GEO 3200 1

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Formation of the Earth and the Moon

Solar Nebula Hypothesis

1. Cold cloud of gas and dust rotates and compresses into a disk-like shape

90% mass concentrated in the center

Turbulence in cloud causes matter to collect in certain locations

2. Clumps of matter begin to form in the disk

Accretion of matter (gas and dust) around clumps by gravitational attraction

Clumps develop into proto-planets

Solar wind drives lighter elements outward

Debris from impact was thrown into orbit around Earth and collected to form the Moon

Moon formed from impact, mantle of impacting body included, proportions of Fe and Mg differ from Earth's mantle ~4.4 Ga

Cold Accretion Model (secondary differentiation)

Earth formed by acccretion of dust and larger particles of metals and silicates

Originally a homogeneous mix throughout

Differentiation: result of heating and partial pressure; possible sources are radioactive decay, gravitational compression, bombardment

Differentiation after Accretion

Iron and nickel sink to form core

Less dense material (Si and O combined with remaining iron and other metals) forms mantle and lighter crust (dominated by silicon and oxygen)

Presence of volatile gases on Earth indicates that complete melting did not occur

Earth was repeatedly partly melted by great impacts, such as the Moon-forming impact

Hot Accretion (primary differentiation)

Internal zonation of planets is a result of hot heterogeneous accretion

Hot solar nebula

Initial crystallization of iron-rich materials forms planet's core

With continued cooling, lower density silicate materials crystallized

Archean Crust

One differentiation occurred, Earth's crust was dominated by Fe and Mg silicate minerals

Earth experienced heating and partial melting, it may have been covered by an extensive magma ocean

Magma cooled to form Komatiites -- ultramafic rocks composed of olivine and pyroxene

Oceanic Crust

4.5 Ga

Formed at mid-ocean ridges

Composed of Komatiite and basalt

Widespread

Formed by partial melting of ultramafic rocks in upper mantle

Continental Crust

4.4 Ga

Formed at subduction zones

Composed of Tonalite, granodiorite, granites

Local

Formed by partial melting of wet, sediment covered mafic rocks in subduction zones

BIFs -- What are they and why are they important?

Anoxic Atmosphere

Earth's early atmosphere was strongly reducing and anoxic (lacked free oxygen)

Evidence is in sedimentary rocks: lack oxidized iron in oldest sed rocks, dark due to presence of carbon, lack carbonate rocks, abundant chert

The chemical building blocks of life could not have formed in the presence of O2!!

Appear During the Precambiran

Includes cherts--red oxidized iron, gray unoxidized iron

Lighter colored stuff is precipitated quartz

Precipitates on shallow sea floor

Iron sourced from:

weathering of iron-bearing rocks on continents

Submarine volcanoes and hydrothermal vents

Stopped forming 1.9 billion years ago

We infer oxygen began to appear in the early atmosphere due to these BIFs being evidence for it

Forming an O2 Rich Atmosphere

Result of photochemical dissociation

UV radiation in upper atmosphere breaks water into H and O

Photosynthesis: bacteria and plants produce oxygen

Volcanic Outgassing

Release of water vapor and other gases from the Earth through volcanism

Most of the water on the surface of the Earth and in the atmosphere was outgassed during the first billion years of Earth history

Formation of the Hydrosphere

Once at the Earth's surface, gases and other volatile elements underwent a variety of changes

water vapor condensed and fell as rain

rain water accumulated in low places to form seas. the seas were originally freshwater

Carbon dioxide and other gases dissolved in the rain and the water was much more acidic than today

this caused rapid chemical weathering (adds Na, Ca, K, and other ions to seawater)

change to more alkaline water may have occured rapidly, this neutrilized the acidic seas

Ions accumulated in the seas, increasing salinity

Seas are less acidic (CaCo3 forms shells of marine organisms and limestones

Two main depositional environments

Submarine volcanic zones

Rift basins

both provide the necessary sources of iron and silica!

The Oxygen Revolution

The earth atmosphere had NO free oxygen, NO ozone layer

Early oceans were rich in dissolved iron and silica, rare today

Silica today is extracted from seawater at the same rate it is dissolved since it is used by organisms for making skeletons

Iron can ONLY be dissolved in oxygen poor water, where only bacteria can survive

BIFs from this time can be traced for hundreds of miles, laid down uniformly and continuously

Can only form by an oxidation reaction

Two models demonstrate how BIFs could form in the early ocean, but neither alone can account for the sheer volume of BIFs worldwide

The formation of BIFs and the composition of our present day atmosphere both require the appearance of free oxygen in the atmosphere

Cyanobacteria (producers of stromatolites) were also present at this time, photosynthesizing away and generating free oxygen

With the presence of free oxygen in the oceans, bacteria had a new niche that could be filled: that of iron-oxidizing bacteria

With the lack of any ozone layer, UV radiation was very high, and with free oxygen in the ocean, inorganic precipitation could also occur

With free oxygen in the ocean and atmosphere, a new ecological niche could be filled RESPIRATION

Oxygen in the atmosphere = UV producing ozone layer to block the UV

Two Models for forming BIFs

1. Inorganic precipitation: UV radiation forms and iron compound in surface water

2. Organic precipitation: bacterial photosynthesis breaks up dissolved iron carbonate to generate energy

BIFs in Rifts

During the Neoproterozoicm part of the Amazon craton tried to rift apart

This forming a 120 triple junction failed rift

At the time of rifting, the area was isolated and covered in ice

Only hydrothermal activity in the glaciated rift deposited minerals, mainly silica, iron and Mg, forming a BIF

Proterozoic Glaciations

During the Neoproterozoic, the Earth experienced at least TWO episodes of global glaciation

One glacial episode was in the Cryogenian period

Other was in the Edicaran period

Resulted in extensive continental glaciations

Seds deposited in basins and shelf areas along the eastern edge of the N.America craton

Most of these rocks deformed during Paleozoic orogenies

Snowball Earth

The entire Earth was ice-covered for long periods of time

Each glacial period lasted for millions of years and ended violently under extreme greenhouse conditions

The Glacial Part

Snow fall+sea ice=increased albedo=more snow fall

Once ice reached critical latitude (~30 degrees) 1/2 Earth's surface covered, the feedback is tremendous and ices the whole Earth

The Warming Part

Heat escaping from Earth's interior prevents oceans from freezing to the bottom

Surface erosion is also prevented

CO2 supply to atmosphere and oceans continues due to volcanism and plate tectonics

But it cannot be absorbed do to the ice cover!

Over millions of years, CO2 levels become so high they offset the ice-albedo feedback

Once melting begings, new water vapor in the air dramatically increases global warming effect (water vapor is the strongest greenhouse gas)

Glaciation reverses and very rapidly enters an extreme greenhouse phase

Evidence for Glaciations

Glacial striations

Tillites (lithified, unsorted conglomerates and boulder beds)

Glacial dropstones

Varved clays

So why did glaciations happen in the first place?

Plate tectonics may have had a role

Continents were located around the equator so there was no tropical ocean

Heat lost by reflection from the rocks on the surface on the continents may have caused global cooling (land plants had not appeared yet)

As continental glaciers and icecaps formed, reflectivity of snow and ice furthered the temperature decrease

Glaciations are associated with:

Decrease in CO2 and increase in O2

CO2 causes the greenhouse effect and global warming

Decrease in CO2 may have caused cooling

Decrease in CO2 was probably caused by increase in the number of photosynthetic organisms

Limestones

Association of limestones with glacial deposits suggests that times of photosynthesis and CO2 removal alternated with times of glaciations

Proterozoic Life

Dominated by prokaryotes

Archae in deep sea hydrothermal vents

Planktonic prokaryotes floated in seas and lakes

Anaerobic prokaryotes in oxy-deficient environments

Cyanobacteria (stromatolites) -- photosynthetic

Eukaryotes started to appear, have distinct masses presumed to be the remains of cell nuclei or organelles

Trace fossils appear about 560 Ma (burrows)

Imprints of soft bodied organisms

Ediacarian (Neoproterozoic) fauna, lived before predators, until this time most organisms were microscopic

Snowball Earth -- what's the hypothesis about and what evidence is used to support it?

Early Paleozoic

Ordovician

Tactonic Orogeny -- Queenston Clastic Wedge; was between Laurentia (N. America, east coast) and Baltica (Europe and western Russia); carbonate platform wedged into subduction zone, exotic terrane

The Five Mass Extinctions

1. End Ordovician: 60%

2. Late Devonian

3. End Permian: 80-95% marine species

4. End Triassic: 50% marine invertebrates, 80% land quadrupeds

5. End Cretaceous

The orogenies of North America - timing and extent

Cambrian

Tommotian stage (early Cambrian)

several skeletal elements

Not part of any living phylum today

Cambrian explosion (middle and late Cambrian) of marine life, few species and genera, many body plans

Bottom-dwellers abundant

Trilobites

Arthropods

Early fish

Softbodied creatures - cnidarians, predatory worms

Huge carnivores - anomalocarids

Enchinoderms, inarticulate brachiopods, mollusks

Stromatolites less abundant

Reefs: Archeocyathids, suspension feeders, probably sponges

Paleozoic Life

Ordovician

Radiation in graptolites and nautiloids

Life in sediments, burrower diversification

Jawless fishes

Grazing snails

Articulate brachiopods

Chrinoids expanded

Reefs: Coral-strome reefs, rugose corals, tabulate corals, stromatoporoids

Plants may have invaded land, not much evidence, restricted to moist habitats

Late Paleozoic

Devonian

Acadian Orogeny -- Catskill Clastic Wedge; famous unconformity Old Red Sandstone (Hutton); second phase of Appalachian mountain building

Mississippian

Antler Orogeny --

Mississippian-Pennsylvanian-Permian

Alleghenian Orogeny -- Formed the Appalachian Mountains; northwestern Africa collided with southeastern N. America

Mesozoic

Triassic

Sonoma Orogeny -- Put exotic terrain on western U.S. (volcanic arc)

Cretaceous

Nevadan Orogeny --

Sevier Orogeny -- Middle Jurassic to earliest Cenozoic

Cenozoic

Laramide Orogeny --

Alpine Orogeny --

Himalayan Orogeny --

Silurian

Wingless insects

Early land plants

Evolutionary radiation of brachipods, bivalve mollusks, graptolites

Trilobites did not recover from extinctions

Swimming animals - ammonoids, euripterids

Jawless fishes to jawed fishes

Devonian

First insects

First amphibians

First forests

First sharks

Ferns

Mosses

Formation of Earth and Moon

BIFs

Snowball Earth

Orogenies of North America

Diversity of Life