Transcript Chapter 11
Chapter 11 The Archean Era of Precambrian Time 1 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? 2 3 Precambrian Archean • Precambrian – Time prior to Phanerozoic Era – Archean Eon • 4.6–2.5 billion years ago – Proterozoic Eon 4 Precambrian Geology • Cratons – Large under-formed portions of continents – Primarily Precambrian • Precambrian shield – Craton exposed at surface – Canadian Shield exposed by glaciation 5 Precambrian Geology • Continental crust formed during Archean • High heat flow required small continents 6 Origin of the Universe • Provide important information concerning age of Earth • Fragments of larger bodies that have undergone collision and broken into pieces 7 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 8 Origin of the Universe • Stars cluster in galaxies – Organized in disks • Milky Way – Our galaxy of stars 9 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 10 Origin of the Universe • Galactic matter is concentrated • Stars form – Our Sun • Supernova – Exploding star • Solar nebula – Dense rotational cloud 11 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 12 Origin of the Solar System • Rocky debris – Collided to form aggregates – Aggregates collided to form asteroids • 40 km diameter – Some coalesced to form planets 13 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 14 Origin of Earth and Moon • Moon formed from impact – Mantle of impacting body – Proportions of Fe and Mg differ from Earth’s mantle 15 Origin of Earth and Moon • Early atmosphere –Degassing from volcanic emissions –CH4 and NH3 abundant –Little O2 •No photosynthesis • Earth’s oceans – Volcanic emissions cooled, condensed – Salts • Carried to sea by rivers and introduced at ridges • Approximately constant through time 16 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 17 – Tilted Earth 23.5° Origin of Earth and Moon • Heat Flow – Decreased through time • Indicates abundant hot spots, small lithospheric fragments 18 Origin of Continents • First crust – Basalt (oceanic) • Felsics differentiated – Formed nuclei of continental crusts – Iceland • Modern analogue 19 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 years20 old Origin of Continents • Greenstone belts – Weakly metamorphosed – Abundant chlorite • Green color – Nested in highgrade felsic metamorphic rocks 21 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 22 Origin of Continents • Banded iron formations – 3 Billion years old – Isua, southern Greenland 23 Origin of Continents • Continental accretion – Deep water sediments accreted to continent – Marine sediments form wedge between continental masses 24 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 25 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 26 Archean Life • Stromatolites – 3.5 Billion years – Suggest photosynthesis • Biomarkers for cyanobacteria – 2.7 Billion years 27 Archean Life • Miller and Urey – Produced amino acids found in proteins • Modeled primitive atmosphere • Added lightning – Included oxygen • Amino acids found in meteorites 28 Archean Life • RNA world – Nucleic acid – Can replicate itself – May have been catalyst for production of key proteins • Foundation for DNA world 29 Archean Life • Mid-ocean ridges • Sulfur reduction – High heat – S + H2 − > H2S + energy – Chemosynthetic organisms • Methane production • Hydrogen oxidation – CO2 + 4H2 − > CH4 + 2H2O + – 2H2 + O2 − > 2H2O + energy energy 30 Archean Life • Ridges offer wide range of temperatures • Organic compounds readily dissolve in warm water • Protection from ultraviolet radiation • Abundant phosphorous • Contain metals 31 • Contain clays 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 32 33 34 35 36