GEO 3200 1

Formation of the Earth and the Moon

Solar Nebula Hypothesis1. Cold cloud of gas and dust rotates and compresses into a disk-like shape90% mass concentrated in the centerTurbulence in cloud causes matter to collect in certain locations2. 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 BIFs1. Inorganic precipitation: UV radiation forms and iron compound in surface water2. 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

LimestonesAssociation 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 PaleozoicOrdovicianTactonic 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 Extinctions1. End Ordovician: 60%2. Late Devonian3. End Permian: 80-95% marine species4. 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 PaleozoicDevonianAcadian Orogeny -- Catskill Clastic Wedge; famous unconformity Old Red Sandstone (Hutton); second phase of Appalachian mountain buildingMississippianAntler Orogeny --Mississippian-Pennsylvanian-PermianAlleghenian Orogeny -- Formed the Appalachian Mountains; northwestern Africa collided with southeastern N. America

MesozoicTriassicSonoma Orogeny -- Put exotic terrain on western U.S. (volcanic arc)CretaceousNevadan Orogeny --Sevier Orogeny -- Middle Jurassic to earliest Cenozoic

CenozoicLaramide Orogeny -- Alpine Orogeny --Himalayan Orogeny --

SilurianWingless insectsEarly land plantsEvolutionary radiation of brachipods, bivalve mollusks, graptolitesTrilobites did not recover from extinctionsSwimming animals - ammonoids, euripteridsJawless fishes to jawed fishes

DevonianFirst insectsFirst amphibiansFirst forestsFirst sharksFernsMosses

Formation of Earth and Moon

BIFs

Snowball Earth

Orogenies of North America

Diversity of Life