Oxygen was well established by this time
• beds of limestone at this time.
End of the Proterozoic - 542 m.a.
• 10% oxygen.
SNOWBALL EARTH