Earth’s Formation
Solar Nebular Hypothesis
Precambrian Earth
Hadean
4.6 to ~4 billion years ago
Archean
4 to ~2.5 billion years ago
Proterozoic
2.5 billion to ~560 million years ago
What was the early earth like?
Scientists believe:
Hotter than today
No crust early on (more in a bit)
No water early on
No atmosphere early on
No magnetic field, strong solar wind removes lighter elements from planet
Atmosphere develops later due to volcanic outgassing (rich in ammonia and methane)
No free oxygen in atmosphere originally
Many meteor impacts
How do we know this
Iron rich minerals (free oxygen indicators)
no oxidation prior to the proterozoic
Need free oxygen to oxidize iron minerals
Use Zircons to show:
Crust probably existed around 4.3 billion years
Oceans formed pre-4 billion years but not before 4.3 billion years
What is a zircon?
Zircons:
Mineral composition: ZrSiO4
High temperature mineral (stable up to 700oC)
Can withstand metamorphism
Doesn’t weather away at earth’s surface easily
Has traces of Uranium in structure
Great for Ur-Pb age dating
Great for Oxygen isotope analysis
Understand information about atmosphere and water conditions at time it formed.
Hadean (4.6 to 4 billion years)
Few rocks exist from this time
Zircons (mineral) from this time
W. Australia: 4.3-4.1 bybp (sandstones)
Canada: 4 bybp (gneiss is 3.8 bybp)
Isotopic analysis shows formed from rocks approximately 200 million years old
A form of plate tectonics was active
Earth was cooling down and crust was forming
How did crust form?
Early earth very hot and molten
Cooling of earth started to cause an ultramafic crust (of sorts) to form. (mantle composition)
Due to heat, convection cells drove a paleo- plate tectonics and modified versions of subduction occurred
Crust today is NOT ultramafic. How did current crust evolve?
Primarily believed to have formed by partial melting.
Partial Melting
In nature, when a rock melts, not all of the rock melts…only a portion of it.
Rocks are made of an accumulation of minerals which all have different melting points.
Minerals rich in silica melt at low temperatures
Minerals rich in iron melt at high temperatures
When partial melting occurs, the minerals richer in silica melt but the others don’t
This creates a melt that is richer in silica than the original rock it came from
Partial melting in reality
Partial melt:
The mantle (ultramafic) and produce Oceanic Crust (Basalt (mafic))
Partial melt:
Basalt and produce andesite and granite (continental crust)
Early earth:
Very Hot
Mostly ultramafic
Cooling down created a thin crust
Convection starts moving crust and a form of plate tectonics starts
Partial melting of utramafic crust leads to an eventual formation of oceanic crust
Partial melting of oceanic crust eventually leads to the formation of continental crust
Pre-Archean: No true continents
Small volcanic islands that later join in accretion to create cratons
Building Continents
What is a continent?
Must have a craton
Core of the continent
Represents oldest rocks
Cratons are made up of
Shields: Ancient igneous rocks that are often metamorphosed
Platforms: sedimentary rocks that accumulate on side of shields that have been metamorphosed
Oldest crust on earth: Canadian shield
Archean Rocks
Granite
Gneiss
Greenstone Belts
Lower part Volcanic and peridotite
Middle volcanic
Top sandstones and shales
Other Archean developments:
Development of the Atmosphere:
Early on:
No free oxygen (actually no atmosphere)
No Ozone layer
Volcanic outgassing produced atmosphere when magnetic field was established
Mostly CO2, Ammonia, and Methane
When did free oxygen occur and how?
Oxygen
2 ways to get “free” oxygen
Photochemical dissociation
H20 Molecules are broken up by ultraviolet radiation to create H2 and O2 molecules
Self limiting process…also produces ozone which forms a barrier to ultraviolet radiation
Photosynthesis
Occurs as organisms combine CO2 with H2O and release O2 as a by-product
3.5 billion years ago: Stomatolites started photosynthesizing.
Cyanobactera
Anaerobic
autotrophic
Prokaryotic cells
Development of Oceans
Early earth:
Too hot for liquid water
As earth cooled, water from outgassing begins to condense and accumulate
Meteorites and comets bring water to surface
Oceans present by early Archean
By the end of the Archean (2.5 bil years) 30-40% of all continental mass existed
Granite, gneiss and greenstone belts
Oceans of a sort existed
Atmosphere existed but still only about 1% of free oxygen (today 21%)
Proterozoic
Evolution of crust changes
More evolved composition of crust
Passive continental margins form and sedimentary rocks become common
Collisions of Archean crust creates larger cratons
Development of Laurentia
North America, Greenland, Scotland and Baltic shield
Several proto-continents collide
Superior, Slave, Rae, Hearne, Wyoming, Nain
Creates mountains and builds N. America continent
Grenville orogeny represents final stage of Laurentia coming together
Supercontinents
1st super continent there is evidence for is Rodina
Formed between 1.3 to 1 billion years ago
Broke apart ~750 million years ago
2nd Super continent called Pannotia
Formed about 650 million years ago a
Broke apart ~550 million years ago
Climate and Atmosphere during the Proterozoic
2 glacial episode occurred
Glacial deposits used as evidence
Locations where the glacial ice covered is approximate
Oxygen in atmosphere increased to ~10%
Banded iron formations (2.3 bybp)
Alternating layers of chert and iron minerals
Red beds (1.8 bybp)
Proterozoic life (2.5bybp-560 mybp)
Archean:
Start:
Prokaryotic, heterotrophic, anaerobic organisms
End:
Prokaryotic, autotrophic, anaerobic organisms
(cyanobacteria)
Proterozoic:
Start
Prokaryotic, autotrophic, anaerobic organisms
(cyanobacteria)
2.1 bybp: Eukaryotic cells form
Larger than Prokaryotic cells
Multicellular with nucleus containing genetic information
Reproduce sexually
At this point still anaerobic
How did eukaryoticcells develop?
New Geologic time suggested and accepted
Ediacaran Period
650-543 mybp
Established in 2004
No fossil record observed earlier
High powered microscopes revealed algae and bacteria of Ediacarian
Create a new Period in the geologic time table
Ediacarian Fauna (670-570 mybp)
Unique group of muticellular soft bodied animals
Preserved as molds and casts
Many thought to have relatives that existed later
Jellyfish, trilobites, etc
Originally found in Australia now found world wide
All known Proterozoic animals were soft bodied
Some evidence early skeletons were developing
True skeletons aren’t found till ~545 million years