precambrian time

Earth History, Ch. 11 1

Precambrian Time

• “Precambian” is the informal term for the interval of time prior to the evolutionary radiation of skeletonized animals at 543 mybp

• “Precambrian” is subdivided into:– Archean Eon, from the origin of the Earth (4.6 bybp)

to the stabilization of Earth’s basic structure (core/mantle/crust) (2.5 bybp)

– Proterozoic Eon, from 2.5 bybp to the beginning of Cambrian time (543 mybp)


Archean43%Phanerozoic

12%

Proterozoic45%

Geologic time


Precambrian rocks

• Although Precambrian time accounts for 88% of Earth’s history, Precambrian rock exposures make up only about 20% of Earth’s land surface

• Most Precambrian rocks have been destroyed in the course of plate tectonic cycles (and most remaining ones are buried beneath the veneer of Phanerozoic rocks)


Precambrian rocks• Cratons are the large, stable, interior regions of

continents that have not undergone major deformation since Precambrian or early Phanerozoic time

• Most Precambrian rocks are confined to cratons, where they may be exposed in a “Precambrian shield”


Precambrianshield area inNW Canada


Archean Time:From the very beginning….

• Age of universe is estimated at ~15 billion years (redshift evidence)

• Oldest radiometrically dated rocks on Earth are ~4.1 billion years old

• But, meteorites and lunar rocks have been dated at 4.6 billion years, suggesting that our solar system is about that old


Origin of our galaxy and solar system

Solar nebula forms(remains of supernova)

Rotation andcontraction to disk

Central concentrationof matter

Formation of discreterings of matter

Condensation of matterinto planets


Origin of our galaxy and solar system (cont.)

• Outer planets are composed largely of volatile compounds

• Denser, less volatile compounds make up the inner planets

• Asteroid belt is a ring of debris that has not coalesced into a planet


Origin of Earth

• Primordial Earth accreted from successive impacts of hot, giant asteroids (some the size of Mars)

• Early Earth was molten because of heat from energy of impacts and radioactive decay

• Dense materials sank to center of planet, with less dense materials rising toward surface

• “Magma ocean” at surface eventually cooled to form oceanic crust


Origin of Earth (cont.)

Homogeneousmolten Earth

Segregation ofmaterials by density

Final differentiationof core/mantle/crust


Earth’s early heat flow

• Earth had greater heat flow in the Archean Eon than today, because Earth’s radioactive “furnace” was hotter

• “Hot spots” were numerous; lithosphere was fragmented into many small plates

• Felsic crust was partitioned into small “protocontinents”


Earth’s internal heat


Origin of the Moon

• Moon originated when a large (Mars-size) body collided with Earth (“glancing blow”)– Core of impacting body was

absorbed into Earth’s core– Remaining mantle of impacting

body and was then captured in Earth’s gravitational field

• Collision caused Earth’s rotation to increase

• Moon has no water; a metallic core and feldspar-rich outer layer; relative abundance of iron and magnesium differ from that in Earth’s mantle


Earth’s early atmosphere

• Earth did not inherit its atmosphere from the initial asteroids that coalesced to form it

• Earliest atmosphere was generated by emission of internal gases (similar to those emitted today from volcanoes):– Water vapor, hydrogen, hydrogen chloride, carbon

monoxide, carbon dioxide, nitrogen

• Note absence of oxygen, which was rare prior to the advent of photosynthetic organisms!


Earth’s early oceans

• Ocean water originated partly from emitted water vapor and partly from icy comets as they melted upon entry into Earth’s atmosphere– 15 million small comets (~12 meters in diameter) enter

Earth’s atmosphere every year!

• Salts were added to the oceans from rivers carrying by-products of chemically weathered rocks– Salinity stabilized very early in Archean time because

salt is removed from the oceans by precipitation of salt minerals


Origin of continents

• Earth’s early crust was entirely oceanic crust of mafic composition

• Earliest continental (felsic) crust must have originated from a mafic parent, but how?– When mafic crust is subducted and melted, the

resulting extrusive volcanics still possess a mafic or intermediate composition

– Igneous activity associated with hot spots can produce felsic volcanics!!


Origin of continents: Iceland example

• Iceland is a volcanic island situated over a hot spot along the mid-Atlantic ridge

• Here, lower oceanic crust contains isolated “pods” of felsic material that have segregated from igneous material in the mantle

• Mafic magma flows to the surface along faults; in doing so it melts felsic bodies along the way felsic volcanics

• As volcanics pile up, isostatic sinking of Iceland causes partial melting and further segregation of felsics more felsic volcanics


Origin of continents:Iceland example

About 10% of Iceland’s crust is felsic in composition


Origin of continents:Iceland example

• Iceland’s crust is 8–10 km thick, about twice the average thickness of oceanic crust

• Iceland is only about 16 million years old and still growing—it’s a protocontinent!

• Archean continents remained small: lithospheric plates were all small because of Earth’s high heat flow

• In Proterozoic time, once the pace of plate tectonics slowed, protocontinents were sutured together to form larger continents


Archean continental crust

• Oldest dated continental crust minerals are ~4.4 billion years old

• Oldest large area of continental crust is ~3.8–4.0 billion years old (NWT Canada)

• Geologists believe that by ~3.5 bybp, total volume of continental crust reached its present level– No net gain or loss since then, because as new felsic

material is added by igneous activity, old felsic material is consumed at subduction zones


Archean rocks

• Archean sedimentary rocks are mostly of deep-water origin– Sandstones, cherts, shales, banded-iron

formations– Very few, if any, limestones or evaporites– No well developed continental shelves for

accumulation of shallow-water deposits


Archean rocks


Archean rocks (cont.)

• Banded iron formations– Alternating bands of iron-rich layers and chert layers– Thought to have precipitated from hot marine water

associated with igneous activity– Iron is weakly oxidized (looks like iron), suggesting

little or no exposure to oxygen• Very few banded iron formations younger than 1.9

billion years old (when atmospheric O2 increased)• Most iron deposits younger than 1.9 billion are

highly oxidized (red beds)– Principal source of world’s iron ore


Banded iron formations

Iron layers

Chert layers(red)


Archean rocks (cont.)• Greenstone belts

– Make up large portions of Archean terranes

– Age of most greenstone belts is ~2.5–3.0 billion years

– Elongate belts of weakly metamorphosed rock separating larger masses of felsic protocontinents

– Include metamorphosed mixtures of mafic and felsic volcanics, volcanic sediments, turbidites

• Assemblage of precursor rocks is characteristic of forearc basins and subduction zones

– Probably formed along subduction zones where protocontinents were sutured together


Formation of greenstone belts

Time 1

Time 2


Greenstone beltsSatellite view ofArchean greenstone beltsand felsic protocontinentsin western Australia

25 mi


Life on Earth

• Why Earth is well suited for harboring life:– Right size

• Gravitational pull of larger planets creates an atmosphere too dense for penetration of sunlight

• Gravitational pull of smaller planets is too weak to retain an atmosphere

– Right temperature• Most H2O is in the form of liquid water, not water

vapor


The Archean fossil record

• All Archean fossils are prokaryotes– Archeobacteria and Eubacteria

• The oldest known forms are bacterial filaments like modern cyanobacteria– 3.2 to 3.5 billion years old, from Western

Australia

• Stromatolites known in rocks 3.4 billion years old and younger


The Archean fossil record (cont.)

3.5 billion year old bacteriapreserved in chert fromWestern Australia

Modern cyanobacterialfilaments


Fossilized bacterial filaments:3.2 billion years old, NW Australia

diameter of filaments = 2 µm


Oldest known stromatolites:3.45 billion years old, W Australia


The Archean fossil record (cont.)3.2 billion year old stromatolitefrom South Africa

Growth of cyanobacterial mats


Origin of life

• Basic attributes of life:– Ability to reproduce– Self-regulation (ability to sustain orderly

internal chemical reactions)

• Proteins are among the compounds required for reproduction and regulation

• Amino acids are the building blocks of proteins


Origin of life• Laboratory synthesis

of amino acids from simulated early atmosphere

• Stanley Miller Soup (1953)– Hydrogen (H)– Ammonia (NH3)– Methane (CH4)– Water vapor (H2O)– Electrical spark– No O2

Amino acidscollected here


Stanley Miller


Origin of life• Miller’s assumption was that no O2 existed in Earth’s

early atmosphere– Incorrect: at least some was there (but not much)

• Experiment did produce many types of amino acids that combined to form simple protein-like compounds

• Amino acids later discovered in Murchison meteorite (1969) in the same relative proportions as in Miller’s soup– Thus, amino acids could have been delivered to the

Archean Earth from space


Origin of life• Nucleic acids

DNA and RNA—also essential for life

• DNA carries genetic code and has ability to replicate itself

phosphate group

sugar

nucleotidebases


Origin of life• RNA also can replicate itself

– Messenger RNA carries information from DNA to sites where proteins are formed

– Transfer RNA ferries amino acids to sites where proteins are formed, and serves as a catalyst in protein growth

• RNA probably was the nucleic acid in the earliest true form of life, with DNA evolving later

• Once RNA and DNA had originated, semipermeable cell membranes evolved that could protect the chemical system of the primitive organism while allowing certain compounds to pass in and out


Origin of life• Where did life form?

– Probably not at the Earth’s surface in shallow pools, as once believed

• Presence of oxygen would have inhibited the “cooking” of “Stanley Miller soup”

– Most likely in the deep sea, away from O2, and probably near a “vent” of hot water

• Modern chemosynthetic bacteria are abundant near vents on mid-ocean ridges

• They derive energy by consuming chemical compounds and allowing reactions to occur within their cell membranes


Mid-ocean ridge “vents”


Origin of life• Mid-ocean ridges are the most likely sites

for origin of life and early bacterial evolution– Enormous size many opportunities for key

events to take place– Anoxic (no O2) water with necessary amino

acid building blocks present– Other key materials present

• Phosphorus, nickel, zinc, clays

– Modern “vent” bacteria are genetically the most primitive archeobacteria known

precambrian time

Documents

earths history

earths mantleearth history

mybpearth history

oldearth history

planetearth history

earths rotation

geologic timeearth history

oceanic crustearth history