Precambrian Time

Archean Eon

(Prekambríum)

Proterozoic Eon(Upphafsöld) (Frumlífsöld)

>4.600 MY-2.500 MY 2.500 MY-540 MY

Jarðsaga 1- Þróun Lífs og Lands –

Ólafur Ingólfsson

Hadean

Archean

First multicelled organism

First one-celled organism

Age of oldest rock

Origin of the earth

Collectively called Precambrian,

comprises about 87% of the

geologic time scale

Proterozoic

Cambrian

First fishes

Trilobites dominant

First organisms with shells

“Age of Invertebrates”Ordovician

Silurian

First insect fossils

Fishes dominant

First land plants

“Age of Fishes”Devonian

Mississippian

Pennsylvanian

Carboniferous

Extinction of trilobites and many other marine animals

First reptiles

Large coal swamps

Amphibian dominant

“Age of Amphibians”

Permian

Pal

eozo

ic

TriassicJurassic

Extinction of Dinosaurs and many other species

First Flowering Plants,

First birds,

Dinosaurs dominant

“Age of Reptiles”

Cretaceous

Mes

ozoi

c

Paleocene

Eocene

OligoceneMiocenePliocene

Tertiary

PleistoceneHumans develop

“Age of Mammals”

HoloceneQuarternary

Cen

ozoi

c

Ph

aner

ozoi

cEpochPeriodEraEon

Development of plants and animalsTime units of the Geologic time scale

4600

3800

2500

Snowball earth ??570

505

438

408

360

320

286245

208

14466.4

57.8

36.6

23.75.3

1.6

0.01Ice ages

warm

The Precambrian – most of Earth´s History

Simplified timeline4,600 million years ago. The Earth has formed, along withthe other bodies in the solar system, from a cloud of dust and gas swirling around the sun. The Precambrian era has begun.

3,750 million years ago. The oldest still-existing rockshave just formed (4.000 million years ago). The very first life is just about to appear in the form of simple, single-celled organisms (Eubacteria, Prokaryotae).

3000 million years ago. The first photosynthetic bacteriahave appeared. All life is restricted to the sea, whichprovides a fairly constant environment and protection from the sun's ultraviolet rays. With photosynthesis, the levels of atmospheric oxygen begin to rise.

2,250 million years ago. Half of Earth history has elapsed, and the first multicellular organisms are just appearing. The first Eukaryotes arise, beginning with the protists.

Simplified timeline, continued1,500 million years ago. World-wide radiation of photosyn-thetic aquatic life has significantly altered the compo-sition of the atmosphere. Free oxygen forms ozone, bloc-king ultraviolet light and paving the way for life on land.

750 million years ago. Brown and red alagae have formed, and the first animals (Annelida) are immediately around the corner.

374 million years ago. The Devonian period, the Age of Fishes, is here. In the last hour, plants and insects havebegun the colonization of the land, and the first amphibians have pulled themselves out of the water. The great Carboniferous forests will shortly arise.

Now. The first reptiles appeared early in the hour, thedinosaurs lived for about 26 minutes later in the hour. The first hominids appeared about 39 seconds ago. Modern humans have been on Earth for the past 6 seconds or so, or about 0.043% of Earth’s history...

In the beginning, Earth and all its systems (including us!) looked like this, just a huge

cloud of gas in space….

Hadean time:>4.600 to ~3.800 million years ago

Hadean time is not a geological period as such. Almost norocks on the Earth are this old - except for meteorites.

Hadean (“hellish”) Earth

• Hadean Earth– Heavy Bombardment of meteors– Outgassing from inner layers of

Earth– Atmosphere and Oceans form– Origin of the Continents

The Nine Planets: A Multimedia Tour of the Solar Systemhttp://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html

Creation of the Solar SystemThe relative abundance of heavy elements in the SolarSystem suggests that it wasformed from gas and dust derived from a supernova -the explosion of an old, massive star.

Heavier elements aregenerated within starsby nuclear fusion of hydrogen, and are otherwise uncommon.

You are composed of:

• A cloud of interstellar gas and dust isdisturbed and collapses under its own gravity. A rotating primordial gas disk forms.

• Once the larger of the dust particles get bigenough to have a nontrivial gravity, their growthaccelerates. Their gravity pulls in more, smaller particles. This goes on until the particles get to the size of boulders and asteroids.

• The trails give way to well-defined spheres of dust and gas; the protosun glows dim red.

• Eventually, after ten to a hundred millionyears, the sun has blazed to life, driving away the residual cloud material and revealing solid planets in stable orbits.

Creation of the Solar System – the brief version

Origin of the Solar SystemThe Nebular Hypothesis (Stjörnuþokukenningin)

Palomar Observatory

painting by William K. Hartmann

Hubble Space Telescape

Origins of the Planets– nebula formed of dust and gas (of

previous star(s))– collapse due to disturbance– slow rotation increases as nebula

collapses– mass collects at center of system

• hot, dense gas begins fusion (sun ignites)

– additional material collects around smaller centers of mass (planetesimals)• higher density elements condense

near primary center of mass• lower density material cleared from

center by solar wind– planetesimals coalesce into planets

Planet Building:the Condensation of Solids

• Different materials condense from the gas cloud onto grains of elements at different temperatures.

• The temperature due to the Sun varied with distance, so different materials condensed at different distances from the Sun.

Close to the Sun: metal oxides and pure metals.• Farther out: silicates and rocky material.• Outer regions: ices (water, methane & ammonia).

Planet-building processes• Dust grains stick together planetesimals• Planetesimals stick together protoplanets

– Terrestrial: • metallic / rocky • but small – not much material

– Jovian:• LOTS OF ICES, so quickly grew more massive• When ~15 x Earth’s mass, gravity strong enough to

attract lots of H/He from solar nebula• got really really big – but not dense

Mercury

VenusEarth

Mars

Jupiter

Saturn

Uranus

NeptunePluto

Terrestrial planets

asteroid belt

The Nine Planets

Jovianplanets

The planets formed and differentiated into Terrestrial and Jovian Planets

Planetary ringsNo ring system

Farther away (from 5.2 to 30 AU)

Close to the Sun (within 1.5 AU)

Many moons (over 60)Few satellites (3)

Faster rotators, differential rotationSlow rotators

Lighter elements, H and HeHeavy gas atmospheres (N2, O2, CO2)

Low density, huge gaseous atmospheresDense, rocky solid surfaces

Large and massiveSmall size, low mass

Jovian PlanetsTerrestrial Planets

The Hadean

DifferentiationHeavier materials inEarth’s core, lighter in mantle and crust

Origin of the EarthGeological Differentiation

Earth melted by:gravitational energy left from formation of the planetmeteor bombardmentRadioactive decay

Gravity concentrated denser materials (Ni & Fe) in center

Less dense materials (silicates) forced to outer layers

Crust became stable after 1 billion years

Compositional ZonesCrust: Continental and OceanicMantleCore: Outer (Liquid), Inner (Solid)

As Earth accreted, frequency of impacts decreased. BUT size of impacts increased due to increasing size of impacting bodies

Period of heavy bombardment from about 4.5 to 4 billion years ago - “magma ocean” due to widepread melting, so no rock record

Oldest rocks around 4 billion years old (Canada, Greenland, Australia) indicating presence of solid crust by that time.

Hadean Period of Heavy Bombardment

Formation of the Moon

The Moon formed as the resultof a collision between Earth anda large meteorite. Moon rocks date to 4.4-4.6 billion years

Evidence of Bombardment by Other Objects

Obliquity of Axis

Uranus tipped on its sideVenus “overturned”

All planets have at least some axial tilt

Forming of Earth’s Atmosphere

Earth’s early atmosphereEarly composition of Earth’s

Atmosphere:

The first atmosphere may haveconsisted of carbon oxides, water vapor, nitrogen, hydrogen cloride, and nobel gases.

Today’s composition of Earths Atmosphere:

21% Oxygen (O2)78% Nitrogen (N2) 0.04% Carbon Dioxide (CO2) ~0.9% Argon (Ar)0-7% water (H2O)0.01% Ozone (O)

Earth probably did not inheritits atmosphere from theasteroids that coalesced toform it; the gases that formedthe early atmosphere must partly have been emitted from within the Earth after it formed, by extensive degassing.

Possible Sources of Early Atmospheric Gases

Remnant gases from solar nebula: H2, He (these gases too light to be held in atmosphere by gravitational field)

Outgassing from Earth’s interior (volcanoes):H2O, CO2, SO2, NH3 (ammonia), N2, NOx (nitric oxides)

Material from impacting comets:H2O, CO (carbon monoxide), CH3OH (methanol), CH4 (methane), C2H2 (acetylene), C2H6 (ethane), HCN cyanide), Ar (argon)

Early atmospheric composition

HCl & Cl

Acid rain led to enhanced weathering

Early atmosphere dominated by N2, CO2 and H2O

Reduced gases from volcanic emissions: HCl and Cl gases were emitted and produce acidity in water which triggers weathering of crust minerals (Na, Mg, & Ca) which along with Claccumulated in early oceans

Atmospheric Development

4.5 4 3.5 3 2.5 2 1.5 1

BYA

100

75

50

25

0

unknown

Methane & Ammonia

Carbon Dioxide

Water Vapor

Oxygen

Evidence of Archean oxygen depletion

The mineral uraninite (UO2)occurs in pebbly sedimentsthat formed by the accumu-lation of gravel in ancientrivers, which are preservedin South Africa. The mineral uraninite cannot survive prolonged contact with free oxygen: it oxidizes.The youngest uraninite-bearing gravels are about 2.6 to 2.4 billion years.

Evidence of Archean oxygen depletionBanded Iron Formation (BIF): these rocks arenot formed on the present earth. They are verythick (thousands of m), widespread (hundreds of km), and consist of finely-laminated rocks very rich in iron, deposited in the Ocean. They started to form about 2.500 MY.

• They are composed of alternating layers of iron-rich material (commonly magnetite) and silica (chert).

•Each layer is relatively thin, varying in thickness from a millimeter or so up to several centimeters.

Absence of banded-ironformations and redbeds, andthe presence of abundanturanite and sulfides in Early Archean sediments indicate that the atmosphere was relatively depleted in oxygen.

Evolution of the Earth’s atmosphere– Initially, hydrogen and helium from the solar nebula

• Expelled after few 10’s of millions of years by Sun’s radiation

– 2nd generation, carbon dioxide (CO2), water (H20), and nitrogen (N2) from volcanic activity of hot, young Earth

• Formation of oceans, from extant water, and from water delivered by comets, began absorbing CO2

– Evolution of plant life in oceans began processing and transforming atmosphere--3rd generation

• carbon dioxide a component of seashells--limestone• over billions of years, massive limestone bedrock form• plants release oxygen which first caused oxidation of

surface (rust!), but eventually stabilized at present levels– 4 to 1 mixture of nitrogen to oxygen

Evolution of Earth’s Atmosphere

1. Initial atmosphere much like Jupiter (rich in Hydrogen and Helium derived from solar nebula)- burned off by Solar Wind / escaped weak gravitational field

2. Second atmosphere much like Venus (dominated by carbon dioxide from the planets interior)- “The Big Burp”

3. Third and present atmosphere (rich in oxygen)- modified from second atmosphere due to rise of anaerobic, photosynthesizing organisms

• Total Volume of Earth’s H2O: 1,398,898,300 cubic kilometers

• Where did all this water come from?

Origin of water and the Oceans

The Early Oceans

• The Oceans formed by water contributed bycomets as well as volcanic emission of water vapor from the planet’s interior.

• Salts were brought to early sea water by riversthat carried the products of weathering on land.

There is an enormousmass of comets “out there”. Every year 15 million small comets (<12 m in diameter) pelt Earth’s atmos-phere! Large comets can contain several km3 of ice. The ice consists of various frozen materials, mainly water, carbon dioxide, methane, and ammonia.

Early Earth

Oceans and liquid water• As the surface of the earth cooled, water

condensed out of the atmosphere. Oceans may have formed and been re-vaporized several times before a stable ocean developed.

• There is evidence for the presence of water on the Earth’s surface around 3.8 BYA.

• Total amount of water in the hydrological circle has been relatively constant.

• Many reactive atmospheric gases (CO2, HCL, SO2) are soluble & were deposited to oceans.

Origin of the Continents- A hotter Earth and Smaller Plates -

Archean Earth muchhotter than present

Only after the finalmagma ocean on Earthhad cooled to form a basaltic crust, didfelsic material begin to segregate from this mafic crust and the mantle to form the nuclei of continental crust.The decline of heat production by

radioactive decay in Earth’s interior

Mafic vs Felsic RocksIgneous rocks can be placed into four groups based

on their chemical compositions:

1. Sialic (or granitic or felsic). Dominated by siliconand aluminum (SiAl). Characteristic of continental crust. (Granite)

2. Intermediate (or andesitic) Intermediate incomposition between sialic and mafic. (Diorite)

3. Mafic (or basaltic). Contains abundantferromagnesian minerals (magnesium and iron silicates). Characteristic of Earth's oceanic crust

4. Ultramafic. Almost entirely magnesium and iron silicates

(ferromagnesian minerals). Believed to be major constituent of Earth's mantle. (Periodite)

Felsic rocks from mafic rocks – Hotspot action needed

Iceland: a modern-day protocontinent:

Earliest continental crust probably formed by segregation of felsic rocksfrom igneous material derived from the mantle, particularily in hugecentral/stratovolcanoes. Presumably numerous protocontinets, resemblingIceland, emerged during Archean time, and later sutured together to form larger protocontinents. Removal of magenesium and iron from mafic rocks by weathering produced sediments that became felsic metamorphic rocks

Archean Contients were small...

About 7% of the modern Continental crust is Archean

...the reason being an extensive network of rifting andsubduction zones and very active plate tectonic engine.

Archaean Eon - Cratons

craton - a part of the Earth's crust that has attained stability and has been little deformed for a long time. The term is restricted to continents, and includes both shield and platform.

shield - a large region of exposed basement rocks, commonly with a very gently convex surface, surrounded by sediment covered platforms.

platform - that part of the continent that is covered by flat-lying or gently tilted sedimentary rocks, underlain by a complexof rocks that were consolidated during earlier deformations.

Shields and Cratons

Protocontinents were apparently small steep-sided island arcs which weresurrounded by deep ocean basins. Evidence: The presence of abundant greenstone belts with pillow structures and the general absence of widespread continental deposits.

ArcheanContinents

Greenstone belts= Large, synclinal bodies with successions of utrabasic, mafic and felsic volcanics, and sediments

Greenstone Belts of the Superior Province

Hundreds of protocontinents...?

Plate Tectonic model for the Archaean

The Forming of Continental Crust• Fragment by fragment, formed in thebeginning from island chains similar to modern-day volcanic island arcs, the continental crust was born, and so the external land cover of the planet.

• This new type of crust had a unique feature of fundamental importance: its low density kept itriding on the surface. Thus it was able toundergo intense transformations, such as mechanical deformation (tectonics) or metamorphism, but remain always in proximity to the surface.

Large Cratons Appear

“Cratonization” about 2.700-2.300 MY, by assimilation of small crustal elements into larger units and incorporation of metamorphized sedimentary rocks (the Greenstones).

Precambrian Provinces of North America

Archean Sedimentary Rocks- Composed dominantly of graywackes ("dirty sand-stones"), conglomerates and sandstones. - Deposited in deltas, tidal flats, shallow marine shelf environments and continental shelf slopes.- Characteristic for tectonically unstable environments

References, web site resources etcStanley, Earth System History, chapter 11

Mjög skemmtileg dagskrá um myndun jarðar: http://www.scctv.net/annenberg/World_of_Chemistry_18.asxGóð mynd (Earth Reveiled Series) um myndun djúpbergs:http://www.scctv.net/annenberg/Earth_Revealed_14.asx

http://www.palaeos.com/Hadean/Hadean.htm

http://www.ucmp.berkeley.edu/precambrian/archaean.html

http://www.astro.washington.edu/labs/clearinghouse/labs/Formss/lab.htmlhttp://facweb.bhc.edu/academics/science/harwoodr/Geol102/Study/Archean.htm

http://zebu.uoregon.edu/disted/astr121.html

http://www.palaeos.com/Archean/Archean.htm

http://www.platetectonics.com/index.asp

http://www.seismo.unr.edu/ftp/pub/louie/class/100/interior.html

http://www.washlee.arlington.k12.va.us/staff/science/rymiller/21

http://www.solarviews.com/eng/comet.htm