a All time prior to Cambrian period which contains conspicuous fossils b Rocks have fossils just inconspicuous c 90 of geologic time d Less than 20 of rocks at Earth s surface Precambrian Time 1 Earth s earliest time 2 Hadean 4 6 3 8 Ga 3 Archean 3 8 2 5 Ga Hadean Key Points 1 Earliest interval of geologic time 2 Earth acquired its basic configuration mantle and crusts surrounding the core Insignificant amount of continental crust in existence 3 4 Very few Hadean rocks exist today Archean Key Points 1 Second oldest interval 2 Substantial continental crust 3 High heat flow from interior was much greater than today so continents remained small because they were constantly rifting apart Precambrian 1 Precambrian rocks on Earth a Craton b Precambrian shield i Craton exposed at surface ii Craton shield is largest exposed by erosion during recent glaciation Solar System Origin 2 Several planets revolve around the Sun 3 Compositions variable 4 Belt of meteorites 5 Orbits in nearly same plane a Nebular theory 6 Radiometric dates of meteorites a Age of Earth b Proxy for the age of planetary material in solar system and therefore approximate age of Earth 7 Dating surficial moon rocks 4 527 Ga a No water rocks aren t weathered and eroded like on Earth b Little tectonics on moon rocks aren t transformed into magma or mm c Thus the moon formed 30 Ma after Earth 8 Age of the universe a Observations of redshift light from galaxies b c Big Bang concept Indicates that the distance between galaxies is increasing i Universe 13 7 Ga ii Galaxy 10 Ga iii Planets 4 6 Ga Hadean Eon 1 Early Earth was very hot due to Impacts from asteroids and meteorites a b Radioactive decay c Initial molten Earth was generally a homogeneously mixed molten mass 2 Differentiation Iron protocore a b Less dense silicate protomantle c Silicates cooled first oceanic crust d Increased concentrations of heavier elements in upper Earth from subsequent meteorite impacts 3 Moon formed after asteroid collided with Earth a Moon primarily composed of mantle material from the impacting body b Observations indicate early Earth experienced many meteorite impacts i ii Craters dated 3 8 4 6 Ga Maria floored by basalts generated by impacts c Decline in frequency of impacts differentiates the Hadean and Archean 4 Atmosphere mostly came from within a Gasses it released and retained by gravitational attraction b Volcanic emissions today give us an indication of Earth s atmosphere 5 Ocean water from volcanoes and comets 6 Salts came from weathering rock Archean Eon granitoids 1 Partial melting of oceanic lithosphere from felsic magma rich in sodium a Formed the nucleus of the first small continental crust b Archean continents remain small high heat rifts 2 Two important rock types from Archean a Greenstone belts i Broad regions of elongate stratified bodies consisting of high grade mm felsic rocks ii Have distinctive green color from mm mineral chlorite 3 Conditions for early life iii Formed by squeezing forearc basin sediments deformed oceanic crust arc volcanics between margins of small protocontinents b Banded iron formations BIF i Archean sedimentary rocks ii Alternation iron oxygen rich layers interbedded with chert 1 Chert iron rich precipitated from sea water a Silica was more concentrated in ocean b Bacteria may have aided precipitation of iron i Precipitation of iron doesn t occur in oceans today a Earth is special just the right size and temperature i Small enough not to attract meteorites plus Jupiter is a good neighbor ii Small enough that the atmosphere isn t too dense so allows sunlight to enter the atmosphere iii Big enough that the gravitational field holds its atmosphere with essential gases such as oxygen needed for life iv Liquid water in sufficient quantities b Life first appeared on Earth sometime after 4 5 Ga i Evidence 1 Stromatolites earliest known fossil life 2 Biomarkers photosynthesis at 3 7 Ga a Small carbon particles in deep water black shales b and turbidities Isotopically light carbon 12C leads researchers to believe it must be from photosynthesis by an unknown primitive life form 3 7 Ga c Actual oldest fossilized organisms prokaryotic life form found in Archean rock 3 5 Ga i Intertwined filaments on pillow lava 1 Similar prokaryotes found today associated with pillow ii Steranes compounds produced by eukaryotes 1 Discovered in Archean rocks 2 7 Ga 2 Suggests eukaryotes were present by the end of the basalts Archean 4 Rock record provides few clues on the actual origin of life 5 Scientist focus more on chemical composition necessary for life and attributes a Self replication ability to reproduce b Self regulation ability to sustain orderly chemical reactions 6 Amino acids C H O N basic building blocks for life 7 Life on Earth likely originated at MOR a Originated in a warm environment isolated from atmosphere warm area adjacent to MOR likely b Chemosynthesizers c Known to inhabit these areas today 8 Atmosphere didn t have oxygen present in large quantities a Soaked up by other sinks oxidizing reduced iron and sulfur b Strong evidence of the buildup of oxygen in the atmosphere just after the Archean