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Chapter 11

The Archean Era

of Precambrian Time

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Guiding Questions • When and how did Earth and its moon come into

being?

• How did the core, mantle, crust form?

• Where did Archean rocks form, and what is their nature?

• When and why did large continents begin to form?

• Where did life arise and what kinds of life existed at the end of Archean time?

• Why did relatively little free oxygen accumulate in Earth’s atmosphere through Archean time?

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• Precambrian

– Time prior to

Phanerozoic Era

– Archean Eon

• 4.6–2.5 billion

years ago

– Proterozoic Eon

Precambrian Archean

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Origin of the Universe

• Provide important

information

concerning age of

Earth

• Fragments of larger

bodies that have

undergone collision

and broken into

pieces

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Origin of the Universe

• Stony meteorites

– Rocky composition

• Iron meteorites

– Metallic composition

• Stony-iron meteorites

– Mixture of rocky and

metallic

– Proxy for core composition

• Most date around 4.6

billion years ago

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Origin of the Universe

• Stars cluster in

galaxies

– Organized in disks

• Milky Way

– Our galaxy of stars

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Origin of the Universe

• Expanding universe

– Galaxies move apart

• Redshift

– Originally concentrated into a single point

• Big Bang

– 15 billion years ago

– Age of universe

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Origin of the

Universe

• Galactic matter is concentrated

• Stars form

– Our Sun

• Supernova

– Exploding star

• Solar nebula

– Dense rotational cloud

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Origin of the Solar System • Planets formed near time of sun’s formation

– 4.6 billion years ago

• Planets far from sun are formed from volatile elements

• Planets close to sun are rocky

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Origin of the Solar System

• Rocky debris

– Collided to form

aggregates

– Aggregates collided

to form asteroids

• 40 km diameter

– Some coalesced to

form planets

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Origin of Earth and Moon • Earth materials

differentiated

– Dense at center

– Less dense

silicates rose to

surface

• Magma

ocean

– Cooled to form

crust

• Meteorite impacts increased

concentrations of some elements in

upper Earth

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Origin of Earth and Moon

• Moon formed from

impact

– Mantle of

impacting body

– Proportions of Fe

and Mg differ from

Earth’s mantle

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Origin of Earth and Moon

• Earth’s oceans

– Volcanic emissions cooled, condensed

– Salts

• Carried to sea by rivers and introduced at ridges

• Approximately constant through time

• Early atmosphere

–Degassing from volcanic emissions

–CH4 and NH3 abundant

–Little O2

•No photosynthesis

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Origin of Earth and Moon • Moon’s maria

– Originally thought

to be seas

– Craters formed by

asteroids

– Floored by basalts

– Craters

• 3.8–4.6 billion

years old

• Earth also impacted

– Tilted Earth 23.5°

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Origin of Earth and Moon

• Heat Flow

– Decreased through

time

• Indicates abundant

hot spots, small

lithospheric

fragments

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Precambrian Geology

• Cratons

– Large under-formed portions of continents

– Primarily Precambrian

• Precambrian shield

– Craton exposed at surface

– Canadian Shield exposed by glaciation

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Precambrian Geology • Continental crust formed during Archean

• High heat flow required small continents

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Origin of Continents • First crust

– Basalt (oceanic)

• Felsics differentiated

– Formed nuclei of continental crusts

– Iceland

• Modern analogue

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Origin of Continents

• Small Archean fragments

– High heat flow limited continental thickness

• Zircon crystals

– 4.1–4.2 B years old

– Weathered from felsic rocks

• Canadian Shield

– 3.8–4.0 B years old

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Origin of Continents

• Greenstone belts

– Weakly

metamorphosed

– Abundant chlorite

• Green color

– Nested in high-

grade felsic

metamorphic rocks

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Origin of Continents

• Greenstone belts

contain igneous rocks

– Volcanics contain

pillow basalts

• Underwater extrusion

– Formation of sediments

in deep water

• Graywackes,

mudstones, iron

formations, volcanic

sediments

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Origin of Continents

• Banded iron

formations

– 3 Billion years old

– Isua, southern

Greenland

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Origin of Continents

• Continental accretion

– Deep water sediments accreted to continent

– Marine sediments form wedge between continental

masses

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Archean Life • Earth is best suited

known planet

– Conditions right by 4.2 Billion years

• Western Australia organic compounds

– 3.5 Billion years

• Mars

– Water flowed once

– Life may have evolved separately

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Archean Life

• South African cherts

contain possible mold

of prokaryotic cell

– 3.4 Billion years

• Oldest unquestionable

life form

– 3.2 Billion years old

– Australia

– Intertwined filaments

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Archean Life

• Stromatolites

– 3.5 Billion years

– Suggest

photosynthesis

• Biomarkers for

cyanobacteria

– 2.7 Billion years

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Archean Life • Miller and Urey

– Produced amino acids found in proteins

• Modeled primitive atmosphere

• Added lightning

– Included oxygen

• Amino acids found in meteorites

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Archean Life

• RNA world

– Nucleic acid

– Can replicate itself

– May have been

catalyst for

production of key

proteins

• Foundation for

DNA world

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Archean Life • Sulfur reduction

– S + H2 − > H2S + energy

• Methane production

– CO2 + 4H2 − > CH4 + 2H2O +

energy

• Mid-ocean ridges

– High heat

– Chemosynthetic organisms

• Hydrogen oxidation

– 2H2 + O2 − > 2H2O + energy

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Archean Life • Ridges offer wide

range of temperatures

• Organic compounds

readily dissolve in

warm water

• Protection from

ultraviolet radiation

• Abundant phosphorous

• Contain metals

• Contain clays

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Archean Life • Atmospheric Oxygen

– Low concentrations early on

– Later, O2 released through photosynthesis

• Sink

– Reservoir that grows so as to take up a chemical as it is produced

• Early crust was sink for O2

– Pyrite (FeS2) transported but not oxidized

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