Chapter 11. The Archean Era of Precambrian Time

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1 Chapter 11 The Archean Era of Precambrian Time 1

2 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 billion years ago Proterozoic Eon 3

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5 Origin of the Universe Provide important information concerning age of Earth Fragments of larger bodies that have undergone collision and broken into pieces 5

Stony-iron meteorites Mixture of rocky and metallic

6 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 6

7 Origin of the Universe Stars cluster in galaxies Organized in disks Milky Way Our galaxy of stars 7

8 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 8

9 Origin of the Universe Galactic matter is concentrated Stars form Our Sun Supernova Exploding star Solar nebula Dense rotational cloud 9

10 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 10

11 Origin of the Solar System Rocky debris Collided to form aggregates Aggregates collided to form asteroids 40 km diameter Some coalesced to form planets 11

12 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 12

13 Origin of Earth and Moon Moon formed from impact Mantle of impacting body Proportions of Fe and Mg differ from Earth s mantle 13

14 Origin of Earth and Moon Early atmosphere Degassing from volcanic emissions CH 4 and NH 3 abundant Little O 2 No photosynthesis Earth s oceans Volcanic emissions cooled, condensed Salts Carried to sea by rivers and introduced at ridges Approximately constant through time 14

15 Origin of Earth and Moon Moon s maria Originally thought to be seas Craters formed by asteroids Floored by basalts Craters billion years old Earth also impacted 15 Tilted Earth 23.5

16 Origin of Earth and Moon Heat Flow Decreased through time Indicates abundant hot spots, small lithospheric fragments 16

17 Precambrian Geology Cratons Large under-formed portions of continents Primarily Precambrian Precambrian shield Craton exposed at surface Canadian Shield exposed by glaciation 17

18 Precambrian Geology Continental crust formed during Archean High heat flow required small continents 18

19 Origin of Continents First crust Basalt (oceanic) Felsics differentiated Formed nuclei of continental crusts Iceland Modern analogue 19

20 Origin of Continents Small Archean fragments High heat flow limited continental thickness Zircon crystals B years old Weathered from felsic rocks Canadian Shield B years old 20

21 Origin of Continents Greenstone belts Weakly metamorphosed Abundant chlorite Green color Nested in highgrade felsic metamorphic rocks 21

22 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

23 Origin of Continents Banded iron formations 3 Billion years old Isua, southern Greenland 23

24 Origin of Continents Continental accretion Deep water sediments accreted to continent Marine sediments form wedge between continental masses 24

25 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

26 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

27 Archean Life Stromatolites 3.5 Billion years Suggest photosynthesis Biomarkers for cyanobacteria 2.7 Billion years 27

28 Archean Life Miller and Urey Produced amino acids found in proteins Modeled primitive atmosphere Added lightning Included oxygen Amino acids found in meteorites 28

29 Archean Life RNA world Nucleic acid Can replicate itself May have been catalyst for production of key proteins Foundation for DNA world 29

30 Mid-ocean ridges High heat Chemosynthetic organisms Hydrogen oxidation 2H 2 + O 2 > 2H 2 O + energy Archean Life Sulfur reduction S + H 2 > H 2 S + energy Methane production CO 2 + 4H 2 > CH 4 + 2H 2 O + energy 30

31 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 31

32 Archean Life Atmospheric Oxygen Low concentrations early on Later, O 2 released through photosynthesis Sink Reservoir that grows so as to take up a chemical as it is produced Early crust was sink for O 2 Pyrite (FeS 2 ) transported but not oxidized 32

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The History of the Earth

The History of the Earth We have talked about how the universe and sun formed, but what about the planets and moons? Review: Origin of the Universe The universe began about 13.7 billion years ago The Big